UNIVERSITY OF CALirOBMIA PPBLICATI0N8 
 
 COLLEGE OF AGRICULTURE 
 
 AGRICULTURAL EXPERIMENT STATION 
 
 BERKELEY, CALIFORNIA 
 
 GUM DISEASES OF CITRUS TREES 
 
 IN CALIFORNIA 
 
 BY 
 
 HOWARD S. FAWCETT 
 
 BULLETIN No. 360 
 
 April, 1923 
 
 UNIVERSITY OF CALIFORNIA PRESS 
 
 BERKELEY, CALIFORNIA 
 
 1923 
 
David P. Barrows, President of the University. 
 
 EXPERIMENT STATION STAFF 
 
 HEADS OF DIVISIONS 
 
 Thomas Forsyth Hunt, Dean. 
 
 Edward J. Wickson, Horticulture (Emeritus). 
 
 , Director of Resident Instruction. 
 
 C. M. Haring, Veterinary Science, Director of Agricultural Experiment Station. 
 
 B. H. Crocheron, Director of Agricultural Extension. 
 
 C. B. Hutchison, Plant Breeding, Director of the Branch of the College of 
 
 Agriculture at Davis. 
 
 H. J. Webber, Sub-tropical Horticulture, Director of Citrus Experiment Station. 
 William A. Setchell, Botany. 
 Mybr E. Jaffa, Nutrition. 
 Ralph E. Smith, Plant Pathology. 
 John W. Gilmore, Agronomy. 
 Charles F. Shaw, Soil Technology. 
 John W. Gregg, Landscape Gardening and Floriculture. 
 Frederic T. Bioletti, Viticulture and Fruit Products. 
 Warren T. Clarke, Agricultural Extension. 
 Ernest B. Babcock, Genetics. 
 Gordon H. True, Animal Husbandry. 
 Walter Mulford, Forestry. 
 James T. Barrett, Plant Pathology. 
 W. P. Kelley, Agricultural Chemistry. 
 H. J. QuAYLE, Entomology 
 Elwood Mead, Rural Institutions. 
 H. S. Reed, Plant Physiology. 
 L. D. Batchelor, Orchard Management. 
 W. L. Howard, Pomology. 
 *Frank Adams, Irrigation Investigations. 
 
 C. L. Roadhouse, Dairy Industry. 
 R. L. Adams, Farm Management. 
 
 W. B. Herms, Entomology and Parasitology. 
 John E. Dougherty, Poultry Husbandry. 
 
 D. R. Hoagland, Plant Nutrition. 
 G. H. Hart, Veterinary Science. 
 
 L. J. Fletcher, Agricultural Engineering. 
 Edwin C. Voorhies, Assistant to the Dean. 
 
 DIVISION OF PLANT PATHOLOGY 
 
 J. T, Barrett E. T. Bartholomew 
 
 II. S. Fawcett C. O. Smith 
 
 * In cooperation with Division of Agricultural Engineering, Bureau of Public Roads, U. S. 
 Department of Agriculture. 
 
GUM DISEASES OF CITRUS TREES 
 IN CALIFORNIA* 
 
 BY 
 HOWAED S. FAWCETT 
 
 CONTENTS PAGE 
 
 Introduction 370 
 
 Pythiacystis (brown rot) gummosis 371 
 
 History 371 
 
 Symptoms 373 
 
 Investigations into the nature and cause of the disease 376 
 
 Resistance of different species and varieties 378 
 
 Conditions faciUtating infection and development of the disease 381 
 
 Fusarium as a secondary aid in the development of Pythiacystis gummosis.. 384 
 
 Methods of control 385 
 
 Prevention 385 
 
 Treatment 387 
 
 Mai Di Gomma or footrot 397 
 
 Symptoms 397 
 
 History 398 
 
 Investigations 398 
 
 Control 400 
 
 Botrytis gummosis 401 
 
 Symptoms and occurrence 401 
 
 Investigations as to nature and cause 402 
 
 Factors favoring the disease 402 
 
 Methods of control 404 
 
 Prevention 404 
 
 Treatm-nt 404 
 
 Sclerotinia gumming due to Sderotinia Ldbertiana 406 
 
 Psorosis (scaly bark) of orange trees 408 
 
 Symptoms 408 
 
 Investigations as to cause and manner of development 408 
 
 Experiments in treatment 410 
 
 Suggestions for treatment 413 
 
 Diplodia gumming 416 
 
 Twig gumming 417 
 
 Exanthema or dieback 418 
 
 Nature and symptoms 418 
 
 Control r. 419 
 
 Minor forms of gumming 419 
 
 Gumming due to Penicillium roseum 419 
 
 Gumming due to Fusarium sp 420 
 
 Gumming due to Alternaria sp 420 
 
 Gumming due to Bacterium citriputeale 420 
 
 Gumming associated with insect injuries 421 
 
 Gumming associated with chemical stimuli 421 
 
 Physical effects of the environment 422 
 
 Summary of directions for prevention and treatment 422 
 
 * Paper Xo. 92, University of California, Graduate School of Tropical 
 Agriculture and Citrus Experiment Station, Riverside, California. 
 
370 UNIVERSITY OF CALIFORNIxV EXPERIMENT STATION 
 
 INTRODUCTION 
 
 The aim of this bulletin is to bring together the information now 
 at hand regarding the so-called gum diseases of citrus and the various 
 minor forms of gumming in citrus trees under California conditions. 
 Special emphasis is here given to contributing conditions and to 
 methods of prevention and treatment. A considerable amount of 
 investigational data not heretofore published is included. The results 
 of investigations in other phases of these diseases are being published 
 in the Journal of Agricultural Research. These latter phases will 
 therefore receive only brief treatment here in order to avoid undue 
 duplication. It is necessary, however, to duplicate in part the descrip- 
 tions of the diseases and a small amount of other data. 
 
 Previous investigators had come to the conclusion that all gum 
 diseases of citrus trees in California originated independently of 
 micro-organisms.^ It was held that these diseases were largely auto- 
 genous in their nature, and frequently induced through the effects of 
 certain climatic or soil conditions alone. It now appears evident 
 that these environmental conditions cannot by themselves initiate 
 the severe forms of gummosis in citrus earlier attributed to them, 
 although certain factors are found to play (as they do in most para- 
 sitic diseases) an important role as contributing conditions which 
 favor infection and invasion of the host hy the causal parasites. 
 
 The investigational work on which the greater part of this bulletin 
 is based was begun in February, 1912, under the direction of the State 
 Commission of Horticulture and continued after October, 1913, at 
 the Citrus Experiment Station of the University of California. 
 Acknowledgment of the assistance rendered during this investigation 
 has been given elsewhere.^ 
 
 The names of species and varieties of Citrus will be used in 
 accordance with Swingle^ as follows : common lemon. Citrus limonia 
 Osbeck; rough lemon, a horticultural variety of C. limonia Osbeck; 
 sweet orange, C. sinensis Osbeck ; sour orange, C. grandis Osbeck ; 
 citron, C. medica Linn.; trifoliate orange, Poncirus trifoHata Raf. 
 The word lemon, when used alone will refer to the common lemon, 
 and the word orange to the sweet orange. 
 
 1 Smith, It. E., and Butler, O. Gum disease of citrus trees in California. 
 Calif. Agr. Exp. Sta. Bull. 200, pp. 235-272, 3908. 
 
 2 Fawcett, 11. S., Two fungi as casual agents in gummosis of lemon trees in 
 California. Monthly Bull. Calif. State Comm. of Horticulture, vol. 2, pp. 
 601-617, 1913. 
 
 3 Swingle, W. T., "Citrus," in Bailey, L. H., Standard Cyclopedia of Horti- 
 culture, vol. 2, pp. 270-785, New York, 1914. 
 
Bulletin 360] gUM DISEASES OF CITRUS TREES IN CALIFORNIA 371 
 
 PYTHIACYSTIS (BROWN ROT) GUMMOSIS 
 
 HISTOEY 
 
 A destructive form of gum disease similar to Pythiacystis gum- 
 mosis first attracted serious attention in the Azores about 1834. A 
 similar gum disease appeared in Italy as early as 1863 ; in Portugal, 
 1865 ; in Australia, 1867 ; in Spain, 1871 ; in the United States, 1865 ; 
 and in most other citrus regions before the year 1890. See accounts 
 and references by Savastano^ Swingle and Webber,^ Butler^ and 
 Fawcett.'^ 
 
 In the early history of citrus growing in California there appears 
 to be no record of the occurrence of gum diseases until about 1875.^ 
 Not long afterwards they became an important hindrance to commer- 
 cial citrus culture, as is evidenced by the horticultural literature of 
 the time. 
 
 A committee of citrus growers appointed to examine the condition 
 of citrus orchards, stated in 1878^ that at that time few localities 
 were free from gum disease, which this committee believed was caused 
 by excessive irrigation and unsuitable cultivation. The orchards 
 most heavily flooded with irrigv.tion water, especially on heavy soils, 
 were found worst affected. The application of manure immediately 
 around the trees was also reported as tending to promote the disease. 
 Lemon roots were found to be more susceptible than orange roots, 
 and in a discussion which followed this report much objection is 
 expressed to lemon and Chinese lemon stock as compared with sweet 
 orange. Light soils were thought by many to be better adapted than 
 hea\'y soils for orange culture since orchards on light soils were 
 healthy while those on heavy soils were rapidly dying out, presumably 
 from gum disease. 
 
 Irrigation by flooding was a common practice at that time and 
 later^*^ it was stated as a settled fact that the lemon (on lemon roots) 
 is peculiarly sensitive to moisture and easily begins to rot if water is 
 
 4''Gommosi degli agrumi, " in Patologia arborea applicata, bp. 127-141. 
 Napoli, 1910. 
 
 5 The principal diseases of citrus fruits in Florida. U. S. Dept. Agr. Div. 
 Yeg. Phys. & Path. Bull. 8, pp. 1-42, 1896. 
 
 6 A Study on Gummosis of Prunus and Citrus, Ann. Bot., vol. 25, pp. 107-153, 
 1911. 
 
 7 Gummosis of Citrus, Jour, of Agr. Eesearch. (In press.) 
 
 8 Mills, J. W., Citrus fruit culture. Calif. Agr. Exp. Station, Bull. 13.8, pp. 
 1-46, 1902. 
 
 9 Southern Calif. Horticulturist, vol. 1, p. 115, Jan., 1878. 
 
 10 Southern California Horticulturist, vol. 1, pp. 314-315, July, 1878. 
 
372 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 left standing* around it. In the following year the statement was 
 made in the same publication^^ that gum disease is induced by mid- 
 summer irrigation and that lemon trees on their own roots were 
 dying rapidly. In 1882 a leading nurseryman^^ regarded gum disease 
 as the only citrus trouble of importance. 
 
 Of the four citrus stocks commonly used at that time (sweet 
 orange, lemon, lime and citron) the lemon and the lime were stated 
 by Gary to be the most fatally affected with this disease. He advises 
 the use of the sweet-orange stock but says nothing of the sour orange 
 which was introduced at a later date. Gary's description of the 
 disease and his reference to the differences in susceptibility of varieties 
 indicate that the type of disease spoken of was Pythiacystis gummosis. 
 Because of the susceptibility of the lemon the growers gradually dis- 
 covered that other stocks must be used. The universal verdict accord- 
 ing to Holt^^ in 1892 was that lemons should not be grown on their 
 own roots. 
 
 Sweet-orange stock though much less susceptible than lemon was 
 also frequently affected with gummosis. This fact drew attention to 
 the sour-orange stock which had been used successfully in southern 
 Europe and in Florida to replace trees affected with mal di gomma. 
 This resistant stock only gradually came into use as a preventive of 
 gum disease in California. Cutter^^ in 1892 stated that his attention 
 w^as called first to the value of the sour-orange as superior to sweet 
 in its resistance to gum disease in 1885. The following year the firm 
 of Twogood, Edwards and Cutter of Riverside, received the first 
 shipment of sour-orange trees from Florida for commercial purposes ; 
 although a few trees had been grown from Florida seeds for test by 
 others before this time. 
 
 In the light of our present knowledge of varietal susceptibility of 
 citrus to gum diseases and of the part played by heaw soils and by 
 excessive irrigation (especially flooding on heavy soils), the failure 
 in growth on lemon stocks, and the dying of trees on heavy soils, 
 but not on lighter soils Avould now appear to have been due to the 
 presence and attack of gum disease organisms, especially Pythiacystis 
 citropththora Smith and Smith. 
 
 The early discontinuance of the use of lemon, lime and citron as 
 stocks, and the adoption of the orange as a general stock for all 
 
 11 Southern Calif. Hort., vol. 2, pp. 83-86, March, 1879. 
 
 12 Gary, Thomas A., Orange culture in California. Pacific Rural Press, 
 pp. 81-82, San Francisco, 1882. 
 
 1-5 Holt, L. M., "Lemon culture for profit," Proc. Calif. Pomological Soc, 
 1892. 
 
 14 Cutter, J. E., Proc. Calif. Pom. Soc, 1892. 
 
Bulletin 360] gUM DISEASES OF CITRUS TREES IN CALIFORNIA 373 
 
 varieties appears to have been largely brought about by this one 
 disease. Sour-orange stocks were introduced from Florida later, as 
 previously indicated, for the purpose of combating this disease on 
 heavy soils, but this stock has not become of Avidespread use in Cali- 
 fornia until recent years. 
 
 SYMPTOMS 
 
 Pythiacystis or brown-rot gummosis with its associated rot of the 
 fruit^^ is probably the most widespread and destructive of the citrus 
 gum diseases. On the lemon (the most susceptible variety) patches 
 of bark on the trunk are kiued and often large quantities of gum are 
 exuded (figs. 1 and 2a.) Infection usually starts at the base of the 
 trunk or on the crown roots and works rapidly both upward and 
 laterally. The bark is killed (not as in cases of Psorosis and shell- 
 bark, merely in the outer cortical layers) but entirely through to the 
 wood, thus including the cambium. A thin layer of wood tissue only 
 about Vrz of an inch thick is visibly affected unless secondary organ- 
 isms enter. The bark above the soil is not softened as it is in the 
 early stages of Botrytis gummosis but remains firm and intact until 
 drying causes it to shrink and crack longitudinally (fig. 2h). Below 
 the surface of the soil secondary organisms frequently set up fermen- 
 tation and moist decay. On the bark of old orange trees and other 
 partially resistant varieties often the progress of the disease is soon 
 arrested and the lesions tend to become self -limited. The loss of large 
 patches of bark is followed by a gradual yellowing and dropping of 
 leaves on the branches leading out from the portion of the trunk 
 aifected. 
 
 Although these are the main characteristics of the disease as seen 
 on superficial examination, there are many special features which 
 appear when more careful study is given to its development under 
 various conditions. In the earlier stages of the disease the exuding 
 gum is usually the only external symptom (fig. 2a). By lightly 
 scraping the bark at this time the margin between the sound and 
 invaded tissue is shown indefinitely, only by the gradual shading of 
 the normal green color into a drab. The bark is not softened but 
 remains firm and only after a considerable time does it shrink and 
 crack longitudinally (fig. 2h). 
 
 On healthy, rapidly growing lemon trees the area of killed and 
 darkened bark, which is elliptical or irregular in outline, is usually 5 
 to 10 inches in vertical length and half that in width, when the gum 
 
 15 Smith, R. E. and others, The Brown Eot of the Lemon. Calif. Agr. Exp. 
 Sta. Bull. 190, 70 pages, 1907. 
 
374 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 first becomes apparent. By that time the fungus has invaded the 
 the bark at this time will show that the outer margin of the invaded 
 zone in the inner tissues is about coextensive with that seen on the 
 surface. The upward and downward extension from the point of 
 infection is usually many times greater than the lateral extension. 
 
 In an irregular zone or band surrounding an actively invaded 
 area, the cambium layer shows an influence extending from the 
 margins of the dead bark. There is a production of clear, watery 
 gum which seems to originate in the region of the embryonic wood 
 among the live cells without any apparent fermentation or decay. 
 This region, not yet darkened, outside the invaded portion, will be 
 spoken of in this paper as the ''outer gummous zone" (fig. 3h). It 
 may in time extend considerable distances upward and downward and 
 small distances laterally from the margin of the invaded zone (fig. 
 3&). It has been traced for 2 and 3 feet upward. The extent of this 
 outer gummous zone varies with the age and rapidity of development 
 of the disease lesion, the condition of the tree, etc. 
 
