• TABLE OF CONTENTS
HIDE
 Title Page
 Credits
 Table of Contents
 Introduction
 Acknowledgement
 General considerations in the prevention...
 Importance of disease control
 Spraying
 Fungicides and their preparati...
 Diseases caused by parasitic fungi...
 Diseases causing fruit decay
 Decay of citrus fruit in transit...
 Diseases and injuries due to non-parasitic...
 Injuries due to meteorological...
 Diseases caused by algae and lichens...
 Spray program
 Index














Group Title: Bulletin - University of Florida. Agricultural Experiment Stations ; no. 229
Title: Diseases of citrus in Florida
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00027352/00001
 Material Information
Title: Diseases of citrus in Florida
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 213 p. : ill. ; 23 cm.
Language: English
Creator: Rhoads, Arthur S ( Arthur Stevens ), b. 1893
DeBusk, E. F
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1931
 Subjects
Subject: Citrus -- Diseases and pests -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: by Arthur S. Rhoads and E.F. Debusk.
General Note: Cover title.
General Note: Includes index.
Funding: Bulletin (University of Florida. Agricultural Experiment Station)
 Record Information
Bibliographic ID: UF00027352
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 002459070
oclc - 13127689
notis - AMG4423

Table of Contents
    Title Page
        Page 1
    Credits
        Page 2
    Table of Contents
        Page 3
        Page 4
    Introduction
        Page 5
    Acknowledgement
        Page 6
    General considerations in the prevention of citrus diseases
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
    Importance of disease control
        Page 13
        Page 14
    Spraying
        Page 15
    Fungicides and their preparation
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
    Diseases caused by parasitic fungi and bacteria
        Page 23
        Page 24
        Page 25
        Page 26
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        Page 112
    Diseases causing fruit decay
        Page 113
        Page 114
        Page 115
        Page 116
        Page 117
        Page 118
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    Decay of citrus fruit in transit and on the market
        Page 131
        Page 132
        Page 133
        Page 134
        Page 135
        Page 136
        Page 137
    Diseases and injuries due to non-parasitic causes
        Page 138
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    Injuries due to meteorological causes
        Page 177
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    Diseases caused by algae and lichens and parasitic and epiphytic flowering plants
        Page 200
        Page 201
        Page 202
        Page 203
        Page 204
        Page 205
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    Spray program
        Page 207
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        Page 210
        Page 211
    Index
        Page 212
        Page 213
Full Text
ROLLIN CUULLLtit LIbKANY
WINTER PARK, FLORIDA


Bulletin 229
r,


UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
Wilmon Newell, Director


DISEASES OF CITRUS

IN FLORIDA

By
ARTHUR S. RHOADS AND E. F. DEBUSK



_^^^ -A5ft 1


Bulletins will be sent free upon application to the
Agricultural Experiment Station,
L GAINESVILLE, FLORIDA.


June, 1931








BOARD OF CONTROL
P. K. YONGE, Chairman, Pensacola RAYMER F. MAGUIRE, Orlando
A. H. BLENDING, Bartow FRANK J. WIDEMAN, West Palm Beach
W. B. DAVIS, Perry J. T. DIAMOND, Secretary, Tallahassee

STATION EXECUTIVE STAFF
JOHN J. TIGERT, M.A., LL.D., President PR. M. FULGHUM, B.S.A., Asst. Editor
WILMON NEWELL, D.Sc., Director IDA KEELING CRESAP, Librarian
H. HAROLD HUME, M.S., Asst. Dir., Re- RUBY NEWHALL, Secretary
search K. H. GRAHAM, Business Manager
S. T. FLEMING, A.B., Asst. Dir., Admin. RACHEL McQUARRIE, Accountant
J. FRANCIS COOPER, M.S.A., Editor

MAIN STATION-DEPARTMENTS AND INVESTIGATORS


AGRONOMY
W. E. STOKES, M.S., Agronomist
W. A. LEUKEL, Ph.D., Associate
G. E. RITCHEY, M.S.A., Assistant*
FRED H. HULL, M.S., Assistant
J. D. WARNER, M.S.. Assistant
JOHN P. CAMP, M.S.A., Assistant
ANIMAL HUSBANDRY
A. L. SHEALY, D.V.M., Veterinarian in
Charge
E. F. THOMAS, D.V.M., Asat. Veterinarian
R. B. BECKER, Ph.D., Associate in Dairy
Husbandry.
W. M. NEAL, Ph.D., Assistant in Animal
Nutrition
C. R. DAWSON, B.S.A., Assistant Dairy
Investigations
CHEMISTRY
R. W. RUPRECHT, Ph.D., Chemist
R. M. BARNETTE, Ph.D., Associate
C. E. BELL, M.S., Assistant
J. M. COLEMAN, B.S., Assistant
H. W. WINSOR, B.S.A., Assistant
H. W. JONES, B.S., Assistant
COTTON INVESTIGATIONS
E. F. GROSSMAN, M.A., Assistant
P. W. CALHOUN, B.S., Assistant


ECONOMICS, AGRICULTURAL
C. V. NOBLE, Ph.D., Agricultural Economist
BRUCE McKINLEY. A.B., B.S.A., Associate
M. A. BROOKER, Ph.D., Assistant
ECONOMICS, HOME
OUIDA DAVIS ABBOTT, Ph.D., Head
L. W. GADDUM, Ph.D., Biochemist
C. F. AHMANN, Ph.D., Physiologist
ENTOMOLOGY
J. R. WATSON, A. M., Entomologist
A. N. TISSOT, M.S., Assistant
H. E. BRATLEY, M.S.A., Assistant
L. W. ZIEGLER, B.S., Assistant
HORTICULTURE
A. F. CAMP, Ph.D., Horticulturist
HAROLD MOWRY, B.S.A., Associate
M. R. ENSIGN, M.S., Assistant
A. L. STAHL, Ph.D., Assistant
G. H. BLACKMON, M.S.A., Pecan Culturist
C. B. VAN CLEEF, M.S.A., Greenhouse
Foreman
PLANT PATHOLOGY
W. B. TISDALE, Ph.D., Plant Pathologist
G. F. WEBER, Ph.D., Associate
A. H. EDDINS, Ph.D., Assistant
K. W. LOUCKS. M.S.. Assistant
ERDMAN WEST, M.S., Mycologist


BRANCH STATION AND FIELD WORKERS
L. O. GRATZ, Ph.D., Asso. Plant Pathologist in charge, Tobacco Exp. Sta. (Quincy)
R. R. KINCAID, M.S., Assistant Plant Pathologist (Quincy)
W. A. CARVER, Ph.D., Assistant, Cotton Investigations (Quincy)
RAYMOND M. CROWN, B.S.A., Field Asst., Cotton Investigations (Quincy)
JESSE REEVES, Farm Superintendent, Tobacco Experiment Station (Quincy)
J. H. JEFFERIES, Superintendent, Citrus Experiment Station (Lake Alfred)
GEO. D. RUEHLE, Ph.D., Assistant Plant Pathologist (Lake Alfred)
W. A. KUNTZ, A.M., Assistant Plant Pathologist (Lake Alfred)
B. R. FUDGE, Ph.D., Assistant Chemist (Lake Alfred)
W. L. THOMPSON, B.S., Assistant Entomologist (Lake Alfred)
R. V. ALLISON, Ph.D., Soils Specialist in charge Everglades Experiment Sta. (Belle Glade)
R. W. KIDDER, B.S., Foreman, Everglades Experiment Station (Belle Glade)
R. N. LOBDELL, M.S., Assistant Entomologist (Belle Glade)
F. D. STEVENS, B.S., Sugarcane Agronomist (Belle Glade)
H. H. WEDGEWORTH, M.S., Associate Plant Pathologist (Belle Glade)
B. A. BOURNE, M.S., Associate Plant Physiologist (Belle Glade)
J. R. NELLER, Ph.D., Associate Biochemist (Belle Glade)
A. DAANE, Ph.D., Associate Agronomist (Belle Glade)
FRED YOUNT, Office Assistant (Belle Glade)
M. R. BEDSOLE, M.S.A., Assistant Chemist (Belle Glade)
A. N. BROOKS, Ph.D., Associate Plant Pathologist (Plant City)
R. E. NOLEN, M.S.A., Field Assistant in Plant Pathology (Plant City)
A. S. RHOADS, Ph.D., Associate Plant Pathologist (Cocoa)
C. M. TUCKER, Ph.D., Associate Plant Pathologist (Hastings)
H. S. WOLFE, Ph.D., Associate Horticulturist (Homestead)
L. R. TOY, B.S.A., Assistant Horticulturist (Homestead)
STACY O. HAWKINS, M.A., Field Assistant in Plant Pathology (Homestead)
D. G. A. KELBERT, Field Assistant in Plant Pathology (Bradenton)
FRED W. WALKER, Assistant Entomologist (Monticello)
D. A. SANDERS, D.V.M., Associate Veterinarian (West Palm Beach)
M. N. WALKER, Ph.D., Associate Plant Pathologist (Leesburg)
W. B. SHIPPY, Ph.D., Assistant Plant Pathologist (Leesburg)
C. C. GOFF, M.S., Assistant Entomologist (Leesburg)
J. W. WILSON, Ph.D., Assistant Entomologist (Pierson)
*In cooperation with U. S. Department of Agriculture.







