UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA AVOCADO DISEASES IN CALIFORNIA W. T. HORNE BULLETIN 585 NOVEMBER, 1934 UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA CONTENTS PAGE Introduction 3 Diseases of green stems and leaves 4 Sun-blotch 4 Tipburn 9 Mottle-leaf or little-leaf 11 Chlorosis 13 Smudgy spot 14 Anthracnose or withertip 16 Burn, desiccation, or sunburn 19 Frost or cold injury 20 Varietal resistance to cold 20 Influence of tree condition on resistance to cold . 22 Prevention and treatment of injury from cold 23 Cankers 24 Dying of twigs and branches 29 Asphyxiation 29 Witches' broom and unusual structures 29 Diseases of trunks 31 Rough bark 31 Diseases of roots 31 Melanorhiza or water injury 32 Asphyxiation, apoplexy, or collapse 35 Oak root fungus 37 Root coil 38 Diseases of flowers and immature fruits 39 Abscission of flowers and fruits 39 Carapace spot 39 Thompson spot 42 Blast or citrus blast 45 Crick-side 48 Embossment 49 Ring-neck 49 Diseases of mature fruit on the tree 51 End-spots 51 Hollow fruit 55 Sunburn 57 Senility or old age of fruit 57 Lenticel cork 57 Diseases of harvested fruit 58 Rhizopus rot, black mold, or bread mold decay 58 Dothiorella rot or surface rot 60 Anthracnose rot 67 Miscellaneous fruit decays 67 Flesh darkening 69 Temperature relations of harvested fruit 72 Acknowledgments 72 AVOCADO DISEASES IN CALIFORNIA 1 2 W. T. HOENE 3 INTRODUCTION This bulletin aims to bring together the principal results secured in the study of avocado diseases. The work was done by the author some years ago in Cuba, and more recently at Berkeley and Riverside. Con- trol measures are suggested whenever information is available. Improvement of transportation facilities, increased information con- cerning handling, the propagation of varieties like the Fuerte adapted to shipping, and of varieties maturing at different seasons, indicate that this fruit may now become a highly important commercial crop. The avocado appears to be more easily injured by unfavorable con- ditions of soil and climate than citrus fruits; and, especially in humid countries, it has its due share of parasitic maladies. Various injuries and diseases which have been observed are not men- tioned here, and some of those not included may prove, on further study, to be of major importance. Injuries evidently caused by insects and other animals are not included though they have a marked signifi- cance and in some cases resemble the troubles discussed. The diseases affecting avocados in other countries may, in some cases, be unlike ours and merit study to the end that quarantine measures may be adopted if the situation appears to justify; but this discussion has not been undertaken in the present publication. The peculiar responses of the avocado to climate, and to soil and water conditions, as well as the variation in behavior by different vari- eties, suggest that studies of the normal physiology of this plant also need to be made. Many avocado troubles already recognized and dealt with are of a unique character and require more extended study. In continuing the work, it is hoped that numerous conditions not now understood may be conclusively explained and that adequate control measures may be developed or further perfected. i Eeceived for publication February 2, 1934. 2 Paper No. 305, University of California Graduate School of Tropical Agricul- ture and Citrus Experiment Station, Eiverside, California. s Associate Professor of Plant Pathology and Associate Plant Pathologist in the Citrus Experiment Station. [3] University of California Experiment Station DISEASES OF GREEN STEMS AND LEAVES Sun-Blotch. — Sun-blotch 4 is now widely distributed in California and was observed by Dr. H. S. Fawcett in Palestine in trees from Cali- fornia. The disease was first described in 1928, but it had existed here for some years. There is as yet no evidence that it was brought in with any of the many importations of scions from the Central and South American countries. The disease is said to exist in Guatemala, but we have not been able to get detailed information. So far as is known, it does not attack any plant other than the avocado. It is not known to have caused the death of any bearing or mature tree, but small plants severely affected may be so depressed in vitality that they succumb to other causes. Where pronounced, the disease profoundly modifies the habit and aspect of the tree and causes most of the fruit to be culls. In old and severe cases trees may become irregularly bent down, the branches having pendent twigs. Trunks, limbs, and older stems are crooked and bent down, from weakness at early stages of growth, with surfaces rough and prematurely aged. Green twigs show an abnormal coloring, ranging from a mere vague mottling which is not distinguishable from some conditions of normal twigs, to pale longitudinal marks or yellow spots on the green bark. The pale areas are mostly a light buff color and are somewhat depressed. These areas frequently extend from a bud for some distance up the stem, or they may appear to extend down- ward from a leaf base. At the beginning of a cycle of growth the whole stem may be pale, the light area being gradually narrowed into a streak above, which blends out into green tissue (fig. 2e). At a very early state, the pale areas of affected twigs may show some bright red color. In some severe cases, such as occur in the very vigorous and succu- lent shoots developed by cutting off large trees and introducing dis- eased scions, extensive development of the buff areas has occurred so that the shoots are strikingly abnormal. In such cases the extensive light-colored areas frequently develop large spots of dead tissue with abundant whitish granular exudations on the surface. This gives a different aspect of the twigs and branches from the usual one which suggests merely a pronounced and premature aging of the bark. This kind of injury might, in some cases, resemble sunburn, but it has oc- curred in experiments where the twigs were fully shaded. Leaves of affected trees usually appear normal, but occasionally a * Coit, J. Eliot. Sun-blotch of the avocado, a serious physiological disease. Cali- fornia Avocado Assoc. Yearbook 1928:27-29. 5 illus. 1928. Bul. 585] Avocado Diseases in California 5 Fig. 1. — Caliente variety with sun-blotch: a, stem not fully normal but not showing distinct sun-blotch grooves, leaves apparently normal; b, large buff sun-blotch area near the stem with the center dead and dark; c, broad sun- blotch area near the stem and two on the side; d, e, f, h, and i, characteristic sun-blotch; g and j, apparently normal fruits on the same tree (the trans- verse cracking is not due to sun-blotch but possibly to moisture fluctuations) ; k to r, leaf distortion (X 0.45). 