 The inner surface of the bark in the invaded zone in a lesion of 
 considerable size varies in color from mineral brown to burnt amber 
 or fawn^^ and the same discolorations will be found on the surface of 
 the wood just at or beneath the cambium (fig. Sh). The discoloration 
 does not extend far (usually only %2 to % of an inch) into the woody 
 layers. The cambium region in the gummous zone is chamois to 
 yellow ochre in color, gradually fading at the margins into the normal 
 color of the sound woody surface. 
 
 Frequently, when the bark is irregular in contour, gum pockets 
 1 to 2 inches in length will be formed. The gum accumulates near 
 the cambium and by pressure separates the bark from the wood at 
 certain places, forming definite pockets. The pressure is usually 
 relieved by a break in the bark before the pockets become large. A 
 few deeper gum pockets of considerable size have also been found, 
 tissue usually for a period of from 2 to 4 months. The removal of 
 situated in the outer gummous zone beneath layers of wood % to i/4 
 of an inch in thickness, showing accumulations of gum under pressure. 
 The gum, which is watery and clear when first formed, hardens as it 
 comes to the surface, apparently through loss of water, and finally 
 becomes brittle. On the surface the hardened gum is usually mahog- 
 any to chestnut in color.^^ The gum accumulates on the surface in 
 long narrow ridges (figs. 1 and 2a) or in oval masses, or runs down 
 and collects in masses on the soil, according to the rapidity of its 
 
 10 Ridgway, Robert, Color standards and color nomenclature, 43 pp., Wash- 
 ington, D. C, 1912. 
 
Bulletin 360] qUM DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 375 
 
 Fig. 1. — Pythiacystis or brown-rot gummosis on lemon trunk. The tree is 
 completely girdled the dead bark resulting from several infections starting near 
 the soil surface. Eidges of exuded gum are characteristic. 
 
376 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 formation and the dryness of the air. During periods of heavy 
 dews and rains it gradually dissolves and disappears. The invading 
 hyphae frequently die out rapidly behind the marginal fringe of 
 advance, and quite often they die out completely over a part or all 
 of this outer margin, so that the progress of the disease is checked or 
 entirely arrested. Such cases are often found among trees having 
 some power of resistance, especially among orange and pomelo trees; 
 or where the weather conditions subsequent to infection become 
 unfavorable to the parasite. 
 
 In trees on which the disease has been present for a long time, the 
 dead bark over the invaded portions dries, shrinks and cracks. The 
 larger cracks are mostly vertical, with smaller horizontal cracks 
 (fig. 2&). A thin layer of the wood immediately under the invaded 
 bark will usually be found to be infiltrated with hardened reddish- 
 brown gum which protects the under layers from rapid drying and 
 to a considerable extent against the entrance of wood rotting fungi. 
 
 On old sweet-orange trees, the invaded areas are usually less exten- 
 sive and more restricted laterally than on the common lemon. There 
 is usually less gum than on the lemon. In the orange there is a greater 
 tendency for the invading fungus to die out and for the invaded 
 area to become self -limited than in the lemon. Frequently the invaded 
 areas on old sweet-orange trunks extend upward from the soil surface 
 as narrow tongues of killed bark. On younger orange trees and 
 frequently on older ones growing vigorously on heavy clay soils the 
 disease may assume much the same characteristics as it does on the 
 common lemon. 
 
 INVESTIGATIONS INTO THE NATUEE AND CAUSE OF THE DISEASE 
 
 Investigations begun in 1912 by the author^^ have led to the dis- 
 covery that the disease is infectious and that the casual agent is a 
 soil inhabiting 'Svater mold," Pythiacystis citrophthora, the same 
 fungus which was previously described as causing brown rot of lemon 
 fruits.^^ The relation of this fungus to the disease was discovered as a 
 result of a series of experiments as follows: 
 
 1. Inoculation into sound trees with bits of diseased tissue trans- 
 mitted the disease with all its characteristic symptoms (fig. 2). It 
 was found, however, that only the diseased tissue from the marginal 
 
 17 Fuwcett, H. S., Two Fungi as Casual Agents in Gummosis of Lemon Trees 
 in California. Month. Bull. Calif. State Comm. of Hort., vol. 2, pp. 601-617, 
 1913. 
 
 18 Smith, R. E. and others. The Brown Rot of the Lemon, Calif. Agr. Exp, 
 Sta. Bull. 190, 1917. 
 
Bulletin 360] quM DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 377 
 
 fringe of the killed bark of active lesions was capable of transmitting 
 the disease. Tissue from places back of this margin toward the 
 center of large lesions or from the outer gummous zone was incapable 
 
 of inducing gummosis. 
 
 Fig. 2. — Pylhiacystis or brown-rot gummosis on lemon tree. Produced by 
 inoculation with diseased bark tissue. 
 
 (a) Tree inoculated February 27, 1912 and photographed April 24, 1912. 
 The large amount of exuded gum indicates the rapid development of a gummosis 
 lesion. A majority of the trees in this same orchard had the same over growths 
 at the union of stock and scion as observed in this illustration. 
 
 (b) Same view as (a) on May 24, 1913, 15 months after inoculation. The 
 gum first formed has been dissolved by winter rains, and the dead bark has 
 dried and shrunken. Gum was exuding (farther around) at this time. 
 
 2. Culture tests made from pieces of bark from various places 
 within and beyond the killed portion of the lesions revealed the 
 presence of Pythiacystis citrophthora but usually only at the marginal 
 fringe of what was subsequently called the invaded zone. This fungus 
 usually could not be isolated from the central portion of the invaded 
 zone and it was never found well out of the outer gummous zone. 
 Isolations of this fungus were made at least 139 times from gummosis 
 trees representing 30 different orchards in 10 different counties of 
 California extending from San Diego on the south to Butte county 
 on the north. It was isolated also from one locality in Arizona. 
 
378 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 3. Inoculations were made into bark of sound trees with pure 
 cultures of this fungus and the disease was induced with all its 
 characteristic symptoms (figs. 3 and 4a). Many repetitions gave 
 the same results. Inoculations with large numbers of other fungi 
 and bacteria found in the older portions of the invaded zone failed 
 to cause the disease. The reason for the fact that only diseased tissue 
 from the marginal fringe of the invaded zone was capable of trans- 
 mitting the disease was now revealed. In this portion only, does 
 the causal organism Pythiacystis citrophthora usually remain alive. 
 
 4. The same fungus was re-isolated 40 different times from 20 of 
 the cases of gummosis produced by inoculation and again found 
 to be alive only at the outer margin of the invaded zones, just as in 
 the naturally occurring cases previously mentioned. The time elapsing 
 between inoculation and re-isolation was from 1 to 12 months in 
 different tests, and the fungus was recovered at distances of 20 inches 
 or more from the original point of inoculation. One strain of this 
 fungus isolated from a diseased tree at Whittier was inoculated into 
 and re-isolated from three different trees in succession during a 
 period of 3 years from 1912 to 1915. During this time the fungus 
 lived in the bark of the three trees for periods of 5, 11, and 6 months 
 respectively and between these periods in cultures for 2, 2, and 10 
 months respective^. Transfers from the original culture which was 
 kept alive for more than 8 years on cornmeal agar medium were capa- 
 ble of producing brown rot of lemon fruits when tested in 1921. 
 
 5. Inoculations were made also with bits of lemon fruits affected 
 with brown rot and with the fungus Pythiacystis citrophthora isolated 
 from diseased fruits, with the same results as those obtained by the 
 use of bits of diseased bark or cultures isolated therefrom. This 
 experiment served to show that the fungus previously known to cause 
 brown rot of lemon fruits and the one capable of inducing this type 
 of gummosis were identical. The detailed experiments on which these 
 statements are based are being published in the Journal of Agricul- 
 tural Research. 
 
 RESISTANCE OF DIFFERENT SPECIES AND VARIETIES 
 
 Among the citrus species and varieties that have been tested the 
 common lemon has the lowest resistance to Pythiacystis gummosis, 
 and the sour orange the highest. The sour orange usually is so 
 resistant to Pythiacystis attack that even when the most favorable 
 conditions are given by inoculation in wounds, there is only a slight 
 gumming with rapid healing of the wounded tissue and with total 
 failure to produce a diseased lesion. The sour orange is also highly 
 resistant to all other infectious gum diseases of importance. Mere 
 
Bulletin 360] qx.tm DISE^VSES OF CITRUS TREES IN CALIFORNIA 
 
 379 
 
 CO 
 
 •rt K tS <^ 
 
 
 M 
 
 T.) Extent 
 laces near 
 ummous z 
 ushing in 
 
 
 W 
 
 ^ 
 
 
 
 
 
 of in 
 chal 
 one, 
 over 
 
 
 
 
 cc 
 
 < 3- — £ 
 
 
 O 
 
 c ----a- 
 
 
 M 
 
 ed poi 
 tie on 
 ch ext 
 und. 
 
 
 
 
 o 
 
 ^^^l^. 
 
 
 ^ 
 
 2- ?< --O 
 
 
 
 
 
 
 '^o' 3 
 
 
 1 
 
 a:a.G-_ 
 
 
 •^ 
 
 3^-5- 
 
 
 o 
 
 3 "C CD 03 
 
 
 
 
 
 
 •-) 1 ^-^ 
 
 
 
 O CC^O 
 
 
 § 
 
 rt „ — ■*• 
 
 
 s ag g 
 
 
 3 
 
 o 
 
 i-i^^o ^ 
 
 
 M 
 
 c o " „ 
 
 
 CC 
 
 S S '^ 3 
 
 ^j 
 
 
 - 5 2 » 
 
 '■< 
 
 O 
 
 ^^a,^ 
 
 fs- 
 
 >-- 
 
 cog P P 
 
 ^* 
 
 
 
 ri. 
 a 
 
 fD 
 
 - 5-t; 
 
 
 3 
 
 ce X crc 
 
 05 
 
 o 
 
 111 fo 
 ark 
 ierli 
 howi 
 
 rs. 
 
 
 0^' 
 
 c-t- 
 
 ^^li 
 
 
 « 
 
 j^O! p 
 
 ■^ 
 
 O 
 
 5-p o ^^• 
 o 2-c 2 
 
 O 
 
 * 
 
 'tS 
 
 
 >i '*' n- 3 
 
 ^*J 
 
 
 on June 
 , showin 
 
 . (r) Sa; 
 ease in 
 
 o 
 
 O 
 
 o 
 
 -! 3 " ^ 
 
 
 h— < 
 
 
 P 
 
 -; . a 
 
 
 <r+- 
 
 i9i; 
 
 ivad 
 tree 
 V b 
 
 
 
 -.3a 
 
 
 o 
 
 Thefu 
 
 zone ( 
 
 Septen 
 
 cover 
 
 
 <; 
 
 s 
 a' 
 
 5' 5-?^ 
 
 Oq O P 5 
 
 
 o 
 
 -IOC 
 
 
 
 iw 2e° p 
 
 
 to 
 
 
 ^ 
 
 <5 j^ 3 CO 
 
 
 (—1 
 
 2,3-?= 2. 
 
 2-3 » 
 
 
 t— ' 
 
 o ^ P S" 
 
 
 
 
 
 ^ 
 
 
 M- 
 
 3 O >i 
 
 
 C^ 
 
 p ""2 
 
 
 t^ 
 
 
 
 
 ^ *? J"" 
 
 
 ^ 
 
 ^ ^ ™ ? 
 
 
 c 
 
 i?y 
 
 
 
 P-^:?tL 
 
 
 P 
 >S 
 <D 
 
 O 
 
 Hi 
 
380 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 gum formation, however, may be induced by suitable stimuli in sour 
 orange as well as other species and varieties. Of the forms which 
 have been most used for stocks in California, the trifoliate orange 
 probably stands next to the sour orange in resistance and the sweet 
 orange next to the common lemon in susceptibility, with the pomelo 
 and the rough lemon standing between these two. Because these stocks 
 are grown from seed there is a possibility of variation in resistance 
 within each variety due to differences between strains and observations 
 have suggested that such variation actually exists. The following 
 observations indicate the relative resistance of some of the common 
 varieties. 
 
 A block of 5,000 sweet-orange seedlings about two and one-half 
 years old, growing in nursery rows on medium heavy clay loam soil, 
 had been planted adjacent to a block of 15,000 sour-orange seedlings 
 of the same age and receiving the same care. All the trees had been 
 irrigated rather frequentlj^ and heavily. On October 21, 1914 four 
 representative rows of sweet-orange trees showed the following per- 
 centage of infection: 
 
 Row 
 
 Number of trees in row 
 
 Number of aflfected trees 
 
 Percentage affected 
 
 
 1 
 
 222 
 
 52 
 
 23 
 
 
 2 
 
 213 
 
 73 
 
 34 
 
 
 3 
 
 212 
 
 63 
 
 29 
 
 
 4 
 
 180 
 
 53 
 
 29 
 
 
 Total 
 
 827 
 
 241 
 
 Average 29 
 
 On some trees only a small lesion was evident with much gum exud- 
 ing, on others the bark was killed to a distance of 6 to 12 inches above 
 the soil, with an abundance of gum, and still other trees were dead. 
 Some trees showed a strong tendency to form ridges of callous tissue 
 along the edges of the dead strips of bark. A thorough search in 
 the block of sour-orange trees failed to reveal a single affected tree. 
 
 Differences in resistance are indicated further by an estimate 
 made by W. M. Mertz, in a nursery of citrus seedlings about two years 
 old, growing at the Citrus Experiment Station. The following is the 
 percentage of gum disease (probably Pythiacystis gummosis) which 
 was recorded. 
 
 Species 
 
 Number of 
 trees 
 
 Per cent with 
 gummosis 
 
 Citrus Aurantium (sour orange) 
 
 1000 
 1000 
 1000 
 2000 
 
 0.3 
 
 Poncirus trifoliata (trifoliate orange) 
 
 1.0 
 
 Citrus grandis (pomelo) 
 
 2.5 
 
 Citrus sinensis (sweet orange) 
 
 10.00 
 
Bulletin 360] qum DISEASES OF citrus trees in California 381 
 
 COXDITIOXS FACILITATING INFECTION AND DEVELOPMENT OF 
 
 THE DISEASE 
 
 A study of the external conditions under which Pythiacystis gum- 
 mosis is most likely to become severe, taken in conjunction with experi- 
 ments with the fungus, indicates that the important factors are: (1) 
 abundant moisture in the soil in contact with the bark over a suffi- 
 ciently long period, (2) favorable soil and air temperature, and (3) 
 wounds or other injuries to the bark. 
 
 Injuries, however, although aids to infection, are not necessary if 
 the two first-named conditions are simultaneously fulfilled. Injuries, 
 especially below the soil surface, aside from inducing infection more 
 readily when the conditions of moisture and temperature are suitable, 
 may even aid in bringing about infection where the soil moisture is 
 not excessive by conveying the organism into the moist inner bark 
 tissue where penetration can start without the addition of further 
 moisture. Wounds or injuries, therefore, while not absolutely neces- 
 sary for the entrance of the parasite when sufficient moisture is present 
 are probably the only means of entrance when the moisture content 
 of the soil immediatelv in contact with the bark surfaces is not favor- 
 able for infection. 
 
 Moisture. — The severe form of Pythiacystis gummosis is most 
 frequently found in California on lemon trees where they are budded 
 low on sweet-orange stocks which are growing on heavy soils and is 
 especiallj^ prevalent in the coastal sections. In the light of what 
 has been stated previously it is readily seen why this should be true. 
 Heavy clay soils, being much more retentive of moisture than lighter, 
 more open soils, furnish the means by which water films may remain 
 adjacent to the bark for periods of time sufficient to permit infection 
 by the fungous parasite. 
 