CONTENTS
PAGE
INTRODUCTION ...................... ---... -- ---- -- ----- 5
ACKNOWLEDGMENTS .....--- -------..... .. ...... --..-... ....-..... ---- -----....-- --------- 6
GENERAL CONSIDERATIONS IN THE PREVENTION OF CITRUS DISEASES....... 7
Grove location ---------...--......-.-. --------.---------- -- --------...........- 7
General care ......-....... .....-- .--- ..-- .--...8.------ ............. 8
Cultivation --.......----..--- ...... ......-----------............. 10
Importance of organic matter in the soil-..........-.....--........ ----... 11
Fertilization .....-- ....--..--....- ..-- ..- .......--- ----.. ..--.......-- 12
Soil m oisture ......... ......-- ...--....... -.....-....... 13
Importance of disease control ....................... ....................... 14
Spraying ...........................-.....-----..---- ..- ..--.. ..--- -- --- -..... 15
FUNGICIDES AND THEIR PREPARATION............. ..... ........................ 16
Bordeaux mixture and bordeaux-oil emulsion................................. 17
Preparing the stock solutions ...............------- -- -----............ 18
M making the m ixture ................. .........- ..... .... ............... 18
Practical field methods for large spraying operations.................. 20
Lime-sulphur solution ....-------........... ---...................-...... 21
DISEASES CAUSED BY PARASITIC FUNGI AND BACTERIA................................ 23
Diseases attacking trees, parts of trees or fruit but not causing
fruit decay ..... .............................. ............ ... 23
Melanose .....- ----...........------- ................. 23
Scab ----------.----..............---------......- 33
C anker ......... .... .. ..... .. ........................... .... ............. 40
Foot rot ......-........... ----..........--....-............ 46
Clitocybe mushroom root rot .................................. ........ .......... 56
Diplodia collar rot and other basal bark diseases............................ 65
Gummosis ..--........-...... ...---.----------------- 66
Psorosis ................... ................. ..... ... .... .... 74
Scaly bark or nail-head rust ................................ ... 84
Withertip and anthracnose ............. -------------.... .. ... ---- 91
Diplodia twig and branch dieback. ................. -----............... 101
Sphaeropsis knot .............. ...................... 104
Damping-off ....------..........--...- .---......... 107
Sooty mold ........--.................. -.............. 108
Felty fungus .---...............-----------------------. 110
Flyspeck and sooty blotch ...........------...... -- .. --------- 110
Thread blight ...-..............------....... ....... 112
Diseases causing fruit decay ....----......------................ 113
Blue and green mold decays .................................... 113
Stem-end rot ......--..-_- .. -. .......... .. ..... 118
Black rot ......... ------.......... ------- ------------... 126
Stylar-end rot of Tahiti limes .......... ---------------................ 128
Charcoal rot of limes ........................................ ----------- ... 130
Aspergillus rot .............. .... .---------. ..... .. ..... .....----- ..---- 130
Decay of citrus fruit in transit and on the market ................... 131
DISEASES AND INJURIES DUE TO NON-PARASITIC CAUSES........................... 138
Diseases due to physiological or unknown causes ................................ 138
Exanthema or dieback ................. ....-------. ..... 138
Mottle-leaf or frenching ------...--......~................. 150
Fruit-splitting -.................. ...... -...... ... 158






PAGE
Internal drying of fruit ............-.....................-- ------- 161
Creasing ..................................----------------- 163
Lumpy rind .............. -- .-.......................--- 164
Green spot ...... ---.~.....---..-. ------------ -----------. 166
Grapefruit blotch ......................... .... -- --- ..----- --.---- 168
Storage spot or "pox" ...........-..------- ......--... -- --------- 170
Greasy spot ......- ....... ............... -------------- --- 172
Yellow spot ....-..---.........-...-...-.. ---...... ----- -- 17-1
Star melanose ............. ...... ..... ... ---- ..- 176
Miscellaneous leaf-spots --............-.......... ------------ -- 177
Injuries due to meteorological causes .........-....--...-------- ---- 177
Cold injury ................................------------- 177
Sunscald ............................. ----------------------- 187
W ind injury .....................................------- -- ----- ----- 191
Lightning injury .......... ...... ------- ----... 195
Hail injury .........-....-...................--------------------------- 197
DISEASES CAUSED BY ALGAE, LICHENS, AND PARASITIC AND EPIPHYTIC
FLOWERING PLANTS ............................------------- ------- 200
A lgae .............. .............. .. 200
Lichens ............... --.. --.. --. .. .. .....-------- ------------- 201
Dodder ....--.. ..------....................... -- ----------- -- .- 203
Cassytha .........................-......--- ....-----. ------ 205
Spanish moss and other air plants ....--....--.---.------------- 205
SPRAY PROGRAM ..........-...-.....-- .-..-- -.......--..... ... ...---- 207








DISEASES OF CITRUS IN FLORIDA

By ARTHUR S. RHOADS AND E. F. DEBUSK'

INTRODUCTION

A strong demand exists for a publication covering the gen-
eral field of citrus diseases occurring in Florida. While this
demand was fulfilled to a large extent by Bulletin 150 of this
Station, published in 1918, this publication is now out of print
and, as might be expected after the lapse of 13 years, largely
out of date. Since the appearance of that bulletin, many opinions
have changed and much new information has been published in
various periodicals and books, most of which are not available
to the average citrus grower. However, regardless of how much
information may be scattered through the literature, this in-
formation is of practical value to the citrus grower only when
brought together and a summary of the authentic informa-
tion on each disease presented in clear and concise form.
The present bulletin has been designed to present to growers,
in logical sequence and standardized form, information concern-
ing the distribution and economic importance, appearance, cause
and control, so far as known, of all Florida citrus diseases on
which definite information exists, with the exception of drought
injury, blight or chronic wilt, and water injury.* In this bulletin
the term "disease" is considered in its broadest sense and in-
cludes the non-parasitic troubles and injuries from various
causes, except where known to be due to insects. This has
been considered advisable, since the lesions or effects produced
not only affect the health of the trees but are frequently mis-
taken for those due to parasitic or other diseases or because
they may serve as contributory factors for infection and de-
velopment of more or less parasitic organisms.
The information given is presented with the view of aiding
the grower to better identify citrus diseases, to realize more
fully their importance, and to employ the best known methods
'Associate plant pathologist, Florida Experiment Station, and citrus
pathologist, Florida Agricultural Extension Service, respectively.
*In view of the fact that it is planned to treat these troubles that are
contingent upon deficits or excesses of soil moisture, or combinations of
the two, in considerable detail in a special publication, they have been
omitted purposely from this bulletin.






Florida Agricultural Experiment Station


for their control. The more important diseases have been dis-
cussed and illustrated in greater detail than the minor ones,
although virtually all the diseases and injuries discussed have
been illustrated, that the grower may be able to identify them
readily. Technical expressions have been avoided as far as
possible.
In the preparation of such a bulletin it is only possible to base
the discussion of the various diseases on the sum total of our
knowledge to date. It is expected that many of our present
views, especially those with respect to the physiological troubles
and our cultural and fertilizer practices, will be modified as ad-
ditional research work advances our knowledge. Unfortunately,
many diseases must be dismissed with but a brief general de-
scription and some must be omitted entirely for lack of definite
information covering them.
Our knowledge of citrus diseases in general is decidedly less
well advanced than our knowledge of the diseases of deciduous
fruit trees. This is, in part at least, due to the fact that pro-
portionately less research work has been devoted to citrus dis-
eases since citrus-growing in the United States is confined to
but a very few favored sections and many of the problems are
peculiar to Florida. However, it is also largely due to the oc-
currence of a number of complex physiological or nutritional
troubles induced by certain soil conditions and cultural and fer-
tilizer practices and to our general lack of knowledge of the
physiology and nutrition of citrus trees, fertilizer requirements,
effect of different rootstocks, soil conditions under which the
different kinds of trees thrive best, and water requirements.
Many of these problems must be worked out individually for
Florida, and only when they have been solved are we likely to
arrive at a better understanding of the exact nature, cause and
control of a number of citrus diseases.
ACKNOWLEDGMENTS
In the preparation of this bulletin the writers have drawn
upon all available sources of information and have made free
use of the publications of this and other experiment stations, the
U. S. Department of Agriculture, and Fawcett and Lee's book,
"Citrus Diseases and Their Control". Much of the information on
certain diseases treated in this bulletin, however, is the result
of personal investigations and observations, some of which have