6 University of California Experiment Station light variegation appears, the paler areas being somewhat imper- fectly developed so that such leaves may be asymmetrical at the petiole, or less often they are distorted throughout, some even having two mid- ribs (fig. lk-r, and fig. 2a). Occasionally a shoot is found on a healthy tree which bears varie- gated leaves; these shoots are true chimeras like many of the variegated- leaved plants grown as ornamentals (fig. 2b). These variegations, though infrequent, might be mistaken for sun-blotch. Fruit on sun-blotched trees may be normal or variously marked with depressed lighter streaks or areas extending parallel with the long axis of the fruit. Most of these streaks and spots start at or near the stem end, but some occur well down the sides of the fruit. In varieties which remain green on ripening, color is as on the stems, but the dark varieties show brilliant purple red in the depressed parts when color develops on the rest of the fruit. Flesh of the affected part may have some lighter streaks, but is apparently not impaired for use as food, the light streaks softening with the rest of the flesh. The intensity of the disease varies greatly. Sometimes the tree is a deformed bush suggesting a weeping mulberry, and bearing only cull fruit. In some cases the tree has good and bad twigs intermingled, and many not distinguishable as to their state. Still, in other cases, there are large and fine trees in which perhaps only a single lightly streaked fruit can be found. Shoots which are vigorous and apparently normal sometimes arise from severely affected limbs. Frequently such shoots develop sun-blotch symptoms at a later date. Cases have been observed where sun-blotched shoots came up from the trunk of ap- parently healthy trees; and cases have been reported where seemingly healthy trees grafted with apparently healthy scions gave sun-blotch growth. In the last two cases the trunks were presumably diseased at the point where shoots originated, and where the grafts were set. Where affected scions are set in healthy trunks they give rise to sun- blotch growth. Also after a time suckers arising from the stock in the vicinity of the scions show the disease. The rate of movement of the inciting agent, or cause, of the disease through the living plant prob- ably varies, but in some cases has been estimated to be about 3 feet per year. Healthy stock grafted with diseased scions, but in which the scions failed to become established, has, in at least some cases, developed the disease. All efforts to transmit the disease by means of sap from af- fected trees have failed thus far. No significant organism has been found by direct examination, or by cultures, though these studies are not exhaustive. Bul. 585] Avocado Diseases in California Fig. 2. — Sun-blotch on fruit, leaves, and stems : a, striking white leaf varie- gation from Kashlan variety with sun-blotch; b, Fuerte variety, possibly a true chimera or variegation not due to sun-blotch; c and d, round-type Fuertes with sun-blotch streaks (in d, the surface of one streak has dried and cracked, and the injuries on the pedicels are not due to sun-blotch but probably to frost) ; e, shoot of Fuerte showing two branches light at beginning of flush of growth, the slender branch above being buff with the leaf petiole at the end green, and the lower branch showing the light area narrowing to a broad streak; /, a shoot similar to the lower one in e but older; g, old sun-blotch shoot showing general roughness. (Photographs of c and d by D. F. Palmer.) 8 University of California Experiment Station No method for transmitting the disease from tree to tree is known except through budding or grafting operations. It is possible, however, that other methods exist. Various sucking insects might be important in this connection as they are known to transmit certain plant diseases. No case is known to the author in which a seed from a sun-blotched tree has produced a seedling which showed the disease from the be- ginning. 5 At one time it was thought that seedlings which had never been grafted showed the disease, but the histories of these trees are not fully clear. The above-mentioned facts suggest that the disease is caused by an invisible agent, i.e., that it is one of the so-called virus diseases. 6 Control measures 7 cannot be recommended with much assurance. Complete eradication appears not practicable on account of the diffi- culty of recognizing the disease in its mild form. It would seem reason- able, in those trees where apparently good limbs and evidently affected limbs are both present, to remove the bad ones. Pronounced cases might be cut back severely to stimulate shoot formation, in hope that some of the shoots may prove healthy and that a new tree may be formed from these. Unless a favorable result is secured, unprofitable sun-blotch trees might well be destroyed. So far as known, a new tree may be replanted where a sun-blotch tree is removed. Occasionally, an apparently good shoot appears in a severely affected tree, and for affected trees of good size and vigor, it may be better to wait for the appearance of such shoots rather than to practice severe cutting back. It will probably be best to remove promptly all young and inferior trees showing symptoms of sun-blotch. It is possible that trees of un- usual value may be conserved, or at least some healthy propagations secured from them, if special care is taken, although this has not yet been demonstrated. Every tree showing any symptoms in fruit or twigs should be care- fully avoided in the usual cutting of scions. Mother trees from which buds and scions are to be taken should be chosen according to a care- fully planned procedure of examination of the tree and fruit; both for avoiding entirely all sun-blotched trees, and to secure propagations of the best types. Nursery trees showing sun-blotch evidently should not be planted. However, the author has not been able, in all cases, to recognize the dis- s Parker, E. R., and W. T. Home. The transmission of avocado sun-blotch. Cali- fornia Avocado Assoc. Yearbook 1932:50-56. 1932. 6 Home, W. T., and E. R. Parker. The avocado disease called sun-blotch. Phyto- pathology 21:235-238. 1931. 7 Home, W. T., and E. R. Parker. The avocado sun-blotch disease. California State Dept. Agr. Mo. Bui. 20(7) : 447-454. figs. 82-85. 