 The great prevalence of fogs, and the comparatively slight fluctu- 
 ations of temperature in the coastal sections also decrease the rate 
 of evaporation in the soil adjacent to the trees that has become exces- 
 sively wet during irrigation or rain. In the interior valleys even the 
 same type of soil usually drys out more quickly after irrigations and 
 rains. If the drying out is rapid there is not sufficient time for 
 infection; if drying is slow, however, the time may be sufficient for 
 the penetration of the fungous hyphae. The effect of a low bud union 
 has already been discussed. The lemon bark above the union may be 
 infected directly when the fungous parasite has communication directly 
 with it through excessively moist soil or by splashing of raindrops, 
 especially when wounds are present. High budded lemon trees have 
 
382 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 less chance of contracting the disease. If budded high on sweet-orange 
 stocks they will have somewhat more chance to escape, because this 
 stock is somewhat more resistant than lemon ; it is nevertheless suscep- 
 tible to infection, especially under conditions highly favorable to the 
 disease. If budded high on sour-orange stock, however, there is 
 practically no danger of the disease even under bad water conditions, 
 because of the great resistance of the sour-orange stock. 
 
 Temperature. — ^Within certain limits temperature appears to be 
 another important factor, especially in the growth of the fungus and 
 its formation of spores and in the infection of the host as a result of 
 the germination of these spores. As has been previously shown,^^ a 
 temperatue somewhere near 25°C. (77°F.) appears to be about the 
 optimum for a sustained growth of the fungous hyphae in cultures, and 
 very little growth takes place above 30°C. (86°F.). If as seems 
 probable similar relations hold for the growth of the fungus in the 
 bark, we have an explanation of the fact that the disease progresses 
 slowly during hot periods, and also for the fact that its progress is 
 sometimes completely checked, the fungus being killed out in many 
 instances. This is especially true in the interior valleys, in the case 
 of parts of lesions entirely above the soil surface. Here the temperature 
 of the bark frequently becomes many degrees above the death point of 
 the fungus. Recent experiments^*^ have shown that a temperature of 
 115 °F. for one minute is sufficient to kill the spores of this fungus on 
 infected fruits and temperatures much lower than this for greater 
 lengths of time will undoubtedly have the same effect. 
 
 In line with this possibility is the observation that the rapid en- 
 largement of invaded areas usually continues much later in the year 
 or may proceed throughout the whole year in the cooler coastal 
 sections, while the activity and progress of the disease are usually 
 limited to the winter, spring and early summer in the hotter interior 
 sections. The following experiment may be given as having a bearing 
 on the influence of temperature. 
 
 On September 10, 1914, at Whittier, inoculation was made by 
 inserting a bit of mycelium of P. citrophthora into a cut in the trunk 
 of a five-year-old lemon tree. On October 9, about one month later, 
 the bark was killed vertically 6 inches above and 6 inches below 
 the ino ulated cut and about 3 inches horizontally at the widest place, 
 with abundant gum on the surface. On cutting off the bark at this 
 time it was noted that there were concentric rings at the cambium 
 layer, corresponding in number approximately to the number of days 
 
 10 Fawcett, H. S., The temperature relations of growth in certain parasitic 
 fungi., Univ. Calif. Publ. Agr. Sci., vol. 4, pp. 183-232, 1921. 
 
 20 Fawcett, II. S., Calif. Citrograph, vol. 7, pp. 233 and 254, May, 1922. 
 
Bulletin 360] qxJM DISEASES OF CITRUS TREES IN CALIFORNIA 383 
 
 from time of inoculation and probably due to differences in rates of 
 advance during night and day. The rate of advance downward and 
 upward averaged about M of an inch per day. The average daily mean 
 temperature at the nearest weather bureau station (Los Angeles) 
 was about 19.5°C. (67°F.) with an average fluctuation of about 5°C. 
 (16°F.). It is of interest that the maintained temperatue at which P. 
 citrophthora advanced at approximately this rate in culture media 
 was 21°C. (71°F.) (See table 1, Univ. Calif. Publ. Agr. Sci., vol. 4, 
 p. 201). Further investigation is needed to determine the relation of 
 temperature to the rapidity of advance of the fungus in invaded 
 tissue. 
 
 Other Conditions. — A further condition which contributes to the 
 chance of infection in the orchard is that of deep planting, or the 
 accumulation of soil next to the trunk of the trees. Under these 
 conditions there is greater surface of the bark in contact with the soil, 
 and the bud union, if low, becomes buried. The bark over the union 
 between stock and scion is usually especially susceptible to infection, 
 probably because of frequent irregularities of growth, suckering, etc. 
 The practice of loosening the soil by digging about the base of the 
 tree, if no injuries are produced, probably helps to prevent infection 
 by hastening the drying out of the soil. This practice may, on the 
 other hand, if carelessly performed, result in injuries to the bark 
 and aid in the entrance of the organism or actually introduce it into 
 the inner bark tissues on bits of soil. Any agencies such as gophers, 
 field mice, or rabbits, or tools used in orchard operations that produce 
 injuries to the bark, may contribute to the ease of infection by the 
 fungus parasite. Infection, however, appears to take place frequently 
 through the sound uninjured bark. 
 
 The following is a concrete example of a set of conditions that 
 resulted in a serious outbreak in a 5-acre 6-year-old lemon orchard at 
 Whittier in 1914, on medium heavy clay soil. 
 
 The season had been one of unusually heavy rainfall. A volunteer 
 crop of barley was cut while green in March and thrown around the 
 trees in contact with their trunks. This operation was followed by 
 heavy rains and cloudy days. The following May 90 trees with 
 brown-rot gummosis in various stages were discovered. The lesions 
 appeared to have started at various places below and above the surface 
 of the soil. Some of those above the surface had started at places 
 where water sprouts had been broken off, but many appeared to have 
 started in sound, uninjured bark. The barley piled around the trunks 
 had undoubtedly aided in retaining films of water for a sufficient time 
 to permit germination of the spores and the penetration of the hyphae. 
 
384 UNIVERSITY OP CALIFORNIA EXPERIMENT STATION 
 
 All the diseased trees were successfully treated under the author's 
 directions and the results will be discussed under the heading of 
 control. 
 
 FUSARIUM AS A SECONDARY AID IN THE DEVELOPMENT OF 
 
 PYTHIACYSTIS GUMMOSIS 
 
 During the examination of a large number of naturally occurring 
 cases of Phythiacystis gummosis it was noticed that a species of 
 Fusarium frequently accompanied and was closely associated with P. 
 citropJithora in the diseased tissue. The question arose as to whether 
 the Fusarium played any part in the development or the severity of 
 the disease. 
 
 Fusarium has been mentioned frequently in the literature as hav- 
 ing some possible relation to certain types of gum disease. Briosi^^ 
 and McAlpin-^ concluded that Fusarium limoni Briosi played an im- 
 portant part in mal di gomma in Italy and in Australia. Earle and 
 Rogers,^^ though not able to produce gummosis by inoculation with 
 Fusarium, believed that under certain conditions it was probably a 
 factor in a certain type of gum disease in Cuba. The present writer 
 also had previously found species of Fusarium repeatedly associated 
 with mal di gomma or footrot in Florida, but inoculations w^ith them 
 had given negative results. 
 
 Although the experiments made with this question in mind have 
 been too few as yet to allow definite conclusions to be drawn, certain 
 results have indicated that the severity of the disease is slightly 
 increased by adding Fusarium along with Pythiacystis citrophthora 
 at the time of inoculation. The characteristics of the disease, however, 
 were the same as when the Pythiacystis was inserted alone, while 
 inoculation with Fusarium alone failed to produce this type of gum- 
 mosis. Fusarium inoculations caused only a slight killing of a narrow 
 layer of tissue along the cut without gumming and later resulted in a 
 cracking of outer layers of bark around the cut. Otherwise the effect 
 was not different in either case from that produced in the inoculated 
 cuts used as checks on the same trees. The details of this experiment 
 will be found elsewhere in this article. 
 
 21 Briosi, G., Intorno al mal di gomma degli agrumi (Fusisporium limoni, 
 Briosi). Atti R. Acad. Lincei, Roma, ser. 3^, vol. 2; Memoria della classe di 
 science fisiche etc., pp. 485-496, 1878. 
 
 22 McAlpine, D., Fungus diseases of citrus trees in Australia and their 
 treatment, Melbourne, p. 132, 1899. 
 
 23 Earle, F. S. and Rogers, J. M., *' Citrus diseases at San Pedro in 1915,** 
 in San Pedro, Isle of Pines, Citrus Path. Lab., Ann. Report 1, pp. 36-38, 1915. 
 
Bulletin 360] qUM DISEASES OF CITRUS TREES IN CALIFORNIA 385 
 
 METHODS OF CONTEOL 
 
 Prevention. — The causative fungus usually infects the bark at or 
 below the surface of the soil. The roots of a susceptible variety are 
 more resistant than the bark of the trunk. As pointed out above, 
 some of the principal conditions of infection are: (1) excessive wet 
 soil in contact with the trunk, as a result of improper irrigation and 
 drainage, or long periods of continuous rains; (2) deep planting, or 
 soil piled up around the trunk, especially when it is highly retentive of 
 moisture; (3) injuries to the bark at the base of the tree, especially in 
 wet weather. 
 
 The methods which have been found effective and are now in com- 
 mon use in preventing infection brought about by these conditions, 
 are as follows : 
 
 The soil is pulled back from the base of the tree to expose the top 
 of the first main roots and is left in a circular ridge to exclude irriga- 
 tion water from the depression next to the trunk. If the depression 
 occasionally becomes filled up with water, the resulting condition is 
 usually not so serious as when the soil is against the bark, because 
 the water after a rain soon percolates into the soil. Under usual 
 weather conditions in California, with most citrus soils, the length of 
 time that water would stand in such a depression is not sufficient to 
 produce infection. 
 
 This pulling away of the soil is most important on the heavier clay 
 soils, while on looser sandy soils this practice is not so necessary. 
 
 As an additional means of prevention the bark at the base of the 
 trunk is painted with Bordeaux paste or other non-injurious fungicide 
 after pulling the soil away. 
 
 Experiments in Prevention. — The following experiment on 15- 
 year-old lemon trees at Chula Vista has been reported by Prizer.^* 
 The trees had low bud unions and the soil (a heavy ''adobe") had 
 not been carefully pulled back from the base. Ten per cent of the 
 trees had new lesions on the trunk the year before. Thirty-six per cent 
 of the trees had already been killed by the disease and were missing. 
 The trunks, in two rows containing 81 trees, were painted with Bor- 
 deaux paste in the fall of 1913, and two other rows next to them 
 containing 72 trees were left unpainted as controls. During the next 
 summer there were 6.9 per cent of the unpainted trees affected with 
 new lesions as compared with 1.2 per cent of the painted. 
 
 24Prizer, J. A., Mouth. Bull. Calif. State Comm. of Hort., vol. 4, pp. 7-19, 
 1915. 
 
386 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 In 1913 and 1914 the San Diego Fruit Company, Cliula Vista, 
 California, instituted methods of prevention. Greater care was used in 
 keeping the soil away from the base of the tree. During the summer 
 and fall of 1913 most of the trunks of the trees were painted with 
 Bordeaux. The following table shows the number of new cases in 6 
 orchards on which a three-year record was available. 
 
 Acres in orchard 
 
 Number of trees newly infected 
 
 1912 
 
 1913 
 
 1914 
 
 10 
 
 20 
 
 7 
 
 
 
 30 
 
 9 
 
 25 
 
 5 
 
 30 
 
 19 
 
 5 
 
 5 
 
 10 
 
 11 
 
 4 
 
 
 
 20 
 
 43 
 
 43 
 
 4 
 
 10 
 
 Total 110 
 
 21 
 
 123 
 
 5 
 
 89 
 
 2 
 
 16 
 
 The marked decrease in the number of new cases of gummosis 
 from 1912 to 1914 will be noted. The total number, 123 in 1912, had 
 decreased to 16 or about one-eighth as many in 1914. 
 
 At Santa Paula records of trees affected with gummosis during 
 1912 to 1914 inclusive, were preventive treatment somewhat similar to 
 that carried out at Chula Vista was applied, show a similar decrease or 
 falling off in new cases.* These records include both Botrytis gum- 
 mosis and pythiacystis gummosis and possibly some other forms of 
 gummosis since these were not separated. The following table of 
 various blocks gives the numbers of the trees with new lesions. 
 
 Table 1 
 
 
 Number of trees in 
 block 
 
 Number of trees newly infected 
 
 Block 
 
 1912 
 
 1913 
 
 1914 
 
 A 
 
 1213 
 
 33 
 
 14 
 
 4 
 
 B 
 
 2588 
 
 150 
 
 132 
 
 5 
 
 C 
 
 1340 
 
 124 
 
 28 
 
 4 
 
 D 
 
 685 
 
 29 
 
 9 
 
 9 
 
 E 
 
 575 
 
 9 
 
 
 
 6 
 
 F 
 
 2925 
 
 103 
 
 31 
 
 
 
 G 
 
 2862 
 
 51 
 
 58 
 
 22 
 
 H 
 
 3368 
 
 62 
 
 62 
 
 30 
 
 I 
 
 4538 
 
 154 
 
 99 
 
 30 
 
 J 
 
 1570 
 
 13 
 
 727 
 
 26 
 459 
 
 
 
 
 21664 
 
 110 
 
 * The author is indebted to the Limoneira Company through J. D. Culbert- 
 son, for the use of these records. 
 
Bulletin 360] gum DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 387 
 
 This record shows that in two years the total number of newly 
 infected trees decreased from 3.3 to 0.5 per cent and this decrease was 
 apparently due to the adoption of preventive measures based on a 
 knowledge of the real nature of the disease, gained from the experi- 
 mental results described in previous sections of this paper. 
 
 The following data on the use of Bordeaux were also taken from 
 the records kept by the Limoneira Company. In one orchard all trees 
 with the exception of 8 rows of about 35 trees each, were sprayed with 
 Bordeaux mixture two years in succession, from 1910 to 1911, and 
 following this the surface of the ground and lower branches of the 
 trees were sprayed each fall during 1912 and 1913. The records show 
 the number of new developments of gummosis in 1914. Each set of 8 
 rows contained approximately 280 trees. 
 
 Number of trees 
 
 Trees with lesions for 
 first time 
 
 Trees newlv gumming 
 at old lesions 
 
 Total 
 
 
 Number 
 
 Per cent 
 
 Number 
 
 Per cent 
 
 Per cent 
 
 Unsprayed 280 
 
 10 
 3 
 
 2.8 
 .9 
 
 49 
 
 24 
 
 17.5 
 
 8.1 
 
 20.3 
 
 Sprayed 280 
 
 9.0 
 
 
 
 It is seen that the unsprayed trees showed 2.8 per cent of newly 
 infected trees, or somewhat the same as the average for 21664 trees in 
 the same locality in 1912 before any preventive treatments were 
 started (table 1). The sprayed trees, however, showed only 0.9 per 
 cent newly infected, or about one-third as many. The continued 
 development of the disease on old lesions was 17.5 per cent on un- 
 sprayed trees as against 8.1 per cent on sprayed trees. 
 
 An effective means of prevention for new plantings, and one es- 
 pecially desirable on heavy soils, is to use trees budded high, 1 to 2 
 feet or more, on resistant stocks, such as the sour-orange. The desira- 
 bility of this stock because of its resistance, not only to Pythiacystis 
 gummosis but also to psorosis and other gum diseases, has become so 
 well recognized that a greater part of the new plantings in California 
 are of trees having this stock. 
 
 Treatment. — Even when careful attention is given to prevention, a 
 few cases of Pythiacystis gummosis will often occur from time to 
 time, especially with trees on susceptible stocks planted in hea\y soils. 
 The following method of treatment has been adopted as the result of 
 much experimental work in which the cooperation of citrus growers 
 has been of great assistance.^^ The extent to which the bark is killed 
 
 25 Fawcett, H. S., Calif. State Comm. of Hort. Monthly Bull. 2, pp. 601-617, 
 1913, and Prizer, J. A., Calif. State Comm. of Hort., Month. Bull. 4, pp. 
 7-19, 1915. 
 