Bulletin 229, Diseases of Citrus in Florida


been published previously in various periodicals, while others
are published here for the first time.
The majority of the illustrations are published for the first
time, most of these having been made by the senior author.
The others, except where special acknowledgment has been made,
have been taken from former publications of this Station and
the files of the department of plant pathology. The illustrations
for figures 82 and 84 have been reproduced from Hume's book,
"The Cultivation of Citrus Fruits," by permission of The Mac-
millan Company.
The authors desire to express their appreciation to Dr. H. S.
Fawcett, of the California Citrus Experiment Station, who read
the preliminary draft of the manuscript for this bulletin in 1927
and kindly offered a number of valuable suggestions. They are
also indebted to a number of colleagues of the Florida Agricul-
tural Experiment Station and State Plant Board who have also
read various drafts of the manuscript and offered helpful sug-
gestions and advice.

GENERAL CONSIDERATIONS IN THE PREVENTION
OF CITRUS DISEASES
The successful control of citrus diseases often depends on
preventive and precautionary measures taken far in advance of
the appearance of the trouble. Before the land is plowed or the
trees are planted, certain measures often can be taken to avoid
troubles in the future. Precautionary measures frequently can
be included in selecting the grove site and rootstock, in providing
drainage or irrigation, and in the general grove practices,
which will greatly reduce the chances for disease invasion. The
more important of these subjects which have some bearing,
either directly or indirectly, upon the probable occurrence of
diseases in the grove have been briefly considered in the follow-
ing subdivisions of this section.

GROVE LOCATION
In selecting the site for a grove, special consideration should
be given to the soil type, subsoil, need for drainage, susceptibil-
ity to drought, and protection from storms and cold, since all of
these factors may have considerable bearing on the disease situ-
ation. Although groves most favorably located will generally be
troubled more or less with certain diseases, those in less favor-






Florida Agricultural Experiment Station


able situations will be further handicapped by the effects of
the adverse factors of site that may stimulate greater disease
development. The growth and development of the citrus tree is
largely influenced by the conditions of its environment. These
may be such as to promote a good to average development of the
trees or they may be such as to produce weak and non-produc-
tive trees or trees that are subject to various injuries and set-
backs from time to time. However, it is not intended to give
the impression that only weakened or injured trees are sub-
ject to attack by diseases, since there are a number of diseases,
such as melanose, scab and canker, that are capable of attacking
the most vigorous and healthy trees. On the other hand, there
are other diseases, such as withertip, that attack only weakened
or injured trees. These diseases are often unable to attack the
vigorously growing tissues but, if a part becomes weakened or
injured, they may greatly expedite the decline of the tree. A
healthy, vigorous tree will have a much greater resistance to
many diseases than one of low vitality, especially when subject
to devitalizing influences periodically.
Among the factors of site that affect the general health and
productivity of the grove may be mentioned soils poorly adapted
to the growth of citrus trees, especially when closely underlaid
by rock, hardpan, or marl, and situations where the trees are
subject to extreme fluctuations of soil moisture or injury from
cold or high winds. In selecting land for the grove, it is highly
important that these factors be considered with regard
to the probable effect they may have on the future welfare of the
grove. Fortunately, some of the adverse factors of site may
be modified to decided advantage even after the grove is es-
tablished. The introduction of drainage, irrigation, frost pro-
tection, wind-breaks, cover-cropping, and changed methods of
cultivation and fertilization, where such are desirable, frequently
will contribute greatly to the general improvement of the grove
condition. It is often difficult and costly, however, to overcome
the bad effects of unfavorable environmental conditions after
the grove has been established.
GENERAL CARE
Widely differing methods are employed in cultural practices
and fertilization, each often giving successful results. This is
not strange, however, when we consider the different soil types,





Bulletin 229, Diseases of Citrus in Florida


soil moisture conditions, locations, rootstocks, and kinds of citrus
trees that come under cultivation. In regard to tillage and the
use of fertilizers, a grove in one locality or on one kind of root-
stock may require a method of treatment quite different from a
grove in some other locality or on a different rootstock. The
method of handling any grove must be determined largely by
the rootstock and local conditions which influence its develop-
ment.
It should be kept in mind that a citrus tree requires a certain
amount of attention regularly and that a generous amount of
suitable plant food is necessary for good growth and productiv-
ity. Applications of fertilizer should be made at timely inter-
vals and should be in proportion to the size and condition of the
trees and the crops they have borne or are capable of bearing.
Applications of fertilizer cannot be withheld any great length
of time in groves on the average sandy soil without causing the
trees to become greatly weakened and die back rapidly. This
is especially true if the trees have been previously stimulated
to vigorous growth and fruit production.-
The care of the grove, however, is not limited to a few cultural
operations each season or to the application of a certain amount
of fertilizer. Other factors of equal importance are the control
of diseases and insects which may attack and lower the grade
of the fruit or cause it to rot before it reaches the market. The
diseases of citrus trees that are caused by fungi and bacteria
are influenced to a large extent in their spread and development
by moisture and temperature. As a rule, they require warm,
moist conditions for their rapid development. Thus, groves lo-
cated in low, shady situations, especially where the trees are
closely crowded, will afford very favorable conditions for the de-
velopment and spread of a number of diseases. In such cases
the disease situation frequently can be greatly improved merely
by thinning out the trees to let in the sunlight and give better
aeration. However, by no means all of the tree and fruit troubles
are caused by fungi or bacteria, insect pests, or meteorological
conditions. Certain factors of site, including moisture-holding
capacity and soil reaction, and cultivation, fertilization, and other
grove practices are often responsible for the development of a
number of complex nutritional disorders of citrus trees, for
which it is difficult to diagnose the cause and to prescribe a
specific remedy.





Florida Agricultural Experiment Station


Cultivation:-Groves probably should never be cultivated to
a greater depth than three inches, since deep cultivation destroys
a large percentage of the fibrous and smaller woody roots, which
all too commonly deranges the growth of the trees and induces
the development of mottle-leaf (frenching), exanthema (die-
back), dropping of fruit, and other troubles. A generation or
so ago it was considered good grove practice by many in Florida
to plow deeply regardless of how many of even the larger woody
roots were torn up. Fortunately, this detrimental practice has
been largely abandoned of late years. Even today, however, we
find an appallingly large amount of excessively deep and un-
necessary cultivation practiced.
The formerly widespread practice of excessive clean cultivation
has been found by experience to be a bad practice, resulting in the
depletion of the generally scanty supply of organic matter in the
soil. Cultivation and soil building must be regarded as antag-
onistic factors in Florida citrus groves. Under Florida con-
ditions, the chief functions of cultivation are to destroy weed
competition for moisture and fertilizer and to cut up and work in
cover crops to reduce the fire hazard. It has been repeatedly
claimed that intensive and frequent cultivation is necessary, es-
pecially on the light, sandy soils of the ridge section, to main-
tain a loose soil mulch and thereby conserve moisture by break-
ing up capillary action. Recent experimental work, however,
tends to minimize the value of this time-honored practice. It
now seems to be pretty well established that cultivation does
not conserve appreciable amounts of moisture in the soil, except
to the extent to which it kills weeds which would rapidly with-
draw moisture from the soil. At any rate, moisture losses by
evaporation from grove soils are confined to the upper few inches
whether the soils are cultivated or not, while moisture losses
through use by trees extends to a depth of a few feet. These
findings are particularly significant to Florida growers since the
majority of our grove soils have a limited capillary action and
by reason of their loose texture are obviously self-mulching.
Moreover, experience seems to bear out the fact that the more
light, sandy soils are stirred the more readily they dry out. It
therefore appears that the cultivation of our sandy grove soils
in excess of that required to keep down the growth of weeds
during dry seasons and to work in cover crops sufficiently to
eliminate the fire hazard is not only a needless waste of time and