1931. Bul. 585] Avocado Diseases in California 9 ease, owing to incomplete knowledge of symptoms. A condition of trunks of nursery trees consisting of obscure depressed longitudinal streaks has been brought to the author's attention, and is being inves- tigated. The effect might be described as ropiness of the trunks of trees which otherwise appear to be normal, also some yellow patches may show on the green bark. Studies have thus far failed to demonstrate that this condition is due to sun-blotch, but the trees are still under observation. Tipburn. — Tipburn consists in the formation of more or less exten- sive dead areas where the leaves have died back from the outer ends. The dry area on a leaf tends to have an uneven outline and to follow back along the leaf margins. More than half of the leaf area may be in- volved and there is a tendency for all the leaves of approximately the same age on a shoot, or sometimes over a large part of the tree, to be about equally affected. Where strongly developed, tipburn appears to be a rather distinct disease. However, some dying back from the tip may be found on old leaves of many trees. Accordingly, it is possible that tipburn should be understood as the name of a symptom, which may be more or less de- veloped, rather than the name of a definite disease. Tipburn is a condition of fully matured or old leaves, but affected leaves probably are not shed very much earlier than normal ones on trees of comparable vigor. A dying of tips and edges of very tender leaves, usually those not yet fully grown, in which the dead part is very dark, is not included in this discussion, as it has not been sufficiently studied. (See "Burn," p. 19.) The original description of tipburn 8 states that the trouble is most pronounced in trees growing in light sandy soils overlying heavy clay subsoils and situated near the coast where they are exposed to ocean winds, but that it may occur in other soils and under other conditions also. Tipburned leaves 9 have been found to contain more salt (NaCl) than normal leaves. Excess of salt in the soil, or in the irrigation water, will undoubtedly increase the condition. Also, tipburned areas evidently have a relation to the water supply of the leaf. Plants in pots and water cultures suffer severely, probably owing to abnormal root conditions and occasional drought. Dean F. Palmer has observed that in the most usual form of tipburn, at least a little mottling precedes the appearance of dying back of the tip and margin (fig. 3). (See "Mottle-Leaf," p. 11.) 8 Coit, J. Eliot. Pests and diseases of the avocado : tip-burn. California Avocado Assoc. Yearbook 1928:19. 1928. 9 Haas, A. R. C. Relation of chlorine content to tip-burn of avocado leaves. Cali- fornia Avocado Assoc. Yearbook 1928:57. 1928. 10 University of California Experiment Station Tipburn apparently is not caused by leaf-killing fungi since the dead areas are often almost without such fungi. Where fungi do occur, they are evidently chance infections, several spots of the same or different fungi being scattered about on the same dead area (fig. 3 and fig. 30b). Fig. 3. — Tipburn on avocado leaves. One area of dothiorella is indicated by the arrow. The leaf at the right was photographed partly by transmitted light to show paler areas between the veins. Apparently all conditions in the orchard from which these leaves came were favorable except that irrigation water con- tained more chlorine than is considered safe for avocados. The dark spots in the dead portions are mainly the fungus Cladosporium sp. It frequently appears that a colony of dothiorella, anthracnose, or other fungus has caused the dead area to be slightly enlarged locally. There is general belief that tipburn is a drought phenomenon, and that it may be intensified by shortage of water, very drying atmospheric conditions, defective functioning of the root system, or general weak- ness of the plant. Tipburn may not come at once with acute lack of water, but may follow after several days, and when abundant water has been supplied. Trees in bad condition, furthermore, do not always show pronounced tipburn, especially in medium or heavy soils. Bul. 585] Avocado Diseases in California 11 Tipburn should be regarded as a state of reduced leaf area, indicat- ing some unfavorable condition — probably a defect in the quality or quantity of water. It is important because it reduces effective leaf area (severely tipburned trees often lack vigor), and also because the dead areas become invaded with such fungi as dothiorella, anthracnose, and alternaria. Spores of these fungi are freely propagated while the tip- burn leaves are still on the tree, and are in favorable position to get on the fruit. While the fruit remains on the tree, or while still hard, it does not often show signs of infection, but when it softens, spots of de- cay appear. This applies particularly to dothiorella in the coastal re- gion; but applies also to other fungi, and to all districts. (See Dothior- ella Rot," p. 60.) Control measures can be suggested only in general terms and based on observations of results in different orchards. The use of irrigation water containing more than a minimum amount of salt should be avoided. About 100 parts per million of chlorine (equals 165 p. p.m. common salt) may be considered a danger line; but other conditions have much to do with the degree of injury to the tree by a specific concentration. 10 In some cases, tipburn has appeared when using water with about 50 parts per million of chlorine while in other cases good trees have been grown with water containing as much as 200 p. p.m. chlorine. Either excessive drying or water-logging of the soil should also be avoided. It has been pointed out that the appearance of the avocado tree is not always a safe guide as to its need of irrigation. The mature leaves are rather stiff and do not show rolling or wilting on slight short- age of water. Small plants have been observed which by accident be- came so dry that the stems shriveled, yet the mature leaves did not show much distress except for some drooping because of the petioles becoming flaccid. When supplied with water the stems became plump again, but the leaves did not entirely resume their former position and in a few days severe tipburn developed. The soil management program should be planned with the purpose of promoting tree vigor. The first measures to be taken are to find out whether an adequate supply of organic material and nitrogen are present. Shelters and windbreaks are beneficial in the more exposed situations. Mottle-Leaf or Little-Leaf. — The name mottle-leaf may probably be applied with propriety to a condition of foliage where the green color- ing matter in leaves is distributed in a spotted or mottled pattern, the io Thomas, E. E. Effects of chlorides in the soil on avocado trees. California Avo- cado Assoc. Yearbook 1932:48-49. 1932. 12 University of California Experiment Station lighter areas being- between the veins (fig. 4). It is also called foliocel- losis, or frenching; in extreme cases frizzels, little-leaf, etc. In severe cases leaves become small in size and light in color and shoot growth is Fig. 4. — A pronounced case of mottle-leaf or little-leaf on Fuerte variety. reduced. Young leaves are abnormal from the beginning and often a bronzing can be noted before the typical mottling is fully developed. Severely affected old leaves often may have many small dead specks Bul. 585] Avocado Diseases in California 13 and spots. Fruit on severely affected shoots is small and abnormal. This disease is not generally common in California, but tends to be severe in some cases. A similar disease known also as mottle-leaf is of much importance on citrus fruits where it has been under investigation for a number of years. 