388 
 
 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 through to the wood is first ascertained by scraping slightly. Then 
 the brownish killed bark (invaded zone) is dissected out with a heavy 
 knife, cutting through to the wood about l^ inch beyond the invaded 
 zone on the sides and 1 to 2 inches beyond at the top and bottom 
 in a manner similar to that shown in figure 4&. The cuts on the two 
 sides are ususally brought together above and below, making an acute 
 angle. No attempt is made to cut beyond the outer gummous zone, 
 since it has been shown that this zone does not contain the invading 
 parasite. The live bark over this zone of gummous influence, not yet 
 invaded by Pythiacystis citrophthora, will usually recover rapidly as 
 
 Fig. 4. — Method of cutting away diseased bark in treatment of a severe 
 case of Pythiacystis gummosis. 
 
 (a) Result of inoculation with a bit of diseased bark on November 16, 
 1912. Photographed September 3, 1913. Invaded area 3x8.5 inches. 
 
 (6) Bark cut away September 3, 1913. Ready for painting with Bordeaux 
 paste. 
 
 (c) Same tree on March 17, 1916, (314 years later) showing growth at 
 edges of wound and asphalt paint on exposed wood. 
 
 soon as the invaded portion has been removed and the further progress 
 of the fungus has been stopped. These cut-out places and the entire 
 trunk are then painted with Bordeaux paste (1 lb. copper sulphate 
 and 2 lbs. rock lime to l^/^ gallons of water) or some other suitable 
 fungicide (see formulae under "Psorosis"). The fungicides other 
 than Bordeaux paste most generally used in California are coal tar 
 products from which the lighter oils have been distilled. Arrow Car- 
 bolineum and Creolineum XXX are trade names of some of those in 
 common use. 
 
Bulletin 360] gum DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 389 
 
 The elimination of the tissue invaded by the causative organism is 
 the most important feature of the treatment, the purpose of the fungi- 
 cide being to prevent reinfection, or to kill the organism in small bits 
 of tissue left behind. 
 
 After treatment the gum usually continues to exude for some time 
 at the edges of the cut-out areas. This is left undisturbed except to 
 examine the edges of the cuts to ascertain whether any bark is being 
 
 Fig. 5.^(a) Young lemon tree into which a sour-orange seedling i/4 inch in 
 diameter had been inarched over a gummosis lesion about 2% years before. The 
 inarched seeding had increased to % inch in diameter and had caused a ridge 
 of growth above as shown alongside the white chalk mark. 
 
 (fo) Old lemon tree showing gummosis lesion bridged by inarching sprouts 
 from .the base. About 5 years after inarching. 
 
 killed by further invasion of the fungus. The gum appears to act as a 
 protective covering under which callous tissue is readily formed along 
 the edges of the cuts. After these cut edges begin to heal, the exposed 
 wood is painted with asphalt paint or other good covering (fig. 4c). 
 In cases where a large part of bark on the trunk has been killed 
 by the rapid invasion of Pythiacystis citrophthora, so that the trunk 
 
390 UNIVERSITY OF CALIFORNIA— EXPERIMENT STATION 
 
 is girdled or nearly so, bridge grafting^^ and inarching^^' ^^' ^^' may 
 often be resorted to in order to save the tree (fig. 5). 
 
 The advisability of this practice, in any particular instance, will 
 depend upon the age and condition of the tree and other factors. 
 A modification of the usual method of bridge grafting has been success- 
 ful with citrus. Instead of a shoot or branch being grafted into the 
 bark both above and below the injured portion, a sprout at or below 
 the lower edge of the wound is allowed to grow. When of sufficient size 
 the top part is cut off, bent over, and grafted into the bark above the 
 diseased area (fig. 5&). 
 
 Another similar method of saving or aiding an injured tree is 
 inarching, in which one or more small seedling trees are planted close 
 to the trunk and the top grafted into the bark above the injury. 
 Where quick relief is necessary and suitable sprouts cannot be stimu- 
 lated, this method has proved beneficial (fig. 5a). 
 
 Experiments in Treatment.-— At Santa Paula, through J. D. 
 Culbertson of the Limoneira Company, and at Chula Vista, through 
 J. A. Prizer of the San Diego Fruit Company, experiments planned in 
 consultation with the author were carried out. 
 
 Previous to these experiments a large number of methods of treat- 
 ment were tested, especially the method of slitting the bark and apply- 
 ing neat's foot oil, all of which gave unsatisfactory results except 
 with trees treated when the lesions were very small. As soon as the 
 investigation began to indicate that the disease was due to some 
 infection the application of Bordeaux paste was used, first in connec- 
 tion with slitting (fig. 6) and later in connection with cutting away 
 of the killed tissue of the lesion (fig. 4&). 
 
 Since a detailed report of these experiments at Chula Vista has 
 been published by Prizer,^^ only a brief summary of the results will be 
 included here. 
 
 In one orchard alternate rows were left without the Bordeaux as 
 checks, but received the same treatment as to cutting away and slitting. 
 The lesions were then divided into three classes called slight, medium 
 and bad. The results of the first year's treatment (1912) before the 
 
 26 Paddock, E. H., Bridge grafting on Citrus. Month. Bull. Calif. State 
 Comm. Hort., vol. 81, pp. 72-73, 1919. 
 
 27 Rundle, H. E., Inarching Citrus Trees. Calif. Cultivator, vol. 44, pp. 
 540-541, 1916. 
 
 28 Hume, H. H., Citrus fruits and their culture, ed. 4, 196 pp., New York, 
 1911. 
 
 20 Fawcett, H. S., Experiments in Bridge Grafting and Inarching in Con- 
 nection with Gummosis of Citrus. Calif. Citrograph, vol. 8, pp. 68 and 95, 1923, 
 
 80 Prizer, J. A., Calif. State. Comm. of Hort., Month. Bull., vol. 4, pp. 
 7-19, 1915. 
 
Bulletin 360] qUM DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 391 
 
 method of cutting out was well perfected (it not yet being known how 
 far cutting away was necessary) were as follows: 
 
 Check (Bordeaux not used) 
 
 Cured 
 
 Still gumming 
 
 Trees dead 
 
 Per cent 
 
 Per cent 
 
 Per cent 
 
 SHght 
 
 81.2 
 
 18.7 
 
 
 
 Medium 
 
 50.0 
 
 50.0 
 
 
 
 Bad 
 
 7.1 
 
 21.2 
 
 71.5 
 
 Bordeaux used 
 
 
 
 
 Slight 
 
 82.3 
 
 17.6 
 
 
 
 Medium 
 
 62.5 
 
 37.0 
 
 
 
 Bad 
 
 23.0 
 
 66.0 
 
 11.0 
 
 It is to be noted that the difference in cured lesions in favor of the 
 application of Bordeaux after cutting is not large in the slight and 
 medium classes. In the bad class, however, the proportion of cures is 
 23 per cent for the Bordeaux against 7.1 per cent for the check, while 
 the dead trees in this class are only 11 per cent for the Bordeaux 
 against 71.5 per cent for the check. 
 
 The results for a period of three years, 1912-1914 inclusive as 
 obtained in treating 209 new lesions on trees which had not been 
 infected before, are of interest. 
 
 Stage of disease 
 
 Trees cured 
 
 Trees still gumming 
 
 Trees dead 
 
 
 Number 
 
 Per cent 
 
 Number 
 
 Per cent 
 
 Number 
 
 Per cent 
 
 Slight 
 
 115 
 37 
 35 
 
 95 
 
 80.4 
 83.3 
 
 5 
 
 7 
 2 
 
 4.1 
 
 15.2 
 
 4.7 
 
 1 
 
 2 
 5 
 
 .9 
 
 Medium 
 
 4.3 
 
 Bad 
 
 11.9 
 
 
 
 The results show that although a large percentage of the trees in 
 the bad classes may be cured, a still larger percentage of those in the 
 slight class yield to treatment. It is important, therefore, to inspect 
 all trees sufficiently often to detect as far as possible all cases when 
 slight, not only because of the decrease in loss of trees themselves, 
 but to prevent severe injury from loss of bark. Prizer considers it 
 necessary in the case of heavy soils near the coast to make not less 
 than three separate tree-to-tree inspections a year, and possibly four 
 on the worst orchards. 
 
 In the Limoneira Company orchards at Santa Paula, similar meth- 
 ods of treatment were carried out with satisfactory results as far as 
 Pythiacystis gummosis was concerned. The Botrytis gummosis was 
 
392 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 more prevalent in these orchards than in the Chula Vista orchard and 
 because of the different character of the disease a method of scraping 
 off the outer cortical layers, as well as cutting out entirely to the wood 
 was worked out. It is described more fully under that disease. 
 
 Later the Limoneira Company desired to displace the Bordeaux, 
 if possible by some other fungicide, partly because the presence of 
 Bordeaux when followed by fumigation with hydrocyanic acid gas 
 was thought, under the climatic conditions at Santa Paula, to empha- 
 size the injury to the foliage. The writer was asked to suggest some 
 substitutes for trial, and the following materials were tried out on 
 6-year-old trees by the Limoneira Company: (1) Carbolic acid (5%) 
 1 part, and liquid whale-oil soap, 1 part, on 2 trees; (2) Avenarius 
 carbolineum, 1 part, emulsified with 3 parts of liquid whale-oil soap, 
 on 2 trees; (3) corrosive sublimate, 1 gram in 1000 c.c. of denatured 
 alcohol, on 1 tree; (4) Rex limesulfur solution, 1 part to 1 part of a 
 paste of lime, to give it body, on 2 trees; (5) Bordeaux paste ( 1-2-1 1/^) 
 on 1 tree; (6) No application, as control, 4 trees. 
 
 These trees were examined from time to time for two years and no 
 marked difference in the recovery of any of them that could be attrib- 
 uted to the application of one substance over another, could be made 
 out. Avenarius carbolineum full strength and corrosive sublimate, 1 
 part to 1000 parts of denatured alcohol, and lime-sulfur solution 
 diluted with equal parts of water, were used in the same way at 
 Whittier in September, 1915, with results similar to those at Santa 
 Paula. 
 
 Since all the trees recovered, even the checks, it is indicated that 
 the proper dissection of the diseased bark is probably more important 
 than the fungicide applied to the wound. The writer^^ had used 
 Avenarius carbolineum emulsified with equal parts of soap and water 
 in Florida for certain diseases of orange trees. In 1916 C. C. Miller, 
 who had used Avenarius carbolineum at full strength without injury, 
 on deciduous trees, began using this substance as a substitute for 
 Bordeaux. Miller 's^^ first report of this treatment is likely to be 
 misleading because he was not then familiar with the difference be- 
 tween the different types of citrus gummosis and because he conducted 
 no extensive, well checked, comparative experiments comparing carbol- 
 ineum and other substances. It is now indicated that any good fungi- 
 cide that is not injurious to the bark is efficient for use in the treat- 
 ment of this disease. 
 
 31 Fawcett, H. S., Florida Agr. Exp. Sta. Bull. 106, 1911. 
 
 32 Miller, C. C, Month. Bull. Calif. State Comm. of Hort., vol, 7, pp. 
 488^93, 1918. 
 
Bulletin 360] gUM DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 893 
 
 An example of what can be done in controlling Pythiacystis gum- 
 mosis when it is treated promptly and with care, is furnished by a 
 five-acre lemon orchard at Whittier on clay loam soil, previously re- 
 
 Fig. 6. — Showing a method formerly used in treating brown-rot gummosis 
 by cutting vertical slits in the bark, one on each side of the diseased lesion 
 and several down through the invaded area. This treatment appeared to be of 
 some value for mild cases, but was useless for more severe outbreaks. 
 
 f erred to, where conditions facilitating infection obtained. About 20 
 per cent of the 6-year-old trees became infected after a period of 
 heavy rains following the application of green, freshly cut barley 
 about the trunks in March, 1914. 
 
394 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 The lesions noticed in May varied in size from those just started 
 to lesions extending one-half to two-thirds around the circumference 
 of the bark on the trunk. On May 25, 1914, the trees were treated by 
 dissecting out all the brown killed bark (invaded zone) and cutting 
 about % inch beyond this invaded zone on the sides and 1 to 2 inches 
 beyond at the top and bottom, as is shown in fig. 4&. The cuts on the 
 two sides were usually brought together above and below, making an 
 acute angle. 
 
 No attempt was made to cut beyond the outer gummosis zone of 
 gummous influence, so that in most cases on larger lesions this zone 
 probably extended into the remaining bark a considerable distance 
 beyond the cuts. As has been pointed out in the description of the 
 disease and elsewhere, the margin of the zone actually invaded by the 
 causal fungus usually lags far behind the zone in which gum formation 
 occurs. These cut-out places and the entire trunks were painted with 
 Bordeaux paste (1 lb. copper sulfate and 2 lbs. rock lime to 1% gallons 
 water). In order to prevent further infection the soil was pulled 
 back from the trunk as far as the top of the first main roots. 
 
 After treatment the gum continued to exude in considerable 
 quantities at the edges of the cut-out areas. This gum was left undis- 
 turbed except for examining the edges of the cuts to ascertain whether 
 the bark was being killed further. The gum appeared to act as a 
 protective covering under which callous tissue was soon seen to be 
 forming rapidly along the edges of the cuts. 
 
 On June 22, 1914, about one month after treatment, an inspection 
 showed only 5 trees with new areas of killed bark, all of which areas 
 were on trees previously treated. On August 3, 7 additional areas 
 of killed bark were found, and on September 15, 8 additional areas, 
 all except 2 of which were on trees previously treated. 
 
 On March 29, 1915, a rapid healing was noted at all the cuts and 
 no effect on the foliage could be detected. Three new lesions, all 
 originating below the surface of the soil, were found. 
 
 On July 30, 1915, 3 trees which had lost bark from areas extending 
 over two-thirds to three-fourths their trunk circumferences, were 
 showing a slight yellowing of the foliage, but only on branches directly 
 above the dissected areas. By March 6, 1916, however, only one tree 
 showed any appreciable effect of the removal of bark and this was one 
 from which about 120 square inches had been removed. The trunks of 
 all the trees had been painted again with Bordeaux paste in the fall 
 of 1915, and no new cases were found during the spring and summer 
 of 1916. 
 
Bulletin 360] qUM DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 395 
 
 The rapidity of healing of cut-out places of various sizes is indi- 
 cated by table 2. These figures were obtained by measuring the areas 
 in two directions and estimating the number of square inches in each. 
 While the figures are only approximate, they give a rough idea, at 
 least, of the rate at which the different sized areas on lemon trees of 
 this age may be expected to close up with new bark tissue. 
 
 Table 2 
 
 Number of areas 
 
 Size of original areas, 
 May 1914 
 
 Average size of areas, 
 June 1916 
 
 Average size of areas, 
 April 1921 
 
 
 sq. in. 
 
 sq. in. 
 
 sq. in. 
 
 5 
 
 .5 
 
 0.0 
 
 0.0 
 
 3 
 
 1.0 
 
 .08 
 
 0.0 
 
 8 
 
 2.0 
 
 .97 
 
 0.0 
 
 4 
 
 3.0 
 
 1.2 
 
 0.0 
 
 10 
 
 4.0 
 
 2.7 
 
 0.6 
 
 2 
 
 5.0 
 
 3.-1 
 
 0.5 
 
 5 
 
 6.0 
 
 3.7 
 
 0.0 
 
 4 
 
 7.0 
 
 2.5 
 
 0.9 
 
 7 
 
 8.0 
 
 4.0 
 
 0.6 
 
 5 
 
 10.0 
 
 4.2 
 
 1.0 
 
 2 
 
 11.0 
 
 8.0 
 
 2.0 
 
 5 
 
 12.0 
 
 9.8 
 
 3.2 
 
 3 
 
 16.0 
 
 11.3 
 
 5.0 
 
 5 
 
 22.0 
 
 17.0 
 
 5.6 
 
 4 
 
 35.0 
 
 27.0 
 
 7.5 
 
 3 
 
 40.0 
 
 22.0 
 
 15.0 
 
 7 
 
 50.0 
 
 42.5 
 
 25.0 
 
 4 
 
 75.5 
 
 55.0 
 
 23.7 
 
 5 
 
 105.6 
 
 69.0 
 
 39.6 
 
 1 
 
 130.0 
 
 80.0 
 
 60.0 
 
 It will be noted that all wounds less than 3 square inches in size 
 at time of treatment were closed, or insignificant, at the end of 2 
 years. Nearly all wounds less than 10 square inches in size at time of 
 treatment were closed in 7 years. Areas larger than 10 square inches 
 at time of treatment were reduced to about one-half to two-thirds of 
 their original size in 2 years and about one-third to one-fourth their 
 original size in 7 years. 
 