Bulletin 229, Diseases of Citrus in Florida


money but greatly expedites the destruction of the organic mat-
ter in the soil and coarsens the texture of the fruit.
Cultivation is rarely practiced in Florida groves on low ham-
mock land, since it is more often necessary in such situations to
get rid of water than to conserve it. The present recommenda-
tions on most lands except the low hammocks are to cultivate
only sufficiently during the dry season of the year to keep down
the growth of grass and other weeds that may compete with
the tree roots for moisture and to grow leguminous cover crops
during the rainy season. With an irrigation system to take care
of the moisture requirements of the trees, mowing can largely
replace cultivation, even on high land, except for what culti-
vation may be necessary to work in cover crops to reduce the
fire hazard.
Importance of Organic Matter in the Soil:-In Florida, where
there is a preponderance of light sandy soil with relatively little
or virtually no organic matter to begin with, the supply, under
existing conditions, tends to become destroyed rapidly, even
under a rational system of cultivation. The bacterial decompo-
sition of organic matter in such soils is very rapid, owing to their
porous condition, the high temperatures prevailing, and the
heavy rainfall. It is evident, therefore, that one of the greatest
benefits that can come to the Florida citrus industry would be
for growers to realize the necessity of growing green manurial
cover crops, especially legumes, as a means of enriching their
soils and replenishing the supply of organic matter. While com-
mercial fertilizers may put back into a soil certain elements that
the trees and fruit crops remove, unless good cover crops are
grown or vegetable matter is hauled into the groves, growers are
not putting back into the soil the organic matter that is being
destroyed by cultivation, with the result that the problem of
grove maintenance increases each year.
An abundance of organic matter is required for the develop-
ment of the beneficial soil bacteria and the maintenance of bio-
logical activity in the soil. The presence of organic matter is
likewise beneficial in that plant food is made soluble by the ac-
tion of the organic materials on the soil constituents. Continu-
ous clean cultivation, however, tends to sterilize the soil and
the too often scanty supply of organic matter originally present
becomes burned out gradually by oxidation, with the result that
the soil is ultimately changed into a lifeless mineral mass, on





Florida Agricultural Experiment Station


which trees fail to thrive and even cover crops cannot be grown.
Growers must realize that they cannot maintain groves in good
condition indefinitely by reliance on commercial fertilizers alone
when there is a deficiency of organic matter in the soil. Groves
kept clean cultivated continuously and merely supplied with com-
mercial fertilizers are almost certain to fail gradually and to de-
velop a number of nutritional troubles of obscure cause. When
there is at least a moderate amount of organic matter in the soil,
however, the citrus tree is not only enabled to utilize commercial
fertilizers to better advantage but benefits in many other ways.
The dominant problem in the production of quality citrus fruit
on our lighter sandy soils is supplying and conserving organic
matter.
Fertilization:-It is well known that the kind, amount, and
method of application of fertilizers exert a considerable influence,
either directly or indirectly, on certain diseases of citrus trees.
Well fed trees are more resistant to cold and to the attacks of
such organisms as the withertip fungus, the Diplodia fungus,
and others. On the other hand, fertilizers, together with the
chemical make-up of the soil, have a very important influence
on the development of mottle-leaf and chlorosis. The ready
tendency for citrus trees to develop exanthema diebackk) or am-
moniation of the fruit under certain often poorly understood
conditions probably is more directly responsible than any other
one thing for the general finicalness of Florida growers in regard
to the composition, proportion, and use of fertilizers. The men-
ace of this disease, through a general misunderstanding and
lack of knowledge of the causes contributing to its development,
has created considerable unjustifiable prejudice to the use of
stable manure and other organic fertilizer materials in past
years.
The question of fertilization is further complicated by the
great diversity of soils, soil moisture conditions, and soil re-
actions. To further increase the complexity of this situation,
repeated attempts are made to grow citrus trees on land utterly
unsuited to them. As a result, the majority of Florida citrus
growers are in a state of constant turmoil and indecision in re-
gard to the proper fertilizer practices. Probably with no other
fruit crop anywhere else in the world is there greater multi-
plicity of opinion relative to fertilization than with citrus in
Florida.





Bulletin 229, Diseases of Citrus in Florida


Since the fertility of the average Florida sandy soil is low,
citrus trees, except under the most favorable conditions, will
not long survive neglect in fertilization. From the disease stand-
point, the aim of fertilization is not only to insure the future
health of the tree but at the same time to produce large crops of
sound fruit that will be resistant to diseases.
Soil Moisture:-Another great drawback to citrus culture on
many soil types in Florida is the tendency for the soil moisture
to fluctuate to either one extreme or the other. A prolonged
dry condition of the soil prevents the trees from making the most
effective use of commercial fertilizers and the heavy rainfall that
occurs at other times results in a heavy leaching of the soluble
fertilizer materials, especially on the lighter sandy soils. Dur-
ing the dry season of the year it is a regular occurrence in many
sections of the state for trees to suffer from lack of moisture at
some time during each year and often the trees may suffer for
months at a time. The frequent, irregular drought periods
in Florida constitute one of the greatest drawbacks to profitable
citrus culture on many soil types.
On the other hand, groves on many soil types in Florida com-
monly suffer from excessively wet soil conditions from time to
time or may suffer first from the soil becoming too wet and
then from its becoming too dry. Extremes of soil moisture may
cause the trees to become weakened to such an extent that they
are readily attacked by disease, if indeed they have not been
killed outright. The annual loss to the citrus industry in Florida
caused by deficits and excesses of soil moisture is truly appall-
ing when fully realized. Recent studies by Rhoads have shown
that the maintenance of soil moisture conditions favorable for
tree growth in many of the groves established in Florida calls
for as much exercise of thought and attention to keep the trees
in a good state of vigor and crop production, and is often of
as much importance as the control of insect pests and the dis-
eases caused by parasitic organisms.

IMPORTANCE OF DISEASE CONTROL
As a rule, very little attention and thought is given, until it
is too late, to the control of either diseases or insects, or to the
prevention of other adverse influences that may have a far-reach-
ing effect on the health and productiveness of a grove. The cone
trol of diseases and insect pests and protection against drought,





Florida Agricultural Experiment Station


excessive moisture, high winds, and cold should be given more
consideration in the general care of the grove and provided for
as any other operation in connection with the production of
citrus fruit. This is especially true of disease control, which is
one of the serious problems that the citrus grower must help to
solve. Diseases are not apt to become less numerous in the grove
or to disappear unless proper control measures are undertaken.
On the contrary, if diseases are allowed to follow their natural
course, they may be expected to become more abundant and
destructive. Certain citrus diseases are already so thoroughly
established in the groves that they are considered permanently
destructive agencies which require control measures at certain
periods each year. Various diseases are responsible each year
for heavy losses in fruit and trees. Such losses are largely pre-
ventable by the use of timely and effective control measures.
As a rule, very little thought or attention is given to a disease
until it is well established in the grove and the damage caused
by it becomes distinctly evident. In the control of any disease
there is generally a period during which preventive or remedial
measures can be applied most effectively and economically. A
delay of a few days or a few weeks may render such measures
ineffective toward preventing injury or checking the disease.
In order to handle intelligently the disease situation in any
citrus grove it is first necessary to understand something of the
nature and causes of diseases, the conditions under which they
develop, the identity and the importance of the particular dis-
eases to be combatted, and the methods most effective for their
control. A similar knowledge should be had of the more common
and destructive insects because the control of the two often goes
hand in hand.
Growers will do well to familiarize themselves with the soil
and soil moisture conditions of their groves and to inspect their
trees periodically for diseases and conditions that need correct-
ing. An annual or semi-annual inspection of the trees is of the
utmost importance in locating cases of bark diseases, such as
foot rot, mushroom root rot, gummosis, and psorosis, so that
they can be treated before the life of the trees becomes threaten-
ed. With a moderate amount of study and observation, growers
may be informed of their most common diseases and the relative
damage caused by each one. They will then be in position to map
out a program for the most effective and economical control of





Bulletin 229, Diseases of Citrus in Florida


these diseases and to make applications of fungicides and the fol-
low-up spray of oil emulsion at the correct time.
The effectiveness of any control measure will depend upon a
proper, intelligent and timely use of the information- given.
Thoroughness is most essential, since very little good will be
accomplished if control measures are applied carelessly or in-
differently. The control of many diseases, such as melanose and
scab for example, must be effected by preventive measures before
the susceptible parts become infected. Even in the case of other
diseases and injuries, however, it will be found easier and less
expensive to prevent their occurrence than to control them after
they develop. This is especially true in starting with a new
grove.
SPRAYING

While spraying operations in the citrus grove are performed
most frequently for the control of insect pests, it is often neces-
sary to spray for the control of certain fungus diseases such as
melanose and scab, which attack the foliage, twigs and fruit.
A fungicide is used for the latter purpose and is applied at in-
tervals to protect any exposed surfaces during the period when
these tissues are susceptible to attack by a disease. The control
of fungous diseases of citrus trees is very intimately tied up with
the control of insect pests, however, and, in many cases, the
two are controlled simultaneously by a combination spray of
bordeaux-oil emulsion, in which the bordeaux mixture is the
fungicide and the oil emulsion the insecticide.
Citrus trees are more difficult to treat successfully with fun-
gicides and insecticides than deciduous fruit trees, since they are
evergreen and retain their foliage throughout and put out new
growth at several times during the year. Moreover, the trees
are frequently allowed to carry their fruit the greater part of
the year. Thus, there is no dormant period when the leaves,
and often the fruit, are absent, which would permit the use of
a strong fungicide or insecticide.
The greatest difficulty involved in the use of fungicides on
citrus trees, however, arises from the indirect effect that they
have on the increase of insect pests. When used at the proper
strength, fungicides kill all fungi with which they come in con-
tact; hence the beneficial (entomogenous) fungi are also killed,
thereby allowing the scales and whiteflies to increase rapidly.