11 On the latter host it is at present considered to be nutritional in nature but the exact cause is as yet unknown. It was previously thought to be related to the inability of citrus leaves to satisfy their calcium re- quirements, 12 but the recent observation showing the beneficial effects of small amounts of zinc indicate that the earlier theories were inadequate. It is now generally believed that an abundant supply of organic ma- terial, together with an adequate supply of nitrogen, and a favorable moisture condition are important in the prevention of mottle-leaf. Re- cent studies have shown that mottle-leaf, in trees of several kinds, has been greatly decreased by the application of several pounds of zinc sul- fate per tree to the soil or by spraying the foliage with a mixture of zinc sulfate and lime. This is being tried on avocados. It is hoped that this treatment may prove beneficial, but attention to the moisture content and organic material of the soil should not be relaxed. Chlorosis. — Chlorosis is the name applied to a condition of foliage which has a uniform very pale-green or yellowish-white color. This is distinguished from mottle-leaf in which the same leaf has light and more or less normal areas alternating giving a mottled or spotted effect. Severely affected chlorotic leaves in time become more or less burned at the tips and edges and some have dead spots. They are evidently less resistant to drying and injury than normal leaves. Since leaves without green color cannot manufacture sugar and starch to nourish the plant, a kind of starvation results from severe chlorosis. The word is used here as the name of a symptom rather than the name of a specific disease. It is believed that the causes and remedies may not be the same in all cases. The explanation usually given for the occurrence of chlorosis is that a certain amount of iron must be present, dissolved in the plant juices, to enable the plant cells to manufacture their green coloring matter. If an excess of lime is present in the soil it may interfere with the iron sup- ply of the normally green parts of the plant, since it makes the iron in- soluble. Apparently there are cases where the above explanation does not apply, since chlorosis sometimes occurs where the lime content of the soil is not excessive. In some cases, at least, the subsoil is impervious and 11 California Agricultural Experiment Station. Eeport of the College of Agricul- ture and Agricultural Experiment Station, 1913-14:62. 1915. 12 Fawcett, Howard S., and H. Atherton Lee. Citrus diseases and their control, p. 336-341. McGraw-Hill Book Co., New York. 1926. 14 University of California Experiment Station percolation of water is very poor. Also new foliage on avocado plants in ordinary soil in pots has come out yellow. In these cases, repotting in very porous soil generally corrects the trouble. Nursery trees of normal color may give rise to a flush of growth which is strongly chlorotic. A bud set in a normally colored stock may grow out into a shoot without chlorophyll. The relation of lime and of drainage to chlorosis needs to be studied further in the avocado. The partly yellow leaf (fig. 26) is not affected with chlorosis, but presumably is a bud sport yielding a chimera or variegated plant. In this case the light color is presumably not related to a soil or water condition. Control may be difficult. Chlorotic plants of other sorts have been restored to normal color by spraying with dilute solutions of iron salts. Injecting solutions of iron salts into the trunk or roots has sometimes restored normal color. The last method, 13 however, has not been found satisfactory since the effect is temporary and a certain injury is caused to the trunk by each injection. Iron sulfate has sometimes been applied in trenches to the roots of affected trees, with favorable results. 14 It is probable that mixing the finely ground material with organic matter is helpful. The most important measure in treating the majority of cases would seem to be to avoid any excess water in the soil. The use of abundant organic material and an adequate supply of nitrogen will probably help. Where soil is very shallow and underlaid with dense layers of soft whitish stone containing much calcium carbonate, control of chlorosis and production of good avocado trees will probably be difficult to ac- complish. (See also "Asphyxiation," p. 35, "Melanorhiza," p. 32, and "Mottle-Leaf," p. 11.) Smudgy Spot. — Smudgy or sooty spots, caused by Helminthosporktm sp., are common on green stems of avocados. Margins of the spots are not sharply outlined but fade out. The size is from very small to large; or numerous spots may be confluent into a nearly continuous area. With a moist cloth the black material may be wiped off leaving the green bark uninjured (fig. 5, at x). Leaf petioles are frequently spotted and less frequently the spots occur on leaves or even on fruits. It has not been observed that the spots are appreciably injurious but they sometimes challenge attention, especially in nursery trees. Specimens were sent to the United States Department of Agricul- ture, Plant Disease Survey, Washington, D. C, and the fungus was is Thomas, E. E., and A. E. C. Haas. Injection method as a means of improving chlorotic orange trees. Bot. Gaz. 86:355-362. 1928. 14 Bennett, J. P. The treatment of lime-induced chlorosis with iron salts. Califor- nia Agr. Exp. Sta. Cir. 321 : 1-12. 1931. Bul. 585] Avocado Diseases in California 15 tentatively identified as belonging to the genus Helminthosporium. Fungi of this genus have been reported on other plants of the laurel famih r . Fig. 5. — Smudgy spot on green, twigs of Fuerte. At x t a number of spots were rubbed off, using a moist cloth. Certain black fungi are common on many plants. These fungi live in the honeydew secreted by various insects. Careful observation has failed to connect the smudgy spot fungus with any insect, and the above name is being used to distinguish it from the so-called "sooty mold fungi" which follow attacks of certain insects. The fungus is widely distributed on avocados in California. It is not 16 University of California Experiment Station much restrained by dry atmosphere, as it is common at Riverside and Escondido. It apparently does no harm to tree or fruit, but may be ob- jectionable on account of its appearance under some very special cir- cumstances, as on nursery trees, or fruit. When bordeaux spray has been applied, the fungus has apparently been destroyed. A colorless spray, such as ammoniacal copper carbonate, might be tried if the stain left by bordeaux is objectionable. A mixture of 2 parts cold water paint with 1 part sulfur, sprayed on the plants and washed off after two months, is said to remove the fungus. (See p. 66 for discussion of copper sprays and cyanide fumigation.) Anthracnose or Withertip. — The word anthracnose is used to desig- nate certain plant diseases which consist in the formation of dead spots of various sizes in living stems, leaves, or fruits. Some anthracnose diseases, such as those affecting the foliage or fruits of sycamores, limes, mangos, and other plants, may be very inju- rious, especially under moist conditions. Anthracnose (withertip) of oranges, grapefruit, and most other citrus plants was formerly some- what misunderstood, owing to confusion with a disease caused by a closely related but distinct fungus affecting limes. A clear account of these diseases as to citrus fruits has been given by Fawcett and Lee. 15 Incomplete studies indicate that in California anthracnose of avocados is caused by Collet otrichum gloeosporioides Penz. and closely parallels anthracnose as it affects common sweet oranges, lemons, and grapefruit. Apparently in California anthracnose of avocados as a disease affecting fruit on the tree, leaves, and twigs is unusual, except following injuries of some kind. The anthracnose fungus may occur abundantly on dead or much weakened leaves and twigs. On the soft (ripe) fruit it is one of the most frequent causes of decay. (See "Anthracnose Rot," p. 67.) In 1928 there appeared to be danger of an outbreak of anthracnose, and as an emergency measure, the following spray program based on experience reported from Florida and other countries was suggested for trial. 