 TEEATMENTS FOR GUMMOSIS TO BE AVOIDED, OR USED WITH CARE 
 A treatment for wounded tree trunks, mention of which is found in 
 very early literature, is the use of fresh cow dung applied to the 
 surface of wounded tissue. Whatever value this practice may have 
 for mechanical wounds, when parasitic organisms happily do not get 
 
296 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 started in such a medium, it is certainly extremely dangerous when 
 applied to tree trunk affected with either Pythiacystis or Botrytis 
 gummosis, for the reason that ideal conditions for growth of the fungus 
 parasites would seem to be furnished by the presence of the fresh, 
 moist cow dung. 
 
 An example of the results of such practice will be briefly mentioned 
 here. 
 
 In February, 1913, on the advice of a foreign expert in citrus 
 culture, a grower treated a large number of trees that had previously 
 had gummosis, by coating their entire trunks with cow dung, then 
 wrapping them first with newspapers and over this, burlap. A bunch 
 of dry weeds and grass was then placed in the forks of the branches. 
 Fifteen pounds of sulfur was added to every 50 gallons of the dung. 
 During the latter part of May it was discovered that a large amount 
 of softening, dying and decajdng of new bark was going on under 
 these wraps. Gum pockets were forming on large limbs above the 
 wrappings under the bunches of grass in the forks. All wraps were 
 then removed as fast as possible, the dung scraped off, the dead, soft, 
 decaying patches of bark cut away and the entire trunk painted with 
 Bordeaux paste. It is not possible to say definitely what would have 
 happened to these trees if the wraps had remained, but many of them 
 would probably have been injured beyond recovery by the rapid 
 increase in the decay of bark under the burlap. 
 
 Records had been kept of 1023 trees so treated. Of these 293 had 
 previously had their trunks painted with Bordeaux paste in connection 
 with gum disease treatment. The remaining 730 had not been painted 
 with Bordeaux previous to wrapping. It was interesting, therefore, 
 on removal of the wraps, to compare the condition of the trunks 
 bordeauxed before wrapping with those not so treated. The following 
 data were obtained : 
 
 Trees not Bordeauxed before wrapping 
 
 Gummosis active on removal of wraps 
 Number 
 
 Per cent 
 
 730 
 
 Trees Bordeauxed before wrapping 
 
 486 
 31 
 
 66.6 
 
 293 
 
 10.6 
 
 These results seem to show that the presence of the Bordeaux under 
 the dung at time of application prevented a greater part of the 
 damage in the case of the trees so treated. 
 
Bulletin 360] g^JM DISEASES OF CITRUS TREES IN CALIFORNIA 397 
 
 Another treatment which frequently results in injury to the trees 
 is the use of neat's foot oil. Considerable evidence has been collected 
 to the effect that this substance has often killed considerable bark or 
 furnished a condition which was favorable to the growth of the para- 
 sites. Experiments in the laboratory showed that Pythiacystis citroph- 
 thora grew as well on the surface of bark previously treated with 
 neat's foot oil, as on that which had not been treated, and Botrytis 
 cinerea grew much better on bark to which neat's foot oil had been 
 applied. 
 
 Light, penetrating oils, such as kerosene, should be used with much 
 caution on citrus bark. A number of cases have been observed where 
 kerosene applied at the base of the tree trunk caused the death of the 
 bark at and below the surface of the soil. 
 
 MAL DI GOMMA, OR FOOTROT 
 
 Mai di gomma, due to Phytophthora terrestria Sherbakoff, is a 
 gum disease with close relationships to Pythiacystis gummosis. Certain 
 phases of the Pythiacystis gummosis occurring on, or near, the main 
 roots of sweet-orange trees, are quite similar to those of mal di gomma, 
 or foot rot. For this reason, certain foot rot-like forms, due to Pythia- 
 cystis citrophthora in California, have previously been referred to as 
 mal di gomma. ^^' ^"^ Since the term mal di gomma was used first in 
 Florida to designate a common Florida gum disease, which is now 
 known to be induced by Phytophthora terrestria, it is proposed to 
 restrict its use (in this country at least) to the disease due to this 
 fungus. 
 
 SYMPTOMS 
 
 This type of gum disease affects, for the most part, the -bark on 
 the lowest portion of the trunk and the upper portion of the highest 
 main roots, mostly below the surface of the soil. Gum usually forms 
 on the trunk of the tree above the soil. The inner bark and finally 
 the wood underneath frequently develop a fetid odor.* The bark 
 
 33 Smith, K. E., and Butler, O., Gum Disease of Citrus Trees in California. 
 Calif. Agr. Exp. Sta. Bull. 200, pp. 235-272, 1908. 
 
 34 Fawcett, H. S., The known distribution of Pythiacystis citrophtJiora, and its 
 probable relation to mal di gomma. Phytopathology, vol. 5, pp. 66-67, 1915. 
 
 * This rotting of the wood, as well as the bark, and the accompanying fetid 
 odor are believed to be due mainly to secondary organisms setting up fermenta- 
 tion and decay below the surface of the soil after killing of the bark by 
 the primary organism. While gum may be formed below as well as above the 
 surface of the soil, it is dissolved readily by moisture and is usually less con- 
 spicuous below the soil surface. This disease, under California conditions, 
 cannot be distinguished from certain phases of brown rot gummosis except by 
 means of culture tests for isolating the causal organisms. 
 
398 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 dies and breaks away in patches, leaving bare, dead areas, wbicli spread 
 in all directions, but mostly downv^ard, on the main crov^n roots and 
 laterally around the trunk (fig. 8). Trees thus affected bear heavy 
 crops of fruit temporarily and the leaves become yellovr. 
 
 HISTOEY 
 
 This gum disease was first known as foot rot and attracted attention 
 in Florida about the same time that Pythiacystis gummosis was noticed 
 in California. Curtiss,-^*'^ in 1888, reported that it appeared in 1876, 
 although few people, he says, remember having observed it before 
 1880. That the disease was not important in Florida before that time 
 is indicated by the fact that Bishop, ^^ writing on citrus culture in 
 Florida in 1875, discusses a number of other diseases but does not 
 mention this one. 
 
 In a book published in 1881, Moore,^^ who made extensive observa- 
 tions on citrus culture in Florida, speaks of this disease under ''foot 
 rot*' as having appeared in "late years" in Florida. In 1896 Swingle 
 and Webber^^ stated that the disease was widely distributed in Florida 
 and seemed to be gradually spreading. 
 
 INVESTIGATIONS 
 
 Only once has the causal fungus been isolated in California. This 
 was from an orange tree at Lindsay in 1912. It was considered by 
 the writer at that time to be only a peculiar strain of the brown rot 
 fungus, Pythiacystis citrophthora, but was later identified by Sherba- 
 koff as the same species which he had previously described.^^ This 
 species, or a closely allied one, appears to be widely distributed, occur- 
 ring in a number of countries. It was found on citrus*^' *^ in Florida, 
 Cuba and Argentine, on coconuts, tobacco and pineapples in Ja- 
 maica,*^ on tomatoes in Florida^^ and castor oil plants and Vinca in 
 India.^^ 
 
 35 Curtiss, Sore Shin or Gum Disease. Fla. Agr. Exp. Sta. Bull. 2, 1888. 
 
 36 Bishop, P. P., Proc. Am. Porno. Soc, p. 48, 1875. 
 
 37 Moore, T. W., Treaties and handbook of orange culture, New Yor^, 1881. 
 
 38 Swingle, W. T., and Webber, H. J., The principal diseases of citrus fruits 
 in Florida. U. S. D. A., Div. of Veg. Phys. and Path. Bull. 8, 1896. 
 
 39 Sherbakoff, C. D., Buckeye rot of tomato fruit. Phytopathology, vol. 7, 
 pp. 119-129, 1917. 
 
 40 Fawcett, H. S., Pythiacystis and Phytophthora. Phytopathology, vol. 10, 
 pp. 397-399, 1920. 
 
 41 Stevens, H. E., Florida citrus diseases. Fla. Agr. Exp. Sta. Bull. 150, 
 110 pp., 1918. 
 
 42 Ashby, S. F., Leaf-stalk rot caused by Phytophthora parasitica. In west 
 Indian Bull. vol. 18, pp. 70-73, 1920. 
 
 43 Dastur, J. F., Phytophthora parasitica new species; a new disease of the 
 castor oil plant. Mem. Dept. Agr. India, Bot. Ser., vol. 5, pp. 177-231, 1913; 
 idem., Phytophthora on Vinca rosea. Mem. Dept. Agr, India, Bot. Ser., vol. 8, 
 pp. 233-242, 1916. 
 
Bulletin 360] gUM DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 399 
 
 Fig. 7. — Mai di gomma or foot rot. (a) On 40-year-old seedling orange trees. 
 White lines indicate boundaries of killed bark, (h) Earth dug away and 
 effected bark and roots being removed before applying Bordeaux paste. 
 
400 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Many comparative inoculations, with Pythiacystis citrophthora, 
 the cause of brown rot gummosis, and Phytophthora terrestria, the 
 cause of mal di gomma, were made under various conditions and the 
 same type of lesion was produced by both fungi. There was an indi- 
 cation in some of the experiments that sweet-orange bark was more 
 susceptible than lemon to Phytophthora terrestria while the reverse 
 was generally true of the fungus of brown-rot gummosis. According 
 to Hume^* in Florida the sweet-orange is more susceptible to mal di 
 gomma than the common lemon, and sour-orange is very resistant. 
 
 The relation of temperature to growth is somewhat different for 
 the two fungi. *^ While the optimum temperature for sustained growth 
 over a period of several days in the laboratory for Pythiacystis citroph- 
 thora is about 77°F. (25°C.), that for Phytophthora terrestria is 
 about 86°F. (30°C.). It is of interest in this connection that the 
 rainy season in California occurs during winter and spring, coinci- 
 dent with moderate temperatures, while the moist season in Florida 
 occurs during the summer, coincident with much higher temperature. 
 
 CONTEOL 
 
 The means of prevention and treatment of mal di gomma, are 
 essentially the same as those for brown-rot gummosis, a certain form of 
 which it so closely resembles that only laboratory examination can 
 distinguish the two diseases. Care regarding excessive moisture in 
 contact with the bark of the crown roots and the base of the trunk, 
 care in keeping the soil away from the base of the tree and in avoiding 
 injuries, the use of sour-orange stocks for new plantings, the use of 
 fungicidal washes on the bark, are all useful in prevention. For 
 detailed suggestions regarding prevention and treatment, * see under 
 brown-rot gummosis. 
 
 44 Hume, H. H., Some Citrus troubles. Fla. Agr. Exp. Sta. Bull. 53, pp. 
 145-173, 1900. 
 
 45 Fawcett, H. S., The temperature relations of growth in certain parasitic 
 fungi. Univ. Calif. Publ. Agr. Sci., vol. 4, pp. 183-232, 1921. 
 
Bulletin 360] qUM DISEASES OF CITRQS TREES IN CALIFORNIA 401 
 
 BOTRYTIS GUMMOSIS 
 
 SYMPTOMS AND OCCUEEENCE 
 
 Botrytis gummosis*^ differs from brown-rot (Pythiacystis) gum- 
 mosis, in that it causes softening of the invaded bark in the early 
 stages and shows a grey color on the surface in damp, cool weather, 
 caused by the conidiophores and spores of the fungus (fig. 9). In 
 the later stages the outer layer of bark is killed and becomes dry and 
 hard much in advance of the inner layer, while there is a greater 
 tendency than in brown-rot gummosis for the tree to renew the bark 
 underneath the dead, hard layer, and there is usually also a less 
 copious flow of gum. Unlike brown-rot gummosis, Botrytis gummosis 
 is confined in California almost exclusively to lemon trees growing 
 in the coastal regions, and usually occurs on trees that are more than 
 10 years of age. This disease should not be confused with ''shell 
 bark, ' ' a desquamated bark condition in which the outer bark of lemon 
 trees dies, cracks and breaks away in longitudinal strips, a condition 
 which is somewhat similar to that frequently brought about in the 
 later stages of Botrytis gummosis. These two diseases are often asso- 
 ciated on the same trees. The conditions favorable to the one are 
 also apt to encourage the other. Neither disease should be confused 
 with psorosis (scaly bark) of sweet-orange trees. 
 
 Unlike Pythiacystis citrophthora, this fungus is not able to gain 
 entrance except through some wound or defect in the bark, and is 
 not able to progress so rapidly in killing the bark through to the wood. 
 A large area is involved, in which only certain outer layers of bark 
 tissue are killed, leaving the cambium alive and capable of renewal. 
 This gummosis produces an outer gummous zone beyond the invaded 
 area, but this is usually less extensive and less rapidly formed than 
 with Pythiacystis gummosis. Other conditions being equal, there is 
 usually somewhat less gum formation in Botrytis gummosis than in 
 Pythiacystis gummosis. 
 
 The writer's attention was first called to this type of gummosis 
 early in February, 1912. After a period of moist, cool weather, patches 
 of bark 6 to 12 inches long and half as wide presented the gray furry 
 appearance characteristic of the fruiting bodies of Botrytis cinerea 
 (fig. 8). In a later survey of th-e citrus districts of California, Botrytis 
 cinerea was always found associated with this type of gummosis and 
 was isolated from a large number of diseased trees. 
 
 46 Fawcett, H. S., "Two fungi as causal agents in gummosis of lemon trees 
 in California," in Month. Bull. Calif. Comm. Hort., vol. 2, pp. 601-617, 1913, 
 and Phytopathology, vol. 4, p. 54, 1914. 
 
402 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 INVESTIGATIONS AS TO NATURE AND CAUSE 
 
 The investigations establishing the relation of the fungus Botrytis 
 cinerea to this disease, were conducted in much the same way as those 
 in connection with brown-rot gummosis. 
 
 (1) Inoculation with diseased tissue showed that the disease could 
 be transmitted to sound bark of healthy trees. 
 
 (2) Cultures from the diseased lesions showed the presence of a 
 fungus, Botrytis cinerea, previously known as the Botrytis rot fungus 
 of the packing houses. 
 
 (3) By inoculation experiments with pure cultures of this fungus 
 the characteristic symptoms were induced on healthy trees (fig. 9). 
 
 (4) Inoculation wdth Botrytis fungus isolated from lemon fruits 
 also produced the same results. 
 
 (5) The Botrytis fungus was again isolated from the artificially 
 induced lesions and was found capable of inducing rot in lemon fruits 
 irrespective of whether it had been found originally in diseased bark 
 or in rotting fruit. Some of the details of this work are being published 
 in the Journal of Agricultural Research. 
 
 The fungus was isolated from the softened invaded area of a large 
 number of these lesions. Attempts to isolate the fungus from the 
 outer gummous zone, however, failed, just as they did in Pythiacystis 
 gummosis. Only rarely was Botrytis isolated from the area where 
 the outer bark was dead and hard. Cultures showed that following 
 Botrytis inoculation this outer dead cortical layer is rapidly occupied 
 under such conditions as prevail at Santa Paula by species of Alter- 
 naria, Cladosporium, Penicillium, Colletotrichum, Fusarium, and other 
 fungi and bacteria. 
 
 FACTORS FAVORING THE DISEASE 
 
 Many contributing conditions tend to favor the occurrence and the 
 severity of this disease. Some of these are similar to those which 
 favor Pythiacystis gummosis, as discussed under that disease. 
 
 Injuries of various kinds to the bark, not only near the soil, but 
 anywhere on the trunk or large branches, may lead the way to infec- 
 tion and development of Botrytis gummosis when the conditions of 
 moisture and temperature are also favorable. This disease is frequently 
 severe on living tissue of trees that have been injured by frost. The 
 fungus may first become established in such trees in a small portion 
 of dead or dying tissue and then advance rapidly into tissue which 
 appears to be sound. 
 