Florida Agricultural Experiment Station


This requires extra spraying with insecticides to control the
insects, thus adding greatly to the necessary labor and expense
of disease control. The situation is further complicated by the
fact that oil emulsions often result in "shadowing" or burning
of the fruit and, if used too freely, exert certain injurious effects
upon the trees and delay the coloring and ripening of the fruit.
In spite of these drawbacks, however, a certain amount of spray-
ing will be necessary for proper disease control and the sooner
the citrus grower realizes this and plans to take care of this
work in a systematic manner the less trouble he will have with
diseases, especially those affecting the fruit and foliage.
The number and frequency of spray applications will vary ac-
cording to the disease, growth conditions of the tree, and cli-
matic conditions. The efficiency of a single spray application may
not extend beyond from 10 to 14 days, even without rainfall,
since, where a rapid growth occurs, considerable new and un-
protected shoots may develop in a few days. To achieve the
best results, the fungicides should be applied often enough to
give complete protection to all exposed surfaces of susceptible
tissues throughout the entire period during which the parts
are liable to infection by the spores of the fungus in question.
The spray should be applied in the form of a fine mist. This
gives a more uniform distribution of the liquid, resulting in a
thin film which covers the surface of the parts to be protected.
If the spray is supplied with a coarse mist it tends to collect in
drops and run off, resulting in a loss of material, an incomplete
covering of the surfaces and the likelihood of burning at the
places where the spray material is concentrated. A good power
sprayer should be provided, preferably one that will maintain
a pressure of from 250 to 300 pounds. The sprayer should be
properly cleaned after use and should be overhauled from time
to time to maintain it in proper working order.

FUNGICIDES AND THEIR PREPARATION
A fungicide is generally used as a preventive and, in order
to be effective, must be applied at the proper time, which is
usually before the disease is visible. A fungicide merely forms
a protective film on the surface of the susceptible parts and it
must be applied before the spores of the disease-producing fungi
have lodged on the surface or, if they have already lodged on
susceptible parts, before they germinate and infect the tissues.





Bulletin 229, Diseases of Citrus in Florida 17

In many cases, spraying after the disease manifests itself is
only a waste of time and material. The fungicide must contain
a poisonous principle of sufficient strength to kill the spores of
the fungus to be combatted and, at the same time, not injure
the tender growth on which it is used.
The preparation and use of fungicides requires care and judg-
ment, as the efficiency of any spray solution will depend upon
proper preparation and the manner and timeliness of its appli-
cation. The discouraging results sometimes obtained from the
use of reliable fungicides are usually due to a carelessness or
lack of timeliness in applying, or to their improper preparation.
It should be remembered that a fungicide is used for a specific
purpose, under certain limited conditions. It is not a "cure-all"
and should not be applied indiscriminately against any and all
diseases that appear in the grove. A better understanding of
fungicides in general, including their preparation, uses, and
limitations will aid materially in obtaining satisfactory results
in the control of diseases in the grove.
The standard fungicides for the control of citrus diseases are
bordeaux mixture and lime-sulphur solution. The former has
proved decidedly more effective than the latter for the control
of most citrus diseases. Other materials are sometimes used
as fungicides, but they are usually employed in special cases
and have not been found satisfactory for general use. The dry,
or dust, fungicides have been found to be less effective than the
liquid ones.
BORDEAUX MIXTURE AND BORDEAUX-OIL EMULSION
Bordeaux mixture is the best known and most widely used
fungicide. It is made from copper sulphate (bluestone), either
fresh quicklime (lump or stone lime) or fresh hydrated lime,
and water in certain definite proportions. Bordeaux mixture made
according to the 3-3-50 formula is generally considered to be
of standard strength for the control of citrus diseases in Florida.
Such a formula indicates that the bordeaux mixture is pre-
pared in the proportion of 3 pounds of copper sulphate and 3
pounds of quicklime to 50 gallons of water. On account of the
fact that insects may increase greatly following the use of bor-
deaux mixture in a grove, it is recommended that 1 percent of
oil in the form of an emulsion be used with it. Hence, we call the
spray bordeaux-oil emulsion and write the formula as 3-3-50-1.





Florida Agricultural Experiment Station


PREPARING THE STOCK SOLUTIONS
For convenience in preparing large quantities of bordeaux
mixture, the bluestone and lime should be made up separately
as stock solutions, each containing one pound of bluestone or
lime respectively to each gallon of water. These separate stock
solutions will keep indefinitely but should be covered tightly to
prevent evaporation which would concentrate the solutions.
Stock Solution A, Bluestone: Dissolve bluestone (copper sul-
phate) at the rate of 1 pound to each gallon of water in a barrel.
An iron vessel should never be used for bluestone solutions or
bordeaux mixture; neither should hoes or other metallic im-
plements be used to stir them. The bluestone may be dissolved
readily by placing it in a clean feed or fertilizer sack and drawing
it about in the water. The granulated or pulverized forms dissolve
much more readily than the coarse crystalline form. ALWAYS
STIR THIS STOCK SOLUTION BEFORE TAKING ANY OUT.
Stock Solution B, Lime: In another vessel, slake an amount
of quicklime equal to the number of pounds of bluestone used
and dilute with water to an equivalent number of gallons. When
slaking the lime, great care should be taken to prevent either
drowning, which results from adding so much water that the
heat of the slaking process is quenched, or burning, which re-
sults from adding so little water that the lime gets too hot.
Owing to the difficulty of getting quicklime and keeping it in
good condition in Florida, fresh hydrated lime may be used, in
which case one-third more lime will be required in a given
amount of stock solution in order to compensate for the water
added during the process of hydration. Quicklime that has be-
come air-slaked, or carbonated, should not be used for making
bordeaux mixture. For the same reason, hydrated lime should
be used only when it is known to be fresh and of high quality.
In case the ordinary plasterers' or finishing grade of hydrated
lime is used, it is preferable to allow it to soak in water over
night before using, in order to secure a bordeaux mixture of
good suspension. ALWAYS STIR THIS STOCK SOLUTION
BEFORE TAKING ANY OUT.
MAKING THE MIXTURE
One of the great drawbacks to the use of bordeaux mixture
lies in the faulty and haphazard methods commonly employed
in its preparation. Unfortunately, bordeaux mixture is not fool-






Bulletin 229, Diseases of Citrus in Florida


proof and the writers are convinced that many of the abuses
heaped upon bordeaux mixture as a fungicide are due to im-
proper preparation and careless application.
In making up small lots of bordeaux mixture, observe the fol-
lowing directions: Dilute the required amount of stock bluestone
solution in a vessel which contains water to half the amount of
spray solution to be made. Dilute the required amount of stock
milk of lime in another vessel which contains water to half the
amount of spray solution to be made. Then pour the contents of
the two containers simultaneously into a third container or the
spray tank, stirring the combined mixture as the two solutions
are poured together. For example, for 50 gallons of 3-3-50 bor-
deaux mixture, use 3 gallons of stock solution A, diluted to 25
gallons, and 3 gallons of stock solution B, diluted to 25 gallons.
For 200 gallons of 3-3-50 bordeaux mixture, use 12 gallons of
stock solution A and 12 gallons of stock solution B. Be sure to
strain all solutions poured into the spray tank through a fine
mesh screen to remove the coarse particles of sediment that may
cause the nozzles to clog. If bordeaux-oil emulsion is desired,
the required amount of oil emulsion may be washed through the
screen before the tank becomes full, using 31/3 quarts to each 50
gallons of bordeaux mixture, or 31/3 gallons in a 200 gallon tank.
The wetting or "spreading" power of bordeaux mixture can be
increased considerably by the addition of a casein spreader at
the rate of 1 pound to 100 gallons of the spray mixture, although
the use of a spreader is not so essential when oil emulsion is com-
bined with the bordeaux mixture. Casein spreader, which is
marketed under a number of trade names, is made from casein,
a milk product, and hydrated lime. The required amount of
spreader should be put into a bucket half full of water and
beaten vigorously with a wire whip and then poured through the
screen into the spray tank containing the bordeaux mixture
while the agitator is running.
TESTING THE MIXTURE
When bordeaux mixture is properly prepared it is of a bril-
liant sky-blue color. If insufficient lime has been used to neutral-
ize the copper sulphate or the lime is air-slaked (carbonated) or
otherwise inferior in quality, resulting in a bad mixture, the
preparation will have a greenish cast and, if this is very pro-
nounced, the mixture is liable to burn tender tissues.