16 Spray three times, beginning early in November and repeating at in- tervals of six to eight weeks. Spray materials suggested were standard bordeaux, ammoniacal copper carbonate, and Burgundy mixture. It is anticipated that the bordeaux will be most effective but that the others will leave less stain on the fruit. James D. Hoffman of Pasadena and C. V. Newman of Tustin carried out this spray program using the bor- deaux mixture on limited areas. Results were significant since they is Fawcett, H. S., and H. A. Lee. Citrus diseases and their control, p. 283-293, 454-464. McGraw-Hill Book Co., New York. 1926. is Home, W. T. Semi-annual field tour, Orange County, October 20, 1928. Califor- nia Avocado Assoc. Yearbook 1929:12-14. 1929. Bul. 585] Avocado Diseases in California 17 showed that no harm was caused to trees or fruit by the bordeaux. How- ever, as no anthracnose appeared in the orchards, we have no evidence concerning the effect of this spray on the anthracnose under California conditions. (See p. 66 for a discussion of cyanide injury following the application of copper sprays. ) During three years of rather careful study of avocado troubles, the author found no case of anthracnose on fruits on the tree until 1931, when fruits of the Northrup and Puebla varieties with spots in which withertip fungus was growing were sent in from Riverside and from Es- condido. In both cases these fruits had been on the trees beyond the proper time for picking, both had been exposed to the November freeze, and at least the Riverside fruit had been subject to hail early in the autumn. It was not possible to state that the unusual circumstances had enabled the withertip fungus to become established, but this was be- lieved to be the case. In the lime, mango, and some other tropical fruits, anthracnose may cause destruction of flowers and young fruits, and it has sometimes been supposed that the excessive shedding of flowers and young fruit which occurs occasionally in the avocado is due to a disease of the same kind. The author has observed with care but has not been able to find anthracnose injuring avocado flowers in California, nor has spotting of young leaves or fruit been seen. It is stated that anthracnose of avocado flowers is rare in Puerto Rico. 17 The author failed to observe it in Cuba, but the studies were by no means exhaustive. (For a discussion of shed- ding of flowers and fruits, see p. 39.) Anthracnose of the avocado merits more thorough study, but for the present it seems safe to conclude that in California it does not constitute a serious menace to avocado trees and young fruit. In other countries, anthracnose has been reported by reliable workers as behaving differently from what is reported here for California. In southern Florida and the West Indies a disease of avocados known as "black spot" and ascribed to a fungus of the genus Gloeosporium or Col- letotrichum occurs. 18 In black spot disease young leaves, stems, and fruits are attacked as indicated by the name and much harm may be done. In Honolulu, a disease known as "rusty blight" and ascribed to a fun- gus of the genus Gloeosporium 19 occurs and causes defoliation of trees. 17 Nolla, J. A. B. The anthracnose of citrus fruits, mango and avocado. Puerto Rico Dept. Agr. Jour. 10:25-43. 1926. is Rorer, J. B. Fungous diseases of the avocado. Trinidad and Tobago Dept. Agr. Bui. 18(3) : 132-133. 2 pi. 1919. Stevens, H. E. Avocado diseases. Florida Agr. Exp. Sta. Bui. 161:3-23. 6 fig. 1922. is Higgins, J. E., Chester J. Ham, and V. S. Holt. Avocado in Hawaii. Hawaii Agr. Exp. Sta. Bui. 25:23-26. fig. 2, pi. 5. 1911. 18 University of California Experiment Station Fig. 6. — The upper leaves, a and b, show a kind of leaf shot-hole which occurs occasionally ; c to g, injuries observed following periods of high temperature on leaves not fully matured ; /, sunburn showing an extensive light area formed on one side of the midvein; d and g, similar to /, but with black dead spots; e, se- vere burn in which a darker band one-eighth inch wide occurs in the dead tissue next to the green. Bul. 585] Avocado Diseases in California 19 The author has not recognized black spot disease, nor the rusty blight disease in California. Temperature, humidity, and varieties of avocados generally grown, are different in California from those in the countries mentioned. It is also possible that the strains of withertip fungi found in California may be different from those occurring in the other coun- tries. Studies in the field following periods of prolonged rain may reveal conditions not yet observed by the author. Burn, Desiccation, or Sunburn. — The avocado obviously is not well adapted to resist conditions which favor extremely rapid evaporation of water, namely, high temperature, very dry air, and rapid air movement. New leaves which have not attained their full maturity (as shown by thickness and stiffness) are susceptible to physical injury and to drying. In the warmer avocado districts, healthy and vigorous trees usually show a certain number of leaves which have turned dark and dried wholly or in part. The drying mostly affects immature leaves and causes them to become deformed as they expand (fig. 6c-g). The period of greatest susceptibility is apparently between the time the leaf is an inch or less in length and when it has reached full size but not attained the thickness and hardness of full maturity. Under diverse conditions of the tree, the injuries produced may show much variation. Sometimes numerous spots or small specks show (fig. 6a, b). A single tree and perhaps only certain leaves are affected. No significant organism has been found in the dead areas, and the blemish is thought to be a desiccation burn occurring under very special condi- tions of tree and environment. The parts played, respectively, by high temperature, by desiccation alone, by wind in bending and injuring the leaf, and by the influence of conditions in the soil, is not clear. The appearance of such injuries is not confined to the times of highest temperature and strong wind, but occurs occasionally with normal summer weather. A vigorous tree is not, apparently, seriously