Bulletin 360] oi^r DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 403 
 
 Fig. 8. — Grey masses of spores of Botrytis cinerea on the surface of bark 
 of a Botrytis gummosis lesion. 
 
404 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 A desquamated condition of bark, fairly common on old lemon 
 trees in the California coastal regions, is also frequently accompanied 
 by Botrytis gummosis. It furnishes dead outer bark tissue from 
 which the fungus may advance. The desquamated condition is similar 
 in appearance to that which usually follows inoculation with Botrytis 
 cinerea on sound tree trunks and Avith which it is often confused. It is 
 thought, however, to be due to other causes. Recent experiments have 
 shown that a species of Phomopsis, to be described as P. calif ornica is 
 probably a factor in causing "shell bark." 
 
 The previous use of neat's foot oil in the treatment of gummosis 
 encouraged the growth of the Botrytis fungus. The trunks of lemon 
 trees previous^ treated at Santa Paula by scoring the bark and 
 painting with neat's foot oil were observed in February and March 
 of 1912 to be fairly well covered with a gray coating consisting of the 
 sporophores and spores of Botrytis. The bark on these trees was 
 found to be in various stages of soft decay with the exudation of 
 large masses of gum. Experiments also showed that this fungus 
 develops better on lemon bark treated with neat 's foot oil either before 
 of after infection by the organism than on bark free from this oil. 
 More recently the application of neat's foot oil to citrus trees has 
 been largely given up, and the more severe stages of this disease, such 
 as were previously seen, have not been observed lately. 
 
 METHODS OF CONTROL 
 
 The control methods used for Botrytis gummosis as in the case 
 of Pythiacystis gummosis are of two kinds, prevention and treatment ; 
 both of which are similar in principle to those discussed in connection 
 with the former disease. A few modifications, however, based on the 
 differences in the latter disease should be pointed out. 
 
 Prevention.— Since Botrytis cinerea appears to be dependent upon 
 abrasions or other injuries for its entrance into lemon bark, especial 
 care is necessary, particularly in moist weather, to avoid injuries in 
 cultivation, picking and other orchard operations. The danger from 
 such injuries may be lessened by painting the tree trunks with Bor- 
 deaux paste or other fungicides or spraying them thoroughly with 
 Bordeaux mixture. The precaution previously mentioned of pulling 
 away soil that is too high against the trunk and of keeping water as 
 much as possible away from the trunk, are also applicable in the case 
 of Botrytis gummosis. 
 
 Treatment. — The principle governing the treatment of this type 
 of gum disease is the same as that for Pythiacystis gummosis, namely, 
 the elimination of the invaded tissue and the prevention of further 
 
Bulletin 360] gUM DISEASES OP CITRUS TREES IN CALIFORNIA 
 
 405 
 
 progress of the disease. As the result of many different experiments 
 in which growers took a prominent part, a method consisting largely 
 of scraping off the outermost layers of bark was found which proved 
 to be best adapted for treatment of this disease (fig. 10), The portion 
 where the bark is totally killed is cut away, but beyond this where 
 usually only the outer layers of bark are dead, these outer layers only 
 are scraped off, leaving intact the live inner layer next to the cambium. 
 
 T'' 
 
 f! 
 
 A 
 
 B 
 
 Fig. 9, — (a) Gum exudation on a lemon trunk as the result of inoculation 
 with the Botrytis fungus. 
 
 (h) Bark over and around the diseased lesion scraped, to show the extent 
 of the dead soft bark. 
 
 To prevent further invasion of the outer bark it is usually found neces- 
 sary to scrape the sound bark several inches beyond the margin of the 
 affected region. A sharp curved tool, described under "psorosis, " 
 which can be easily controlled in scraping, is in general use for this 
 purpose (fig. 13). This modification of the method used for Pythia- 
 cystis gummosis is advisable because of the different nature of the 
 disease, in order to preserve as much as possible of the inner live 
 bark. Where both types of gummosis are present on the same trees, 
 as is frequently the case, this method is still applicable to the com- 
 bined lesions produced. The cut or scraped portions are then painted 
 
406 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 with a fungicide (fig. lOh). Bordeaux paste, and some of the coal tar 
 products, such as Avenarius Carbolineum, Arrow Carbolineum and 
 Creolineum XXX, which contain only the heavier oils, have given 
 good results. 
 
 The following is a typical example of the many experiments in 
 treatment of this disease : 
 
 On October 8, 1912, at Santa Paula, a lesion on the trunk of an 
 18-year-old lemon tree was treated by cutting away the bark entirely 
 in a few small places where it was killed to the wood, but scraping 
 away only the dead outer layers and leaving the cambium still attached 
 (fig. 10a). The scraping was done with a sharp curved instrument 
 made on the principle of a box scraper, and the scraped portion was 
 then painted with Bordeaux paste (fig. 10&). 
 
 On May 10, 1913, new bark was building up over the scraped 
 portion, but the disease had spread slightly on the margins where 
 scraping had not been continued out far enough. 
 
 On May 17, 1914, new bark was seen to have formed over the 
 entire scraped portion and the disease appeared to be entirely stopped. 
 
 SCLEROTINTA GUMMING DUE TO SCLEROTINIA 
 
 LIBEKTIANA 
 
 This disease, usually of minor importance, occasionally has been 
 found associated with rapid dying of bark on the roots and trunks of 
 citrus trees growing in damp, cool locations especially after periods of 
 severe frost. The bark is at first soft just as in the case of attack by 
 Botrytis cinerea. Though this fungus usually advances more rapidly 
 than Botrytis, it is soon checked, and callus begins to form as soon as 
 gum accumulates. Later, as the bark dries, it is left in shreds (fig. 11) 
 and large black sclerotia are found within and under this bark. Its 
 effect on citrus twigs has been described by C. 0. Smith, *^ who refers 
 to the gumming usually accompanying its attack. It appears to 
 infect the young growth, usually at the blossoming period, and fre- 
 quently extends back into larger branches. 
 
 When the fungus is found on the trunk or roots, observations have 
 indicated that previous injury of the bark was usually necessary for 
 its entrance. It has frequently been found on young trees following 
 frost injuries, apparently advancing from frost injured tissue into 
 tissue not killed by frost. It has been observed on a lemon tree 20 
 years old where all the roots had been infected, probably from injuries 
 made in digging about them and placing vetch straw near the crown 
 
 47 Smith, C. O., Cottony rot of lemons in California. Calif. Agr. Exp. Sta. 
 Bull. 265, pp. 237-258, 1916. 
 
Bulletin 360] gxJM DISEASES OF CITP.US TREES IN CALIFORNIA 
 
 407 
 
 in damp cool weather. An old seedling orange tree also was observed 
 with the bark on one side of the trunk killed by the fungus, which 
 had apparently gained entrance through a small sunburned area and 
 had advanced into the live bark for some distance. 
 
 Fig. 10. — Treatment of Botn'tis gummosis. 
 
 (a) Lesion being scraped. 
 
 (&) Painting scraped area with Bordeaux paste. 
 
 This fungus has also been shown by inoculation experiments to be 
 capable of advancing rapidly into lemon bark, inducing copious gum 
 formation for a short time only. Observation shows that the halting 
 of the invasion of the fungus is usually coincident with the exudation 
 of considerable quantities of gum. The prevention and treatment of 
 this disease is the same as that for Botrytis gummosis. (See directions 
 under Botrytis gummosis, page 404.) 
 
408 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 PSOROSIS (SCALY BARK) OF ORANGE TREES 
 
 Althoiigh psorosis is generally classed as a gum disease, gum 
 exudes only at certain seasons, or during certain phases of develop- 
 ment of the disease, and then the gum is usually not so conspicuous a 
 feature as it is with brown-rot gummosis. 
 
 This disease was briefly described by W. T. Swingle and H. J. 
 Webber in 1896 in Florida and given the name of psorosis. 
 
 It is now known in California as ''scaly bark," but must not be 
 confused with another disease known in Florida as scaly bark (nail- 
 head rust). This last is distinct from psorosis and does not occur in 
 California. 
 
 The slowness of the development of the disease, and its inconspicu- 
 ous appearance and lack of effect upon the foliage in the earlier stages, 
 frequently prevent it from being noticed until it is far advanced. 
 
 SYMPTOMS 
 
 The most conspicuous feature of the scaly-bark disease is the 
 appearance on the trunk or large limbs, of irregular scales of bark 
 % to 1 inch in diameter, standing out as if pushed off from the 
 surface (fig. 12). It usually begins with a very small area in which 
 only a thin outer layer of bark dies, hardens, and is raised from the 
 surface, leaving a layer of bark underneath still alive. This first area 
 slowly enlarges from year to year until finally it encircles the trunk 
 or limb. Often a number of small areas begin at the same time and 
 thus cover the surface more rapidly. Later the deeper layers of bark 
 and even the wood may be affected. Cum may exude as the disease 
 advances, but gum is not a necessary accompaniment of the disease 
 and its presence and amount depend on growth conditions, season of 
 the year, etc. 
 
 INVESTIGATIONS AS TO CAUSE AND MANNEE OF DEVELOPMENT 
 
 An investigation into the cause and manner of development and 
 control of this disease, has been carried on for years. Many difficulties 
 which were not encountered in the work with brown-rot gummosis 
 have arisen in the investigation of the cause of psorosis. One of these 
 has been the extreme slowness with which the disease develops in its 
 incipient stages. 
 
 In certain cases it has been possible to transmit the disease to sound 
 trees by inoculations with bits of tissue from diseased lesions, but 
 many of the attempts have failed. In one successful case two years 
 elapsed after the diseased tissue was placed in a wound in sound bark. 
 
Bulletin 360] quM DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 409 
 
 before any sign of the disease was noted. This suggests that an 
 organism of some kind which is able to advance very slowly may be 
 the immediate cause of the disease. Experiments planned to determine 
 
 Fig 11. — Eoot of lemon tree showing shredded appearance of the bark and 
 the black sclerotia as the result of the attack of Sclerotinia libertiani . Arrows 
 point to sclerotia. 
 
 whether this hypothesis is correct have been under way for some time, 
 but because of the slowness of development of Psorosis from the early 
 stages there has not been time for proof of this conjecture to be 
 established. 
 
410 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 As examples of the slowness with which the disease often develops, 
 the following cases may be mentioned. At the Experiment Station 
 two incipient lesions not yet gumming-, and each about 1 inch in 
 diameter were outlined on a Valencia orange tree 12 years old. These 
 lesions had progressed only Y^ inch beyond the original mark in 21/2 
 years. A slightly larger area on another tree, which was 1% inches in 
 diameter when first observed became only 4 inches in diameter in 3I/2 
 years. These areas were so small and inconspicuous when outlined that 
 they would not have been noted at all at that stage by the average 
 grower. 
 
 After the lesions become larger, however, they develop somewhat 
 more rapidly. The following example represents a rather rapidly 
 developing lesion. An area on a trunk of a 12-year-old Valencia tree 
 was 3x7 inches when marked. Six months later it was 8x12 inches; 
 after 9 months, 10x15 inches; after 2 years, 12x16 inches (with much 
 gumming), and after 3I/2 years 14x17 inches. While this increase 
 from 3x7 inches to 14x17 inches seems considerable in itself, it repre- 
 sents an average of only about li/^ inches of advance per year in any 
 one direction. The lesion was probably 3 to 4 years old when first 
 measured. Scaly bark thus differs from any other gum diseases, 
 whose whole course of development can be followed out in 1 or 2 
 years. 
 
 The visible advance of the disease depends greatly upon the season 
 of the year. In the majority of cases it seems to be most active during 
 the growing season and especially in summer and early fall, and to be 
 quiescent during the winter and early spring. This great show of 
 activity is due in part to the breaking away of scales of bark, probably 
 brought about not only by drying of the outer bark that has previously 
 died, but also by the increase in growth of the inner live bark beneath. 
 Gum formation and exudation also take place at this season, giving 
 further indication of activity. Most of the advance for the year, 
 however, appears to be finished when the gum formation is at its 
 maximum. The gum is the result and not the cause of the death of 
 the outer bark. The gum probably tends to hinder more than to aid 
 the advance of the disease at this time. 
 
 EXPERIMENTS IN TREATMENT 
 Although the investigation as to the cause and manner of develop- 
 ment of the disease received first attention, a number of experiments 
 in control have also been carried on. Various methods of cutting, 
 scraping and slitting the bark have been compared with no cutting or 
 scraping. Comparisons have also been made of the application of a 
 considerable number of substances as fungicides or coverings. 
 
Bulletin 360] gUM DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 411 
 
 Without going into detail, it may be said that a method of scrap- 
 ing the outer bark to depths varying according to circumstances, to be 
 described later, has been found more effective than any other method. 
 Scraping has been found to be much more important than the use of 
 any kind of fungicide or disinfectant. Slitting has proven of little 
 or no value in these experiments. These experiments have also shown 
 the advisability of scraping not only the area visibly affected, but also 
 beyond the area to a distance of from 6 to 8 inches to head off the 
 advance of the disease in bark not yet visibly affected. The spread of a 
 lesion in its early stages appears to take place within the outermost 
 layers of bark. This accounts for the beneficial results of light scrap- 
 ing beyond the visible edge of the lesions. 
 
 A large number of disinfectants and coverings were tested with 
 and without scraping or other method of cutting the bark. In each 
 case similar areas on the same or different trees were left without the 
 application of the disinfectant for comparison. Almost without excep- 
 tion the diseased trees on which the bark was carefully scraped showed 
 the best recovery. The disinfectant applied seemed to have little 
 effect. Lesions of the same kind not scraped continued to develop in 
 size at the same rate, whether painted or unpainted with the various 
 disinfectants. 
 
 A cooperative experiment was also carefully conducted under com- 
 mercial conditions by M. B. Rounds*^ of the Azusa Foothill Citrus 
 Company. Methods of scraping and slitting the bark, combined with 
 applications of Bordeaux paste, Creolineum XXX and emulsified 
 cresol (liquor cresolis compositus U. S. P.), respectively, with check 
 trees for comparison, were instituted according to the following plan : 
 Most of the trees treated were in the first and second stages and were 
 chosen with an attempt to get sets of like specimens for each combina- 
 tion of treatment. . 
 
 
 Check 
 
 Bordeaux 
 
 Creolineum 
 
 Emulsified 
 cresol 
 
 Check, not scraped 
 
 4 trees 
 
 5 trees 
 5 trees 
 5 trees 
 
 5 trees 
 5 trees 
 5 trees 
 5 trees 
 
 5 trees 
 5 trees 
 5 trees 
 5 trees 
 
 5 trees 
 
 Scraped 
 
 6 trees 
 
 Scraped and slit 
 
 5 trees 
 
 Slit 
 
 5 trees 
 
 
 
 The scraping was done according to the method described below 
 and the slitting by making long vertical slits with a heavy knife, 
 through and on either side of the diseased lesions. The trees were 
 
 48 Eounds, M. B., California Cultivator, vol. 8, p. 222, 1922. 
 
412 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 treated in October, 1921, and the following results are reported by Mr. 
 Bounds for August, 1922. 
 
 Disease still 
 
 progressing 
 
 Number of trees 
 
 Disease not 
 
 progressing 
 
 Number of trees 
 
 Not scraped or slit. 
 
 Check 
 
 Bordeaux 
 
 Creolineum XXX. 
 [ *Emulsified cresoL. 
 
 Scraped. 
 
 f Check 
 
 J Bordeaux 
 
 ] Creolineum XXX. 
 Emulsified cresol.. 
 
 Scraped and slit. 
 
 Check 
 
 Bordeaux 
 
 Creolineum XXX. 
 Emulsified cresol.. 
 
 Slit only. 
 
 Check 
 
 Bordeaux 
 
 Creolineum XXX. 
 Emulsified cresol... 
 
 3 
 4 
 5 
 4 
 
 16 
 
 
 
 
 
 
 
 
 
 
 2 
 
 
 3 
 3 
 
 4 
 3 
 
 13 
 
 1 
 
 1 
 
 1 
 
 5 
 5 
 5 
 6 
 
 21 
 
 5 
 5 
 3 
 5 
 
 18 
 
 2 
 2 
 1 
 
 2 
 
 * Liquor creFolis compositus U. S. P., full strength. 
 
