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
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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.