Florida Agricultural Experiment Station


Since mixtures containing free copper sulphate are liable to
cause burning, it is advisable to test them to determine whether
or not this copper salt has been completely neutralized by the
lime. One method of testing in the field is to place a new nail,
bright knife blade (not a stainless steel one), or other bright
piece of steel in the bordeaux mixture for a few minutes. If no
copper is deposited and the metal remains bright the test in-
dicates that the mixture is either neutral or alkaline and, there-
fore, safe to use.
Another, and more reliable, test can be performed with a so-
lution of yellow prussiate of potash (potassium ferrocyanide).
A small bottle of a 10 percent solution of this poisonous salt can
be secured from a druggist. After stirring the bordeaux mixture
thoroughly, take out a sample in a clean glass or white cup and
add a drop of the testing solution. If the copper sulphate has
been neutralized, the test solution will remain a clear yellow
until it blends with the mixture. If free copper be present, how-
ever, the drop of the test solution will immediately turn to a
reddish-brown color when it strikes the mixture. The latter
reaction indicates that more milk of lime should be added to the
bordeaux mixture until the test solution fails to produce the
reddish-brown color.
Bordeaux mixture should be used shortly after it is prepared,
since it deteriorates rapidly during warm weather. If delays
are unavoidable, the good qualities of the spray mixture may be
preserved temporarily by adding 1/ pound of sugar to each 100
gallons of the bordeaux mixture and agitating thoroughly.
PRACTICAL FIELD METHODS OF PREPARING BORDEAUX
MIXTURE FOR LARGE SPRAYING OPERATIONS
For commercial spraying with modern power spraying outfits,
the following method of preparing the bordeaux mixture has been
found the simplest in field practice, since it eliminates the need
of dilution barrels. Start filling the tank with water and pour
in the required amount of stock bluestone solution and start the
agitator. When the tank is about three-fourths full, stir up
the stock milk of lime and pour the required amount slowly
through the tank screen. Shortly before the tank becomes full,
pour in the required amount of oil emulsion and, when the tank
is filled, the operator is ready to start spraying.
A new method of preparing bordeaux mixture instantaneously






Bulletin 229, Diseases of Citrus in Florida


by the use of chemical hydrated lime and powdered bluestone
has recently come into prominence among fruit growers. The
chemical hydrated lime is an exceedingly fine hydrate, so called
because it is used largely in the chemical industries. The use of
this kind of hydrated lime gives a much better bordeaux mixture
and eliminates the difficulties of securing and slaking quick-
lime and the necessity for setting up a series of stock and di-
lution barrels and other cumbersome equipment. The required
amount of ingredients can be weighed out and poured into the
tank screen and washed into the tank by the inflowing water
according to the method just described for the use of stock so-
lutions. The use of chemical hydrated lime, which is an ex-
ceedingly finely divided product free from grit and containing a
much higher percent of calcium hydroxide than the ordinary
hydrated lime of the building trade, will undoubtedly supplant
the use of quicklime and ordinary hydrated lime in the commer-
cial preparation of bordeaux mixture.
LIME-SULPHUR SOLUTION
Lime-sulphur solution has the properties of both a fungicide
and an insecticide when used at certain strengths. Lime-sul-
phur is used in the citrus grove chiefly to control rust mites,
red spiders, and scab. Concentrated solutions are obtainable
from two sources and are usually distinguished as commercial
lime-sulphur and home-made lime-sulphur. Within late years,
lime-sulphur has been put on the market in dry form to elim-
inate the freight on the water.
Commercial liquid lime-sulphur is a concentrated product
which is more expensive than the home-made product but keeps
better and eliminates the trouble of making and storing the
home-made solution. It is also more convenient to handle and
varies less in its strength. These concentrated solutions are
generally used at dilutions of 1 part to from 25 to 40 parts of
water for scab control and of 1 part to from 50 to 65 parts of
water for rust mite control. For scale control on trifoliate
orange and other trees that shed their leaves and become com-
pletely dormant, a dilution of 1 part to 8 of water is customary.
These dilutions are usually made on the basis of a concentration
of 320 Baum6 as shown by the hydrometer reading. The com-
mercial brands of lime-sulphur solution will vary in concentra-
tion or density from 31 to 34 degrees Baum6 and the concentra-







TABLE I.-LIME-SULPHUR DILUTION CHART, SHOWING THE AMOUNT OF CONCENTRATED LIME-SULPHUR OF DIFFERENT DENSI-
TIES WHICH MUST BE USED TO MAKE 50 GALLONS OF SPRAY MIXTURE OF THE CONCENTRATIONS INDICATED.


Hydrometer
Reading


BaumB

36

35

34

33

32

31

30

29

28

27

26

25

24

23

22

21

20


ISp. Gr.I

1.330

1.318

1.306

1.295

1.283

1.272

1.261

1.250

1.239

1.229

1.218

1.208

1.198

1.188

1.179

1.169

1.160


1 to 8 1 to 15 1 to 20 1 to 25


Quarts

20

20%

211/

22

23

24

251/

27

28

29%

311/

33

35

37

40

44

48


Quarts

11

11

12

12

13

14

14

15

16

17

18

19

20

211/

221/

24

24%


Quarts | Quarts


Strength of Spray Solution Desired


1 to 30 11 to 35
Quarts I Quarts

6 5


1 to 40 1 to 45
Quarts I Quarts

4% 4

4% 4

5 41/2

5 4%

5 4/2

5% 5


5

5%

5%2

6

6%

7

7%
8

8%

9

9%


5%2
6

6%

7

7%

7

8

8%

9

10

10%


1 to 50
Quarts

3%

3%

4

4

4

4%

4%
5

5

5%

5%

6

6

62

7

7

8


1 to 55 1 to 60 11 to 65


Quarts I Quarts


Quarts


___





Bulletin 229, Diseases of Citrus in Florida


tion of the home-made solutions will often vary from 22 to 28
degrees Baume. Such differences in density will make impor-
tant differences in the amount of water which should be added
to secure the proper strength of spray solution at given dilutions.
The density of lime-sulphur concentrates should therefore be
determined by a hydrometer with either the Baum6 or specific
gravity scales, preferably the former. This testing instrument,
which can be obtained from some of the firms that handle lime-
sulphur or spray supplies, will enable one to prepare accurately
spray solutions of lime-sulphur of a definite strength regard-
less of the variation in density of the concentrated solution.
Table I gives the figures for diluting lime-sulphur solutions of
varying degrees of density. The first column gives the range
of hydrometer readings of the concentrated solutions for both
the Baum6 and specific gravity scales. The other columns give
the amounts of the concentrated solution to use in order to pre-
pare 50 gallon lots of spray solutions diluted to a range of
strengths varying from 1 to 8 to 1 to 65. Thus, if a concentrated
solution shows a Baum6 reading of 29 degrees and it is desired
to use this at a strength of 1 to 30, in the column under 1 to 30,
opposite 29 degrees, will be found 8 quarts. This indicates that
8 quarts of a solution of this concentration will be required to
make 50 gallons of spray solution of a 1 to 30 strength.

DISEASES CAUSED BY PARASITIC FUNGI AND BACTERIA
DISEASES ATTACKING TREES, PARTS OF TREES OR
FRUIT BUT NOT CAUSING FRUIT DECAY
MELANOSE
Caused by Diaporthe (Phomopsis) citri (Fawcett) Wolf
Melanose occurs throughout the citrus-growing sections of
Florida and the Gulf states and has been reported from most
of the other citrus-growing countries of the world. This disease
was first observed at Citra, Florida, in 1892, and was first de-
scribed and named by Swingle and Webber in 1896. Within
recent years, melanose has become widespread and prevalent
to the extent that, in the older citrus sections of Florida, it is
the most important cause for lowering the grade of the fruit.
In fact melanose, together with the Phomopsis type of stem-end
rot, impairs the market value of more citrus fruit than all other
parasitic diseases combined.