 While it is too soon to draw definite conclusions from this experi- 
 ment alone, because of the slowness of development of the disease, the 
 results, taken together with experiments made at the Experiment 
 Station and many other observations, indicate that the particular 
 fungicide to be applied is of less importance than the manner of scrap- 
 ing or treating the bark. None of the trees scraped alone showed any- 
 advance of the disease, while 65 per cent of those slit alone, and 
 approximately 80 per cent of those not scraped or slit, showed an 
 an advance in the disease at the time here recorded. 
 
Bulletin 360] quM DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 413 
 
 SUGGESTIONS FOR TREATMENT 
 
 The stage of the disease largely determines how each tree should be 
 handled. Each tree affected presents an individual problem. Certain 
 tentative suggestions, however, will be made here, it being understood 
 that these apply to typical conditions and that they may need modifi- 
 cation when applied to any particular case. 
 
 Fig, 12. — About the second stage of psorosis (scaly bark) on orange trees, 
 showing formation of scales of bark, giving the surface a roughened scaly 
 appearance. 
 
 First Stage. — At the very beginning only an outer layer of bark 
 appears to be injured or changed, an inner layer next to the cambium 
 still being alive and active and free from discoloration except that it 
 
414 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 frequently presents a slight greenish appearance. Later a yellowish 
 discoloration may appear, due. to the development of gum within the 
 tissue. The trunk and large limbs of all trees should be inspected 
 carefully to detect the disease at its very first beginning. When these 
 beginning areas are small and do not cover more than 1/4 ^^ "the circum- 
 ference, the affected bark may be scraped rather deeply and the sur- 
 rounding apparently unaffected bark scraped very lightly for 4 to 6 
 inches in all directions beyond the margin of the affected areas. 
 
 Second Stage. — When the disease has progressed further, so as to 
 cover more than about % of the circumference of the entire bark on 
 the trunk but has not yet seriously injured the wood, it may for 
 convenience be considered to be in the second stage. At this stage 
 the affected portion often presents a roughened surface (fig. 12). 
 
 Fresh scaling of the bark on the advancing edges continues and 
 exudation of gum takes place at certain seasons of the year. In such 
 cases the cure is more uncertain, but the disease may often be checked 
 and sometimes cured by a light to medium scraping. Particular atten- 
 tion should be given to the advancing edges. Care must be taken not 
 to scrape deeply enough to kill the inner layer of bark. The affected 
 surface may then be treated with a disinfectant as previously men- 
 tioned. Six months or a year later, these should be treated again, 
 scraping only where the disease is still active. The progress of the 
 disease is so slow that usually one cannot discern within less than six 
 months or a year, whether it has progressed or not. 
 
 Third Stage. — Where the disease has been present for a number 
 of years (5 to 10 or more) or until a greater part of the bark of the 
 trunk is affected and the wood underneath is killed and beginning to 
 decay, there is little hope for a permanent recovery. A tree of this 
 kind, however, sometimes remains surprisingly productive for a num- 
 ber of years, so that it becomes a question whether to replace it at 
 once or to treat it superficially with the idea of preventing possible 
 spread to other trees, and of taking it out later. If only a part of the 
 branches show the disease in the second and third stages, these may 
 be cut out entirely and the remaining part of the tree frequently in- 
 spected for further outbreaks. If the wood is just beginning to be 
 discolored and killed, this should be chisled out and benzene-asphalt 
 paint or other good wood protecting covering applied to the exposed 
 surfaces. A severe cutting back of a badly affected tree may be 
 helpful. If the trunk is too badly decayed and the entire tree appears 
 stunted and unproductive, the tree should be dug out at once. 
 
 Season of the Year. — Where there is much frost hazard, bark 
 scraped too late in the fall or during the winter months is likely to be 
 
Bulletin 360] qUM DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 415 
 
 killed by low temperatures. Experiments at Riverside indicate that 
 bark treated in the late spring and summer months recovers most 
 rapidly. 
 
 Fungicides. — As was previously stated, the kind of fungicide 
 employed appeared to be of minor importance in certain of our experi- 
 ments. Some kind should be applied, how- 
 ever, to the scraped areas, as a matter of 
 precaution. Except for the slight danger 
 to the foliage when followed by fumigation 
 there is nothing better than Bordeaux paste. 
 Other substances of good fungicidal value 
 are some of the high-boiling coal tar prod- 
 ucts such as Arrow Carbolineum, Creolin- 
 eum XXX, etc. Mercuric cyanid, 1 part 
 dissolved in 500 parts of water and 500 
 parts alcohol, as used for pear blight, is 
 also an excellent disinfectant. There are 
 many others that might be used. 
 
 For Bordeaux paste, dissolve 1 pound 
 of bluest one (copper sulfate) in 3 quarts of 
 water in a wooden, earthen or glass vessel 
 and slake 2 pounds of lime in 3 quarts of 
 water. The bluestone is most easily dis- 
 solved by suspending it in a sack at the top 
 of the water overnight. If the bluestone is 
 pulverized and suspended in warm water 
 it dissolves rapidly. Good lime that is not 
 air-slaked should be used. If covered to 
 avoid evaporation the dissolved ingredients 
 will keep indefinitely in separate vessels. 
 \Yhere the paste is being used over a num- 
 ber of days or weeks, just enough of the wet 
 slaked lime and the bluestone solution 
 should be mixed to last for one or two days. 
 It may be applied with large whitewash 
 brushes. Commercial Bordeaux pastes 
 brought to equivalent strength may also be 
 used. 
 
 Tools. — A number of different kinds of 
 
 « ■ .1 T ^ Fiff. 13. — Tool for scrap- 
 
 scrapers are in use lor scraping the bark, jj^g ^gg ^ark or for goug- 
 
 Several types devised by Mr. Culbertson of i^g o^^t small diseased areas 
 
 ■ IT- • /-< e • ji caused bv psorosis or other 
 
 the Limoneira Company for use m the ^j^j.^ diseases. 
 
416 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 treatment of Botrytis gummosis are shown in figure 10. A modifi.ca- 
 tion of one of these, first made under the direction of Dr. J. T. Barrett 
 of the Citrus Experiment Station, is now in common use. It may be 
 made of spring steel and consists of a curved blade sharpened on both 
 edges and at the end and set in a wooden handle. The handle is 
 714 inches long and 1% inches in diameter and the steel part con- 
 taining the curved blade is 5% inches long (fig. 13). These may be 
 made by a local blacksmith or obtained through the farm advisor. 
 
 DIPLODIA GUMMING*^ 
 
 In a former publication^® it was stated that this form of gumming 
 had not been seen in California, but was common in southern Florida 
 and Cuba. Since then a Diplodia fungus similar to the Florida one 
 has frequently been found in California especially in San Diego county 
 associated with a gumming of large branches. A gummosis due to a 
 Diplodia has been found recently in the Philippines also.^^ This fungus 
 appears to be especially active in California in connection with the 
 so-called ''heart rot" following severe freezes. After the freeze of 
 1913 this fungus was found advancing into the yet unkilled wood of 
 large branches Avhich had been cut off after the freeze, resulting in 
 frequent gum formation. The fungus advanced much more rapidly 
 in branches whose cut ends were sealed with grafting wax than in 
 those not so sealed. The Diplodia attack resulted indirectly in the 
 death of areas of bark, but the fungus advanced much more rapidly 
 in the woody tissue (fig. 14). 
 
 Control. — The heart rot due to Diplodia following a severe freeze is 
 difficult to control. A non air-tight disinfectant, such as Bordeaux 
 paste, mercuric cyanid (1 part in 1000 parts of denatured alcohol) 
 or other non-injurious substance, should be used to disinfect the 
 tools and cut surfaces. This treatment may be followed by the sealed 
 covering sometime later when the wound has thoroughly dried out. 
 If only one application is to be used, a thin substance like the higher 
 boiling coal tar products, as Avenarius carbolineum, Arrow carbolin- 
 eum, or Creolineum XXX, may be used. All parts of the tree cut 
 back should be thoroughly whitewashed to prevent sunburning. 
 
 49 Fawcett, H. S., Diplodia natalensis as a gum-inducing and fruit-rotting 
 fungus. Report of Plant Pathologist, Fla. Agr. Exp. Sta. Ann. Report, 1911, 
 pp. 61-67, 1912. 
 
 Gumming, Report of the former Plant Pathologist, Fla. Agr. Exp. Sta. Ann. 
 Report, 1912, pp. 77-92, 1913. 
 
 50 Fawcett, H. S., Citrus diseases of Florida and Cuba compared with those 
 of California. Univ. of Calif. Agr. Exp. Sta. Bull. 262, p. 210, 1915. 
 
 81 Reinking, O. A., Philippine Agriculturist, vol. 9, 123-127, 1921. 
 
Bulletin 360] quM DISEASES OF CITRUS TREES IN CALIFORNIA 
 
 417 
 
 TWIG GUMMING 
 
 A gumming and dying of a few scattered twigs, especially on naval 
 orange trees, in the late summer or fall, is usually of minor importance. 
 It occurs in both California and Arizona. The cause is unknown. It 
 is characterized by sudden wilting of leaves and dying back of twigs 
 
 Fig. 14. — Gumming (at the point of the arrow) on lemon bark, due to 
 Diplodia sp. following a severe frost injury. 
 
 or small branches to a distance of 12 to 24 inches from their tips. At 
 the base of the dead portion the bark splits and gum oozes out in 
 considerable quantities. It appears to occur most often after periods 
 of hot dry weather and has been known in California for many years 
 as a minor trouble. It is quite distinct from citrus blast though some- 
 times confused with the latter disease. 
 
418 
 
 UNIVERSITY OP CALIFORNIA EXPERIMENT STATION 
 
 A somewhat similar gumming, associated with the dying back of 
 twigs and branches but without the characteristic splitting of the bark, 
 has frequently been noted in good sized nursery trees. Although certain 
 organisms have been found in the lesions none of them have reproduced 
 the disease on inoculation. These forms of twig gumming differ from 
 twig blight caused by the cottony rot fungus, Sclerotinia lihertiana, 
 in the absence of a whitening or shredding of the bark and of the 
 black sclerotia frequently formed in the bark attacked by that fungus. 
 
 Fig. 15. — Cross-section of a green orange from a tree affected with exan- 
 thema. The darked areas between the septa next to the core indicate the 
 presence of clear gum that has taken the place of the normal tissue. 
 
 EXANTHEMA 
 
 Nature and Symptoms. — Exanthema, or die-back as it is commonly 
 called in Florida, is often classed as a gum disease although gum for- 
 mation is not always a conspicuous feature. The clear gum seen in 
 connection with exanthema exudes only from gum pockets on the 
 twigs, or is formed internally near the center of the fruit at the 
 angles of the segments (fig. 15). Dark excrescences and multiple buds 
 on the branches, dying back of terminal branches, compact shortened 
 growth and dark irregular reddish brown patches on the surface of 
 the fruit are other symptoms. 
 
Bulletin 360] quM DISEASES OF CITRUS TREES IN CALIFORNIA 419 
 
 The disease has been considered to be the result of a nutritional 
 disturbance, but the actual cause is unknown. Localized areas in 
 California orchards showing exanthema have frequently been noted 
 where sheep corrals or cattle barns had been located in previous years. 
 It has also been found in other instances to correspond to spots where 
 the top soil had been graded off in leveling the orchard before plant- 
 ing. Although the use of large amounts of organic nitrogenous fertil- 
 izers, such as stable manure, dried blood and cottonseed meal have 
 been considered to be contributing factors in inducing exanthema on 
 some soils in Florida, these fertilizers have not been observed to 
 encourage the disease under California conditions. 
 
 CONTKOL 
 
 In many cases in California where exanthema has been trouble- 
 some in young trees in localized areas, the trees have outgrown the 
 trouble without any special treatment. In cases where the contribut- 
 ing conditions are such as may be economically removed by the grower 
 the remedy is, of course, obvious. 
 
 Exanthema is of such minor importance in California that no study 
 has yet been made of special methods of control. The methods used 
 in Florida cannot be recommended for California without trial as the 
 disease is here manifested under such widely different conditions. 
 
 OTHER MINOR FORMS OF GUMMING 
 
 In addition to the gum diseases already discussed there frequently 
 occur minor forms of gumming, some of which have been shown to be 
 induced by microorganisms while other forms are brought about by 
 insect injuries, chemical stimuli, or in certain cases by physical effects 
 of the environment. 
 
 Most of these forms of gumming cannot be classed as definite 
 diseases and are of minor importance commercially as compared with 
 the preceding diseases. 
 
 PENICILLIUM EOSEUM 
 
 Gumming due to Penicillium roseum. This fungus, which forms 
 small pink tufts of hyphae and spores on the surface of lemon bark, 
 is capable of inducing a small amount of gum exudation and death of 
 small areas of bark, as has been shown by inoculations with pure 
 cultures. It has been found as a secondary fungus associated with 
 Botrytis gummosis and shell bark on lemon trunks, mostly in the 
 moister coastal sections of California. 
 
420 UNIVERSITY OF CALIFORNIA FJiPERIMENT STATION 
 
 FUSAEIUM 
 
 Many species of this genus of fungus produce pinkish to red pus- 
 tules or masses of spores within the surface layer of dead bark of 
 lemon trees. In general appearance and color these red masses look 
 to the unaided eye much like those of Penicillium roseum. This fungus 
 also has been shown to be capable of inducing the formation of a small 
 amount of gum, and of causing a very limited amount of injury to 
 bark tissue adjoining a wound when inserted into cuts on sound tissue, 
 but did not produce definite diseased lesions. It has frequently been 
 found associated with brown rot (Pythiacystis) gummosis and appears 
 to be capable of increasing the severity of this gummosis when associ- 
 ated with Pythiacystis citroplitliora (as has previously been mentioned 
 under ''brown rot gummosis") but when acting alone, Fusarium 
 appears to be of minor importance in gummosis. It should be stated, 
 however, that Barrett^^ has found species of Fusarium constantly 
 associated with the 'dry root rot' of citrus in connection with which 
 considerable gumming is frequently noted. Although the disease has 
 not been reproduced by inoculation, this fungus is thought to be an 
 important factor in the development of dry root rot of citrus. 
 
 ALTEENAEIA 
 
 A species of Alternaria similar to Alternaria citri has very 
 frequently been found in bark tissue associated with minor forms of 
 gumming on lemon trees. Inoculation with pure cultures has shown 
 that it may sometimes induce slight gum exudation and very slight 
 injury to tissue adjacent to cuts into which spores were inserted. 
 Small green immature navel oranges affected with black rot frequently 
 show gum exudation at the navel cavity apparently due to the 
 presence of Alternaria citri. 
 
 Gumming due to Bacterium citriputeale C. 0. Smith. If, during 
 the season for citrus blast and black pit attack, the weather becomes 
 warmer than usual, small drops of gum are apt to form at the edges 
 of the twig lesions caused by the citrus blast bacterium. Slight gum- 
 ming has been noted also on cuttings inoculated with this bacterium 
 and held in moist chambers over a free water surface at constant 
 temperatures of 80° and 93.4°F. (30° and 34°C.), but not at tempera- 
 tures below this. Control punctures produced no gum. Large light- 
 green lemon fruits also develop gum by inoculation with the same 
 organism under the same conditions, but not in the uninoculated 
 punctures. 
 
 52 Barrett, J. T., Dry root rot. Calif. Citrus Inst., First Ann. Kept. 1919- 
 1920, p. 157, 1920. 
 
Bulletin 360] Gr:M DISEASES OF CITRUS TREES IN CALIFORNIA 421 
 
 It is probable that various other fungi are responsible for minor 
 forms of gumming in citrus. Observation has indicated that under 
 certain conditions ArmiUaria mellea, Schizophyllus commune and 
 other wood rotting fungi are minor agents in bringing about small 
 gum exudations. 
 