injured by having a few burned leaves. However, shelter is one of the most important considerations in avocado culture. Trees close to heavy shelter, such as a home, often show a very healthy aspect. For newly planted trees, this protection is more important than for citrus. Small plants which have not become fully established, or which are low in vigor, may show pale, yellowish color in the most exposed twigs, a brown, corky layer may be developed over the surface and even a kill- ing of the bark may result. Where limbs on trunks are weakened by an excessive crop, or other- wise, and are brought to a new or more severe exposure, the bark may be killed. Varieties like the Lyon and Puebla frequently are injured in 20 University op California Experiment Station this way. Timely thinning of fruit appears to be indicated in this case. The avocado tree may be cut back severely when well grown, if it be- comes desirable to get rid of injured limbs, and it will restore itself rather promptly. Protection of newly-set trees until they are established, whitewashing exposed trunks and branches, and supplying abundant water at critical times, are accepted measures for reducing injury to a minimum. 20 (See also "Cold Injury," p. 20; "Tipburn,"p. 9; "Sunburn," p. 57; "Abscission of Flowers and Fruits," p. 39; and "Sun-Blotch," p. 4.) Frost or Cold Injury. — No extensive studies of the effects of cold on avocados have been undertaken by the author but a number of papers have been published on this matter. 21 It appears safe to make the fol- lowing statements : 1. Symptoms on foliage and stems : where freezing is slight leaves may show small dead spots or merely specks; or lower and outer more ex- posed leaves may be killed at the tips. Where freezing is severe, varying degrees of leaf injury will be apparent, sometimes causing complete defoliation. Where this occurs, many twigs will be killed back from the tips. Bark of stems may be split and separated from the wood so that in healing, grotesque cankers are formed. With increasing degrees of cold, larger stems are affected until the main limbs may die and even the trunk may be killed to the ground. 2. Symptoms on flower buds and flowers : these have been reported killed when trees were otherwise not much injured, causing loss of one year's crop. 3. Symptoms on fruit : the fruit pedicels are more easily frozen than either the leafy stems or the fruit itself, the most slender part of the pedicel being most liable to injury (figs. 7 and 8). The pedicels wither and the fruit drops. If used when the pedicel first shows injury, such fruit should be perfectly good. Varietal Resistance to Cold. — The Mexican varieties, including Fuerte, show cold resistance, varying from somewhat more pronounced 20 California Avocado Association. Symposium on heat injury. California Avo- cado Assoc. Ann. Eept. 1917:94-99. 1918. 2i Hertrich, Wm. Effect of the recent cold weather on the different varieties of avocados in different localities. California Avocado Assoc. Ann. Kept. 1921-22:16- 27. 1922. Hodgson, Eobert W. The California avocado industry. California Agr. Ext. Ser. Cir. 43:38-41. Revised 1934. Kinman, C. F. Observations on frost injury to avocados. California Avocado Assoc. Ann. Eept. 1918-19:56-58. 1919. Eyerson, Knowles. Avocado culture in California. Part I. History, culture, va- rieties, and marketing. California Agr. Exp. Sta. Bui. 365:602-604. 1923. Eyerson, Knowles. Eecovery of the avocado tree after the 1922 freeze. California Avocado Assoc. Ann. Eept. 1922-23:26-29. 1923. Webber, H. J. Cold resistance of the avocado. California Avocado Assoc. Ann. Eept. 1917-18:49-50. 1918. Bul. 585] Avocado Diseases in California 21 than the orange to about that of the lemon. Injury may be expected even on trees of good size where conditions otherwise are favorable for resistance at 20° F. The Fuerte variety, in some ways intermediate be- tween Mexican and Guatemalan types, is generally rated with the Mex- ican varieties in cold resistance. Fig. 7 — Two Itzamna fruits showing what is thought to bo typical cold injury. The pedicel is injured but the fruit is unhurt. At the left the whole pedicel is darkened but the fruit was still living tissue as shown by normal abscission or cut- ting off of the pedicel four days after picking. At the right the lower segment of the pedicel is shown as uninjured. The Guatemalan types present a wide range of resistance, but expe- rience indicates that they are slightly less resistant, about the same as the lemon. Severe injury may be expected at 25° F. The West Indian types, which occur only as occasional trees in Cali- fornia, are considered too tender for commercial planting. Severe in- jury to them may be expected at 27° F. Seeds of hardy Mexican types are now used almost exclusively for producing rootstocks in the nurseries. If seeds of tender Guatemalan and West Indian varieties should be used, the stocks might be frozen 22 University of California Experiment Station when the tops are not. This is especially true since temperature is likely to be lowest at the surface of the ground, and it is not customary in California to mound young trees for winter protection. Fig. 8. — Artificial cold injury, produced by exposure f or 4% hours to a temperature reaching 26° F. The fruit was uninjured but part of the pedicel was killed. Influence of Tree Condition on Resistance to Cold. — Owing to ex- treme local variations of intensity and duration of cold, both varietal resistance and resistance due to tree condition may be difficult to judge. Tree condition itself may also be a very obscure matter. Young, actively Bul. 585] Avocado Diseases in California 23 growing leaves are easily injured. The mature leaves on trees in active growth are reported to be more easily injured than comparable leaves on trees in a dormant condition. The young foliage, flowers, and tender fruits of all varieties and under all conditions do not have the same degree of resistance; but all are very tender. Trees in any way weakened, or not in full vigor, are said to suffer much more than robust trees. Trees in poor or unfavorable soils, those which have become too dry (and especially if they have been dry and then heavily irrigated), and trees with bad unions, or which have been wounded or recently pruned, may be injured. Girdling is said to have a rather enduring effect in this way. Wind injury increases cold damage. Small plants and young trees suffer much more than older ones. In robust trees, recovery from cold injury is more rapid and com- plete than in citrus trees. Where only leaves and small branches are lost, harm seems to be slight except for loss of the succeeding crop. Trees may make rapid recovery when frozen to the main trunk or even to the ground. However, some trees fail to recover satisfactorily and may as well be removed when it is seen that they are not doing well. Prevention and Treatment of Injury from Cold. — For commercial groves, the matter of prevention of cold injury is probably more impor- tant than for citrus. The extensive information developed in