Florida Agricultural Experiment Station


The 1923-24 citrus crop of 20,000,000 boxes graded about as
follows: Bright and fancy, 20 percent or 4,000,000 boxes; golden,
45 percent or 9,000,000 boxes; russet, 35 percent or 7,000,000
boxes. In a survey of 32 representative packinghouses through-
out six of the leading citrus producing counties, made by the
junior author, it was found that 59 percent of the golden and
61 percent of the russet fruit were due to melanose. Assuming
that this condition represents the average for the state, it was
estimated that, with golden selling at a handicap of 30 cents
per box and russets and plains bringing 70 cents a box less than
brights, melanose losses for that year were about as follows:-
In golden fruit, 5,310,000 boxes at 30 cents ............$ 1,593,000
In russet fruit, 4,270,000 boxes at 70 cents .-................. 2,989,000
Invested in spraying and pruning for melanose control 318,000
Off sizes and cull fruit ...... .......... ...... .......................... 100,000
Approximate loss due to melanose .................................. 5,000,000
All commercial varieties of Citrus grown in Florida are at-
tacked by melanose but grapefruit appear more susceptible than
oranges. Melanose is rarely important in young groves but
ordinarily increases progressively with the advancing age of
the trees. In general, those conditions which promote the ac-
cumulation of dead wood may be considered predisposing fac-
tors to melanose development.
SYMPTOMS OF MELANOSE
Melanose affects the foliage, twigs and fruit of citrus trees,
attacking these parts only while they are sufficiently young to
be susceptible to infection.
On the Leaves:-Melanose first appears on the tender young
leaves as minute, dark, circular depressions with yellowish mar-
gins (Fig. 1). As the leaves harden, the spots become flush with
the surface and later become distinctly raised, with somewhat
irregular margins. They are then rough and mahogany brown
in colnr and the discolorations in advance of them are no longer
evident (Fig. 2). If the infection is severe, the leaf partially
loses its natural green color and becomes pale green or yellow-
ish-green. The lesions occur on both surfaces but are most num-
erous on the upper. The markings quite often assume cir-
cular or ring-like patterns varying in diameter from one-eighth
to one-fourth inch. In case of severe infection, solid scar tissue
of various forms may result. The foliage on the watersprouts






Bulletin 229, Diseases of Citrus in Florida


and other succulent shoots is frequently so severely attacked
that it is permanently distorted (Fig. 3).
On the Twigs:-The disease on the twigs (Fig. 4) appears at
first very much as it does on the leaves but the minute corky
outgrowths ultimately become more raised than on the leaves
or fruits. If the infection
is severe, the twigs may
become more or less com-
pletely covered by scar
tissue and may die.
On the Fruit:-Lesions
on the fruit (Fig. 5) are
similar to those on the
leaves. They are at first
dark and sunken, later be-
coming brown, distinctly
raised, and rough to the
touch. When the infec-
tion is slight, the mela-
nose spots are usually
scattered, raised specks
ranging from 1/50 to 1/25
inch in diameter. They are
round and dark brown in
appearance, resembling
minute drops of burned
sugar. If the infection is
more severe the melanose
specks are much more
numerous and sometimes
closely resemble the erup. Fig. 1.-Early stage of melanose on
tions resulting from exan- grapefruit leaf, showing the yellow-
thema ammoniationn). As bordered spots.
on the other susceptible parts, however, the spots may form lines,
curves, circles, and other patterns. They sometimes appear in
tear-streaked patterns (Fig. 6) resulting from infections caused
by spores being washed down over the fruits by drops of water.
In other cases, a solid scar tissue, roughened, or with a "shark-
skin" effect, may involve a large part, or even all, of the fruit.
Where shallow cracks of more or less regular pattern develop
in this superficial scar tissue the "mud-cake" type of melanose






Florida Agricultural Experiment Station


results (Fig. 7). Fruits moderately to heavily infected by mel-
anose have a rough, sandpaper-like surface, the rough russeting
being termed "rust" by some growers. In severe cases of in-
fection the fruit may be dwarfed and, where badly scarred, may
crack open as it approaches maturity.


Fig. 2.-Portion of grapefruit leaf showing character of mature melanose
spots. (Twice natural size.)
CAUSE OF MELANOSE
Melanose was shown by Stevens, in 1912, to be due to Pho-
mopsis citri Fawcett, the same organism that produces one of
the stem-end rots of citrus fruits in Florida. In 1926, Wolf
reported the finding of the perfect, or ascospore, stage of this
fungus occurring on dead citrus twigs lying on the ground and
named it Diaporthe citri. The minute, pustule-like fruiting bodies
of the fungus develop abundantly on recently dead twigs and
fruit stems. The life-cycle of the fungus appears to be completed






Bulletin 229, Diseases of Citrus in Florida


ordinarily on the dead wood, since spores do not seem to be pro-
duced in the corky specks developing from infections on leaves,
twigs, and fruit. It will thus be seen why the presence of re-
cently dead wood in citrus trees is one of the important factors
in contributing to melanose infection. In Florida, drought, in-
adequate fertilization, root pruning through deep cultivation,


Fig. 3.-Distortion of foliage on young grapefruit shoot as a result of
severe melanose infection.

scale-insects, and freezes are responsible for the great majority
of the dead wood in citrus trees.
The fruiting pustules of the fungus causing melanose may be
found at any time of the year but usually are not abundant in
winter. With the advent of spring, innumerable pustules begin
to develop and, about the middle or the latter part of April, they
are found in abundance. At this time many of the pustules are
filled with spores, whereas others may be only in the first stages
of spore development, so that successive crops of spores may be
produced while the growing parts of the trees are still suscept-
ible. The length of time that twigs have been dead has an im-
portant bearing on their production of melanose spores, those
that have been dead but a few months being much more likely





Florida Agricultural Experiment Station


Fig. 4.-Melanose on orange twigs. (Enlarged.)
to harbor the organism than those that have been dead for sev-
eral months.
Another important factor necessary for the development of
melanose is moisture, in the form of either rain or heavy dew or





Bulletin 229, Diseases of Citrus in Florida


fog. The spores of the causal fungus developed in the pustules
are imbedded in a gelatinous matrix, which swells when moist-
ened. The spores are then forced out of the microscopic mouths
of the pustules in minute, tendril-like masses, after which rains


Fig. 5.-Melanose on grapefruit.
or the drip of water from dew or fog wash them down over the
young leaves, twigs, and fruits, thereby leading to the infection
of these susceptible parts. Given frequent rains or heavy dews
or fogs, trees with dead wood of relatively recent origin may
thus continue to infect the growing tissues as long as they re-
main susceptible.
It has been found that citrus foliage and shoots are very sus-
ceptible to infection from the time they emerge from the bud
until they harden distinctly, which usually takes place before
the leaves take on a deep green color. This means that, or-





Florida Agricultural Experiment Station


dinarily, the leaves are susceptible to infection for from two
to three weeks in periods of good growing weather. Melanose
infection of the leafy parts of the tree, however, ordinarily is
not of sufficient economic importance to justify special control
measures.
The fruits are likewise very susceptible to infection when
first formed but become progressively resistant with increasing
age. The approximate size at which they develop practical im-
munity from mel-
anose, based on
diameter m e a s-
urements, varies
from an inch for
p: n llF tangerines, 11/2
Inches for or-
,. I anges, and 21/2
inches for grape-
a e fruit.
The severe
outbreaks of mel-
anose can be
Sr l traced definitely
Sto rainy periods
.. occurring as a
rule in May or
l' c l early June after
Fig. 6.-Melanose on grapefruit, showing tear- the melanose
streaked effect.
spores have had
a chance to develop in abundance and before the young fruits
have had a chance to outgrow the susceptible stage. The sud-
den showers, heavy dews and fogs that come earlier do not ap-
pear nearly so favorable for infection as do cloudy, rainy periods
of two or three days' duration. The rainy periods that com-
monly occur late in March come well after most of the spring
flush of growth has hardened but long before even the fruit from
the earliest bloom has become resistant and sometimes before
much of the bloom has set fruit. While a moderate amount of
fruit infection almost invariably takes place when these March
rains occur, ordinarily, the principal infections do not occur be-
fore the latter part of April or early May, since April usually
is one of the driest months of the year.