 GUMMING ASSOCIATED WITH INSECT INJUEIES 
 The injuries to citrus made by a number of insects are frequently 
 followed by gumming, usually slight in amount. Small drops of gum 
 may form on fruit at injuries produced by the orange tortrix {Tortrix 
 citrana), and on small tree trunks and limbs from grasshopper, katy- 
 did and other insect injuries. Gumming has frequently been noted 
 on twigs badly infested with the California red scale. To what extent 
 this gumming may be due to secretions of the insects or to the entrance 
 of microorganisms at the time of injury is uncertain. Our negative 
 results from mechanical injuries to citrus kept sterile and free from 
 chemical stimuli would indicate that this gumming was probably not 
 due to the injury or wound in itself. 
 
 GUMMING ASSOCIATED WITH CHEMICAL STIMULI 
 
 Among the chemical stimuli that have been seen to result in gum 
 formation occasionally in citrus orchards may be mentioned: (1) 
 liquid hydrocyanic acid spilled on the soil near the roots of trees; (2) 
 hydrocyanic acid gas used in fumigation; (3) spray mixtures contain- 
 ing copper sulfate not properly neutralized with lime or containing 
 other toxic substances; (4) a poison containing arsenic in contact with 
 the bark, etc. 
 
 Hydrocyanic acid coming in contact with a large main root usually 
 causes the- death of a strip of limited section of the bark on the trunk, 
 and even on the limbs and branches leading up from this portion of 
 the trunk. This frequently results in considerable gumming adjacent 
 to the killed strip and is sometimes mistaken for a definite gum disease. 
 Under some conditions the shock to citrus trees produced by fumiga- 
 tion with hydrocyanic acid gas appears to be responsible for initiating 
 gumming on the tree trunks of orange trees especially. The pressure 
 in small gum pockets formed near the cambium produces small rifts 
 in the bark through which the gum exudes in small drops, sometimes 
 in numerous places on the trunk. Sometimes coils or strings of gum 
 will be pushed out of small openings one to two weeks after fumiga- 
 tion. Such gumming is usually temporary and the places where the 
 gum has formed quickly close up and fill with new tissue without 
 noticeable injury to the tree. 
 
422 UNIVERSITY OF CALIFORNIA F^PERIMENT STATION 
 
 Copper sulfate solution if spilled on the soil over a root will act 
 in much the same way as the hydrocyanic acid in killing strips of bark 
 and resulting in gumming. 
 
 Ant poisons containing arsenic when allowed to leak out of the 
 containers frequently kill patches of bark and set up gumming. 
 
 PHYSICAL EFFECTS OF THE ENVIEONMENT 
 
 Physical effects do not appear to be as a rule more than contribut- 
 ing conditions for gumming in citrus under California conditions, 
 other stimuli acting as the immediate cause. 
 
 Mechanical Injuries. — The writer has not been able to induce gum 
 exudation on healthy citrus trees by mechanical injuries alone, pro- 
 vided the injuried portions are kept clean and reasonably free from 
 contamination with microorganisms or unusual chemical substances. 
 The following kinds of wounds were made : cuts vertically or hori- 
 zontally through the bark, augur holes with and without glass or 
 wooden plugs, bruises made by heavy and light blows from a black- 
 smith 's hammer, long horizontal and vertical slits through the bark 
 areas, of bark cut away, etc. All such injuries kept clean, healed in 
 the usual way without gumming. When purposely infected with 
 Botrytis cinerea or other injurious fungi, however, gumming resulted. 
 
 Burning and Freezing. — Sunburning and freezing are not import- 
 ant factors in themselves in initiating gum formation in citrus, al- 
 though they do act as contributing conditions by allowing the entrance 
 of organisms such as Botrytis cinerea, Sclerotinia lihertiana or other 
 fungi which, after becoming established in the injured tissue, may 
 advance rapidly and induce gumming in tissue apparently sound. 
 Freezing and sunburning often get credit for initiating the gumming 
 when they have merely opened up the way for its initiation by other 
 agencies. 
 
 SUMMARY 
 
 For the sake of brevity this summary deals with average conditions. 
 Allowance must therefore be made for variations in treatment for 
 unusual or abnormal conditions. Consult the discussions in the text 
 for more detailed statements. 
 
 Pythiacystis (Brown Rot) Gummosis. — Prevented by pulling the 
 soil away from the base of the tree trunk until the tops of the first 
 main roots are exposed; by keeping the soil next to the trunk from 
 becoming excessively wet ; by avoiding injuries to the bark ; by paint- 
 ing the trunks with fungicide such as Bordeaux paste, and by using 
 
Bulletin 360] qxJM DISEASES OF CITRUS TREES IN CALIFORNIA 423 
 
 sour-orange stocks budded high for all new plantings, especially on 
 heavy clay soils. Treated, when not too far gone, by cutting away the 
 invaded killed bark, but not necessarily the outer gummous zone, and 
 painting the wound with a suitable fungicide ; by scraping away any 
 outer layers of dead bark ; by painting exposed portions of wood (after 
 healing of edges begins) with benzine-asphalt paint or other suitable 
 covering ; by cutting back the tops on trees severely affected ; by 
 inarching or bridge grafting in certain cases. 
 
 Botrytis and Sclerotinia Gummosis. — Prevented by the same 
 methods as to soil, water, injuries, fungicides and use of sour-orange 
 stocks as in case of previous disease. Treated by cutting out the dead 
 bark to the wood and by scraping off only the outer bark beyond this 
 where the inner layer is not killed, and by painting with fungicide 
 as in the previous disease. 
 
 Psorosis (Scaly Bark) of Orange Trees. — Methods of prevention 
 not definitely known. Treated by scraping away the outer affected 
 bark of lesions in the first and second stages, scraping lightly the 
 bark not j^et visibly affected to a distance of 6 to 8 inches beyond the 
 lesion in each direction and applying Bordeaux paste or other suitable 
 fungicides ; by cutting out certain badly affected limbs altogether ; by 
 eliminating certain of the worst trees in the third stage of the disease 
 or cutting them off below the diseased part where this is possible. 
 Treatment must vary much according to the stage of the disease. ^ ( See 
 discussion of various stages.) 
 
 Diplodia Gumming and Twig Gumming. — Treated by eliminating 
 the parts affected and by treating cuts with fungicide followed by 
 paint where wounds are large. 
 
 Exanthema. — Little is known as to its real cause, and no one defin- 
 ite method of prevention or control can be suggested for California 
 conditions except to eliminate certain supposed contributing condi- 
 tions where this is possible. (See previous discussion.) 
 
 Minor Forms of Gumming. — Not many of these are sufficiently 
 important to require special attention either as to prevention of treat- 
 ment. Where they are due to organisms most of the same principles 
 apply as are given for the previous diseases. Some forms are depend- 
 ent upon conditions that cannot be controlled but recovery often 
 follows a change in the contributing conditions. 
 
STATION PUBLICATIONS AVAILABLE FOR FREE DISTRIBUTION 
 
 BULLETINS 
 
 No. 
 
 253. Irrigation and Soil Conditions in the 
 Sierra Nevada Foothills, California. 
 
 261. Melaxuma of the Walnut, "Juglans 
 
 regia." 
 
 262. Citrus Diseases of Florida and Cuba 
 
 Compared with these of California. 
 
 263. Size Grades for Ripe Olives. 
 
 268. Growing and Grafting Olive Seedlings. 
 
 270. A Comparison of Annual Cropping, Bi- 
 ennial Cropping, and Green Manures 
 on the Yield of Wheat. 
 
 273. Preliminary Report on Kearney Vine- 
 yard Experimental Drain. 
 
 275. The Cultivation of Belladonna in Cali- 
 
 f-ornia. 
 
 276. The Pomegranate. 
 
 278. Grain Sorghums. 
 
 279. Irrigation of Rice in California. 
 
 280. Irrigation of Alfalfa in the Sacramento 
 
 Valley. 
 283. The Olive Insects of California. 
 
 285. The Milk Goat in California. 
 
 286. Commercial Fertilizers. 
 
 287. Vinegar from Waste Fruits. 
 294. Bean Culture in California. 
 
 297. The Almond in California. 
 
 298. Seedless Raisin Grapes. 
 
 299. The Use of Lumber on California Farms. 
 304. A study on the Effects of Freezes on 
 
 Citrus in California. 
 308. I. Fumigation with Liquid Hydrocyanic 
 Acid. II. Physical and Chemical Prop- 
 erties of Liquid Hydrocyanic Acid. 
 
 312. Mariout Barley. 
 
 313. Pruning Young Deciduous Fruit Trees. 
 
 316. The Kaki or Oriental Persimmon. 
 
 317. Selections of Stocks in Citrus Propa- 
 
 gation. 
 319. Caprifigs and Caprification. 
 321. Commercial Production of Grape Syrup. 
 
 324. Storage of Perishable Fruit at Freezing 
 
 Temperatures. 
 
 325. Rice Irrigation Measurements and Ex- 
 
 periments in Sacramento Valley, 
 1914-1919. 
 
 No. 
 
 328. 
 331. 
 332. 
 334. 
 
 335. 
 
 336. 
 
 337. 
 339. 
 
 340. 
 
 341. 
 342. 
 343. 
 344. 
 
 345. 
 
 346. 
 347. 
 
 348. 
 349. 
 
 350. 
 351. 
 352. 
 
 353. 
 354. 
 355. 
 356. 
 
 357. 
 
 358. 
 
 359. 
 360. 
 
 Prune Growing in California. 
 
 Phylloxera-Resistant Stocks. 
 
 Walnut Culture in California. 
 
 Preliminary Volume Tables for Second- 
 Growth Redwoods. 
 
 Cocoanut Meal as a Feed for Dairy 
 Cows and Other Livestock. 
 
 The Preparation of Nicotine Dust as 
 an Insecticide. 
 
 Some Factors of Dehydrater Efficiency. 
 
 The Relative Cost of Making Logs from 
 Small and Large Timber. 
 
 Control of the Pocket Gopher in Cali- 
 fornia. 
 
 Studies on Irrigation of Citrus Groves. 
 
 Hog Feeding Experiments. 
 
 Cheese Pests and Their Control. 
 
 Cold Storage as an Aid to the Market- 
 ing of Plums. 
 
 Fertilizer Experiments with Citrus 
 Trees. 
 
 Almond Pollination. 
 
 The Control of Red Spiders in Decidu- 
 ous Orchards. 
 
 Pruning Young Olive Trees. 
 
 A Study of Sidedraft and Tractor 
 Hitches. 
 
 Agriculture in Cut-over Redwood Lands. 
 
 California State Dairy Cow Competition. 
 
 Further Experiments in Plum Pollina 
 tion. 
 
 Bovine Infectious Abortion, 
 
 Results of Rice Experiments in 1922. 
 
 The Peach Twig Borer. 
 
 Observations on Some Rice Weeds in 
 California. 
 
 A Self-mixing Dusting Machine for 
 Applying Dry Insecticides and 
 Fungicides. 
 
 Black Measles, Water Berries, and 
 Related Vine Troubles. 
 
 Fruit Beverage Investigations. 
 
 Glim Diseases of Citrus Trees in Cali- 
 fornia. 
 
 CIRCULARS 
 
 No. No. 
 
 70. Observations on the Status of Corn 166. 
 
 Growing in California. 167. 
 
 82. The Common Ground Squirrel of Call- 169. 
 
 fornia. 170. 
 87. Alfalfa. 
 
 110. Green Manuring in California. 172. 
 
 111. The Use of Lime and Gypsum on Cali- 173. 
 
 fornia Soils. 
 
 113. Correspondence Courses in Agriculture. 174. 
 
 117. The Selection and Cost of a Small 175. 
 
 Pumping Plant. 
 
 127. House Fumigation. 178. 
 
 136. Melilotus indica as a Green-Manure 179. 
 
 Crop for California. 
 
 144. Oidium or Powdery Mildew of the Vine. 182. 
 148. "Lungworms." 
 
 151. Feeding and Management of Hogs. 183. 
 
 152. Some Observations on the Bulk Hand- 184. 
 
 ling of Grain in California. 188. 
 
 155. Bovine Tuberculosis. 190. 
 
 157. Control of the Pear Scab. 193. 
 
 159. Agriculture in the Imperial Valley. 198. 
 
 160. Lettuce Growing in California. 199. 
 
 161. Potatoes in California. 201, 
 165, Fundamentals of Sugar Beet Culture 202, 
 
 under California Conditions. 
 
 The Country Farm Bureau. 
 Feeding Stuffs of Minor Importance. 
 The 1918 Grain Crop. 
 Fertilizing California Soils for the 1918 
 
 Crop. 
 Wheat Culture. 
 The Construction of the Wood-Hoop 
 
 Silo. 
 Farm Drainage Methods. 
 Progress Report on the Marketing and 
 
 Distribution of Milk, 
 The Packing of Apples in California. 
 Factors of Importance in Producing 
 
 Milk of Low Bacterial Count. 
 Extending the Area of Irrigated Wheat 
 
 in California for 1918. 
 Infectious Abortion in Cows. 
 A Flock of Sheep on the Farm. 
 Lambing Sheds. 
 
 Agriculture Clubs in California. 
 A Study of Farm Labor in California. 
 Syrup from Sweet Sorghum, 
 Onion Growing in California, 
 Helpful Hints to Hog Raisers. 
 County Organizations for Rural Fire 
 
 Control. 
 
CIRCULARS— Continued 
 
 No. 
 
 203. Peat as a Manure Substitute. . 
 
 205. Blackleg. 
 
 206. Jack Cheese. 
 
 208. Summary of the Annual Reports of the 
 
 Farm Advisors of California. 
 
 209. The Function of the Farm Bureau. 
 
 210. Suggestions to the Settler in California. 
 212. Salvaging Rain-Damaged Prunes. 
 
 214. Seed Treatment for the Prevention of 
 
 Cereal Smuts. 
 
 215. Feeding Dairy Cows in California. 
 
 217. Methods for Marketing Vegetables in 
 
 California. 
 
 218. Advanced Registry Testing of Dairy 
 
 Cows. 
 
 219. The Present Status of Alkali. 
 
 224, Control of the Brown Apricot Scale 
 
 and the Italian Pear Scale on Decid- 
 uous Fruit Trees. 
 
 225. Propagation of Vines. 
 
 228. Vineyard Irrigation in Arid Climates. 
 230. Testing Milk, Cream, and Skim Milk 
 for Butterfat. 
 
 232. Harvesting and Handling California 
 
 Cherries for Eastern Shipment. 
 
 233. Artificial Incubation. 
 
 234. Winter Injurv to Young Walnut Trees 
 
 during 1921-22. 
 
 235. Soil Analysis and Soil and Plant Inter- 
 
 relations. 
 
 236. The Common Hawks and Owls of Cali- 
 
 fornia from the Standpoint of the 
 Rancher. 
 
 No, 
 
 237. Directions for the Tanning and Dress- 
 
 ing of Furs. 
 
 238. The Apricot in California. 
 
 239. Harvesting and Handling Apricots and 
 
 Plums for Eastern Shipment. 
 
 240. Harvesting and Handling Pears for 
 
 Eastern Shipment. 
 
 241. Harvesting and Handling Peaches for 
 
 Eastern Shipment. 
 
 242. Poultry Feeding. 
 
 244. Central Wire Bracing for Fruit Trees. 
 
 245. Vine Pruning Systems. 
 
 246. Desirable Qualities of California Bar- 
 
 ley for Export. 
 
 247. Colonization and Rural Development. 
 
 248. Some Common Errors in Vine Pruning 
 
 and Their Remedies. 
 
 249. Replacing Missing Vines. 
 
 250. Measurement of Irrigation Water on 
 
 the Farm. 
 
 251. Recommendations Concerning the Com- 
 
 mon Diseases and Parasites of 
 Poultry in California. 
 
 252. Supports for Vines. 
 
 253. Vineyard Plans. 
 
 254. The Use of Artificial Light to Increase 
 
 Winter Egg Production. 
 
 255. Leguminous Plants as Organic Fertil- 
 
 izer in California Agriculture. 
 
 256. The Control of Wild Morning Glory, 
 
 257. The Small-Seeded Horse Bean. 
 
 258. Thinning Deciduous Fruits. 
 
 259. Pear By-products.