connec- tion with citrus groves should apply as to wrapping the trees and the use of orchard heaters. For home-lot trees and very small plantings, orchard heating may not be practicable, but the young trees should have the trunks protected by loosely wrapping with cornstalks or similar material for several years after planting or until they become robust. The Fuerte and Mex- ican varieties are preferable to more tender kinds for planting in a wide variety of situations. Young seedlings of promise should be protected as they might easily be lost while young, but later the danger is much reduced. Where trees have been severely frozen they should be whitewashed, and it will be best to disturb the new growth as little as possible for the first year. If shaping of new growth is to be done, probably the best method would be to pinch out the tips. Very little foliage should be cut away. In general, the procedure for frozen citrus trees may safely be followed for the avocado. When the tree is reestablished, dead branches should be cut away, the wounds disinfected and covered to prevent wood decay, and to facili- tate healing. The new growth will need somewhat more care in training to secure a desirable tree than in starting with a nursery tree. 24 University of California Experiment Station Authorities agree that the avocado may not safely be "hardened off" by severe drying in preparation for winter, but that a favorable mois- ture condition of the soil should be continuously maintained. The California Agricultural Extension Service might well be con- sulted in making plans for frost protection in any given locality. ( See "Temperature Relations of Harvested Fruit," p. 71.) Cankers. — The word canker is used for a condition affecting stems, trunks, or roots, in which there is a dead spot of varying size and shape, often surrounded by a zone of diseased or more or less abnormal tissue. Cankers in plants are frequently caused by parasitic fungi which ac- tively invade and kill the plant tissue when certain conditions of tem- perature and moisture prevail. When conditions become less favorable, the fungi may become inactive and die out. Cankers may, however, be due to other causes. Cankers affecting avocado bark, 22 usually near the base of the tree, have been studied by Dr. Fawcett. He was able to isolate a fungus which was first thought to be a variety of brown rot organism, Phytophthora citrophthora, but which on later study by Dr. Barrett was found to be Phytophthora cactorum (Leb. and Cohn) Schroet. 23 This fungus, when placed below the bark of healthy avocado trees, caused the same sort of canker. It did not, however, readity attack orange trees when inoculated into the living bark. Cankers which the author has studied on avocado appear to be prin- cipally of two kinds, those affecting the base of the trees and roots, and those found on limbs and green stems. A number of avocado cankers at the base of the tree have been ob- served, but only bacteria and miscellaneous fungi were found, none of which produced disease when inoculated into normal avocado bark. A case of this kind was studied with M. H. Kimball, Assistant Farm Ad- visor of Los Angeles County, and a considerable number of cultures were made. It is probable that in the cases investigated the primary cause of the canker was not in an active state, or had died out. 22 Fawcett, Howard S. A Pythiacystis on avocado trees. Phytopathology 6:432- 435. 1916. Fawcett, Howard S. Bark diseases of avocado trees. California Avocado Assoc. Ann. Eept. 1916:152-154. 1917. Also in: California Citrograph 2(3) :22-23. 1916. 23 The genus Phytophthora has recently been studied by a number of authors with regard to the segregation of species and the names these should bear. According to generally accepted usage at this time a form causing crown rot of walnut in Cali- fornia belongs to the species cactorum. It has considerable similarity in structure, and effect on the tree attacked, to one of the fungi which causes lemon gum disease, first called Pythiacystis citrophthora but now more generally Phytophthora citroph- thora (S. and S.) Leonian. Still a third closely related fungus is Phytophthora para- sitica Dastur which causes a similar effect known as citrus foot rot disease in Florida and in other countries. Bul. 585] Avocado Diseases in California 25 Fig. 9. — Inoculations made in studying avocado cankers (xl.l) : a, inocula- tion with dotliiorella on a healthy avocado stem in the greenhouse ; b, inoculation using Phytophthora parasitica (Fawcett's culture 1792), the fungus of citrus foot-rot ; c, inoculation using fresh tissue from an avocado stem canker ; d, check showing manner in which inoculations were wrapped ; e, check unwrapped ; /, in- oculation using Phytophthora citrophthora (Fawcett's culture 1309A) the fun- gus of lemon brown rot. 26 University of California Experiment Station Both the citrus foot rot fungus, Phytophthora parasitica Dastur, and the fungus causing lemon gum disease, P. citrophthora, have been in- oculated into avocado stems in the greenhouse and very decided cankers Fig. 10. — Canker and withering of twigs in Itzamna variety. The white points on the surface of the canker at x suggest that fungus bodies are present; but the white points are small masses of sugary material which exudes and dries. The three twigs at the right show about the condition found in melanorhiza. have been produced (fig. 9b, /). From this it appears probable that any one of the three species of Phytophthora mentioned might cause can- kers at the base of avocado trees, under favorable conditions. Cankers found by the author at the crown of avocado trees have not been very large, and have not been observed to kill the tree. What is apparently the second kind of canker has been observed on green stems. This consists of black, slightly sunken areas often an inch Bul. 585] Avocado Diseases in California 27 Fig. 11. — Two cankers on an avocado limb before rough swelling occurred (x0.6) : a, surface aspect; b as at a, but at x the surface has been cut away over the left half of the canker to show the blackened tissue beneath. At xx the left half of the canker has been cut away somewhat more deeply than at x, but not to the wood. The deeper layers of bark appear normal. The right half of the can- ker has had the surface of the upper portion cut away as with the left half of x to show the blackened subsurface tissue. At x and xx sap has exuded and dried to a white deposit. or two long and a quarter of an inch wide. Some of these cankers show an accumulation of white granular material which is frequently re- ferred to as avocado sugar. This may exude from small eruptions, giv- ing the suggestion of fungus bodies. This whitish, sugary material is very common in the avocado, in connection with abnormal conditions of 28 University of California Experiment Station many kinds, so that its presence does not usually give much assistance in recognizing the cause of the injury (fig. 10 x, x) . In early stages these cankers appear to be entirely free from significant fungi, or bac- teria, and inoculations made by placing some of the affected tissue in wounds in healthy avocado plants in the greenhouse have all proved negative (fig. 9c). Such cankers have been rather common in the more succulent varieties such as Anaheim, Panchoy, Itzamna, and Nabal, in the sandy lands of northern San Diego County. Some of these lands are now known to have an excess of water at certain times, and some have a high content of common salt. Associated with stem cankers of this kind, large, black areas with flowing outlines have appeared in green fruits. Some of the more severely affected trees with the twig cankers have developed twig symptoms suggesting water injury or melanorhiza (see p. 32 and fig. 10) . At the times when the soil was examined it appeared to be in excellent condition. The black, rough root condition of melano- rhiza was not found, but a large number of the smallest roots were dead. In some cases the limbs and trunk are very much roughened with ab- normal swellings formed about centers of injury. In figure 11 two can- kers are shown, apparently of this kind, but before the rough, swollen stage has been reached. A sugary material collects about such areas, exuding as liquid, and drying in the air. The fungus dothiorella (see "Dothiorella Rot," p. 60) has been found in cankers, and is abundant in dead twigs (fig. 30a, c) ; but when it is inoculated into healthy twigs no canker has been produced, in the au- thor's experiments 24 (fig. 9c). From the preceding it will be evident that cankers of avocados in California are not fully understood. However, where rapidly spreading cankers are found penetrating to the wood at the crown, these should be treated by excision, as recommended for lemon gummosis. Remove the soil about the crown so as to expose the base of the trunk and upper roots and form a ridge to exclude irrigation water from the trunk. Cut away the brown, diseased bark and a strip of healthy outer bark V2 i ncn wide at the sides and 1 to 2 inches at the top and bottom of the diseased area. Paint the wounds and lower part of the tree with bordeaux paste. Cankers on limbs and trunk (fig. 11) which are not actively spreading might well be cleaned out and disinfected, where practicable. Attention should be given to soil moisture, and to the possible presence of injurious amounts of alkali or salt in the soil or irrigation water. 24 A series of inoculations was made by Clayton O. Smith during the spring and early summer of 1933. A number of cultures of the Dothiorella stage of Botryos- phaeria ribis from different host plants were used and a large number of species of plants, including avocado, were inoculated. Many of the inoculations, including some of those on avocado, gave positive results. Bul. 585] Avocado Diseases in California 29 Dying of Twigs and Branches. — Where the avocado tree is favorably situated it usually makes a dense and very heavy foliage. There is a ten- dency for branches to bend down so that the mass of foliage becomes much crowded. In some cases the trees drop nearly all their leaves at blossoming time, but apparently this is not the case with very vigorous shoots. No careful studies are known to the author on longevity of avo- cado leaves, but apparently leaves on strong shoots may persist for more than one year. It seems probable that many twigs die from shading in competition with other twigs. Some dead twigs evidently have not died from lack of light. Whatever the cause, a thrifty avocado tree is usually found to accumulate much interior dead brush. While the twigs are dying, many puzzling conditions appear. Some- times there is a strong suggestion that canker-producing fungi are active. However, considerable time spent in examining these twigs con- vinces the author that their dying is a normal occurrence, not caused by harmful agents such as fungi, but something in the nature of natural pruning. The dying of twigs usually occurs more rapidly at the time of blossoming and during desiccating winds. Dothiorella, anthracnose, alter naria, and many other fungi quickly become established in the dead twigs and form their spores abundantly for infection of the fruit or for invasion of other weakened .tissues. (See "Dothiorella Rot," p. 60.) Asphyxiation. — Asphyxiation affects leaves and small twigs, and when severe, the whole plant, so that it might seem proper to discuss it at this point; but since it is most probably caused by conditions in the soil, the discussion is given with the root diseases. (See p. 35.) Witches' Broom and Unusual Structures. — Abnormal growths are found occasionally in avocado trees (fig. 12). The infrequent occur- rence suggests that these are not important, but represent deviations from normal development. Their cause is not understood. The witches' broom (fig. 12i) occurred on one of the older Fuerte trees at the Citrus Experiment Station. The broom consisted of twigs somewhat thicker than normal with small leaves and making up a rather dense mass of foliage about 2 feet broad by 4 feet long. This tree has a small amount of sun-blotch and it appears that a twig, abnormal and constricted by sun-blotch, may have developed in this way owing to dis- turbance of nutrition in the terminal part. Scions from the abnormal growth failed, and the whole broom was lost through breaking of the stem on which it was formed. There is reason to believe that these witches' brooms are not caused by parasites but are of the nature of bud sports, and need no treatment beyond cutting out. The abnormal growths (fig. 12e, /) may be of different character and 30 University of California Experiment Station Fig. 12. — Abnormal or peculiar structures of the avocado: a, fasciation of stems of the Egas variety; fc, fasciation on the Carchi variety; c, seedless fruit ("cuke," "unfertilized fruit," "pickle") which has developed peculiar pointed talon-like outgrowths ; d, Fuerte fruit which has been transformed into a large misshapen and woody body ; e, witches' broom of avocado suggesting tubers of artichokes ; /, avocado shoot intermediate in character between the forms shown at e and i; g, avocado seedling showing normal fleshy white scales on the lower part of the stem, indicated by the arrow, and older roots with white rounded outgrowths ; h, four seedlings having all or portions of certain leaves composed of thick white tissue; i, parts of an extensively branched witches' broom on a Fuerte tree ; x, tip of the stem shown at the left of this group of four ; xx, stems showing abrupt changes in diameter. Bul. 585] Avocado Diseases in California 31 due to different causes. The peculiar irregular leaf growth 25 (fig. 12h) appears to be a kind of tissue usually confined to the scales on the earli- est formed part of the stem but occasionally formed on later leaves also. The structures indicated by the arrow (fig. 12g) are normal and their correspondence with the irregular white structures (fig. 12h) has not been proved. Certain abnormalities are shown in figure 12c and d. Their occurrence is very rare. The white, rounded, callus-like outgrowths (fig. 12