Bulletin 229, Diseases of Citrus in Florida


CONTROL OF MELANOSE
As early as 1896, Swingle and Webber showed that melanose
outbreaks could be greatly reduced by applications of copper
sprays. However, in spite of the fact that these investigators
had demonstrated this point, few, if any, growers attempted


Fig. 7.-Melanose on grapefruit, showing mud-cake type.
to control melanose by spraying because the injurious effects
resulting in a great increase in scale-insects following applica-
tions of copper sprays were usually more damaging to the trees
and fruit than the melanose.
The control of melanose was considered impractical by the
growers until 1912, when Stevens found that the disease was
caused by a fungus (Phomopsis citri) that produces its spores
on dead twigs and branches. Following this, it became the gen-
eral practice to prune for melanose control. However, it later
became evident that thorough pruning to eliminate the dead and


liiP~L"L~!

-
s;..~





Florida Agricultural Experiment Station


dying twigs and branches did not prove to be a consistently
effective, practical or economical method of controlling this
disease in groves of any considerable size. In view of this it has
become customary to rely upon spraying in conjunction with
pruning.
A single application of 3-3-50 bordeaux mixture plus 1 percent
of oil as emulsion, applied shortly before the May rains set in,
has been found by several years of experimental work to give
an excellent control of melanose as a rule. This application, in
ordinary years, should not be made before April 15 and should
be completed by the fifth day of May. With the repeated dem-
onstration of the effectiveness of bordeaux-oil emulsion as a
combined fungicide and insecticide, spraying for the control
of melanose and other fungous diseases of citrus trees has been
put on a commercial basis anti the former objections to the use
of bordeaux mixture have been largely overcome. It has been
found that 1-40 lime-sulphur solution has but little effect in con-
trolling melanose and that dusting with either copper or sulphur
mixtures has failed to give satisfactory control.
Extra Spraying for Insect Control Following Melanose Spray-
ing:-Unfortunately, spraying with bordeaux mixture or bor-
deaux-oil emulsion destroys the spores of the beneficial fungi
parasitic on certain citrus insects as well as the spores of the
fungi that are being combatted. This favors a considerable and
usually rapid increase of scale and certain other insects, which
may do serious damage by fall unless measures are taken to
prevent an excessive infestation. It is largely to assist in keep-
ing the scale-insects in check that bordeaux-oil emulsion gen-
erally is recommended for citrus disease control instead of plain
bordeaux mixture. The addition of the oil is also effective
against the whitefly. Experience has shown, however, that even
when oil emulsion is used in conjunction with the bordeaux mix-
ture or other copper fungicide, it is usually necessary to apply
a "follow-up" spray of oil emulsion to assist in the control of
scale-insects. This application should be made between May 20
and June 15, the period when the scale-insects are hatching in
greatest numbers.
Secondary Benefits from Melanose Spraying:-There are two
important secondary benefits from spraying with bordeaux mix-
ture or bordeaux-oil emulsion for melanose control. One is the





Bulletin 229, Diseases of Citrus in Florida


reduction of the amount of stem-end rot that may be expected
to develop in the fruit during marketing. This reduction may
be as high as 50 percent from a single application that produces
satisfactory results in the control of melanose. The other is the
reduction in the tendency of the fruit to develop the symptoms
of exanthema known as ammoniationn". Spraying with bor-
deaux mixture or bordeaux-oil emulsion will completely elimi-
nate this trouble in many instances. A marked beneficial effect
commonly occurs also in the trees themselves that may be suf-
fering from exanthema diebackk).

SCAB
Caused by Sphaceloma fawcettii Jenkins
Citrus scab, lemon scab, sour scab, or verrucosis, as the dis-
ease has been variously termed, is widely distributed through-
out Florida and the Gulf States, where it causes serious injury
to a number of species and varieties of citrus fruits. This dis-
ease appears to have been unknown in the United States until
about 1885, when it was discovered near Ocala, Florida, on sour
orange nursery stock. It is believed to have been introduced into
this country on Satsuma orange trees from Japan, where it has
been present from ancient times. It was reported on lemon trees
in Florida in 1886 and gradually spread to all citrus-growing
sections of Florida and, later, to the Gulf States. Scab early
assumed commercial importance in Florida in disfiguring lemon
fruits and it was the pronounced susceptibility of the lemon to
both scab and cold that resulted in the general abandonment of
its commercial culture before 1910 in this state. Scab has now
spread to many citrus-growing countries of the world where the
moisture and temperature conditions are favorable to its de-
velopment.
Scab may attack, to a greater or less extent, the leaves, twigs
and fruits of many kinds of Citrus. Within a given species,
certain varieties may be very susceptible to the disease while
others are practically immune. Among the commercial species
and varieties grown in Florida the sour orange, lemon, and
some of the tangelos are extremely susceptible. The grapefruit
and shaddock are quite susceptible to infection, as are also the
King orange, Temple orange, mandarin orange, and Satsuma






Florida Agricultural Experiment Station


orange, which belong to the mandarin group of oranges. The
sweet or round oranges are rarely attacked and may be con-
sidered quite resistant. The Mexican or Key lime, kumquat,
citron and Cleopatra mandarin are apparently immune.
The greatest commercial injury caused by scab is to the grape-
fruit crop. It is owing to the large increase in grapefruit plant-
ings that this disease has attracted more attention in recent
years than formerly in Florida. All varieties of grapefruit,
however, are not attacked to the same degree. According to
Hume, the Hall is the most susceptible variety, while Marsh is
the least susceptible. He states that Duncan, Walters, and Fos-
ter are less affected than Hall. According to Winston, the Royal
and Triumph grapefruits (supposed to be hybrids of orange and
grapefruit) appear to be immune.
In general importance as a source of loss to the Florida citrus
industry, scab is probably surpassed only by such diseases as
melanose, blue and green mold decays, and stem-end rot. Scab
is not uniformly serious throughout the citrus-growing sections
of Florida, but may be of the greatest importance in certain well-
defined localities, especially in the interior of the peninsula. The
losses occasioned by this disease result primarily from heavy
premature dropping of young fruit and a marked reduction in
the grade of the fruit that reaches maturity, much of which is
rated as culls. In severe attacks of the disease, from 50 to 90
percent of the fruit of susceptible varieties in an affected grove
may be injured. Winston states that an average estimate of the
scab losses to Florida growers, made by conservative business
men familiar with the citrus industry, is about 1,000,000 boxes
of fruit annually. This means a considerable increase in the
cost of producing grapefruit, tangerines, mandarin oranges,
tangelos, and lemons of first quality in districts where scab is
a serious factor and has to be controlled by spraying.
With the exception of damping-off of seedlings, scab is the
only parasitic disease of consequence occurring in the citrus
nursery. There it is extremely serious and produces a marked
stunting effect upon seedling stocks of sour orange and rough
lemon for budding, frequently reducing the growth from 40 to
50 percent.
SYMPTOMS OF SCAB
Scab occurs on the leaves, twigs and fruits of susceptible spe-
cies and varieties of Citrus. These parts are attacked only while






Bulletin 229, Diseases of Citrus in Florida


very young and, with their maturity, irregular, raised scabby
or wart-like lesions are formed.
On the Leaves:-The disease starts as small, pale orange,
usually somewhat circular, elevated spots. As the leaves develop,
these infections become well-defined warts or excrescences on
one side of the leaf, often with a conical depression on the op-
posite side. The lesions occur on either surface of the leaf but
most commonly on the under side. The lesions may not be
visible on the opposite side of the leaf or may penetrate it. They
may be single or
irregularly
grouped. The
crests of these
wart-like growths
usually become
covered with a
scabby, corky tis-
sue, at first pale
yellowish orange
in color, later be-
coming pink,
then drab and,
finally, dark olive
drab as the leaf
matures and
hardens. The in-
fected spots often
run together and
cover large areas
with a corky,
scabby growth.
B a d 1 y infected
leaves become
very much crin-
k I e d, distorted, Fig. 8.-Scab on leaves and twig of sour orange.
and stunted, hav-
ing very little resemblance to normal foliage (Fig. 8).
On the Twigs, the characteristic effects of the scab disease
are the development of small masses of similar corky outgrowths
on their surfaces. The centers of infection may remain entirely
separate or may run together to form larger, somewhat elevated






Florida Agricultural Experiment Station


growths. In severe cases of attack, as on the sour and bitter-
sweet orange trees, the twigs may be killed.
On the Fruit:-The disease starts on the fruit in much the
same way as on the leaves and twigs, forming irregular scabby
spots or caked masses which vary from cream-colored to pale
yellow in young fruits to drab or dark olive-gray with age. This
change in color is accentuated by saprophytic fungi growing on
the surface of the scabby tissue. Fruits severely attacked
when very young often become misshapen, with prominent warty


Fig. 9.-Scab on Temple orange, showing warty projections.
projections or conical growths extending from the surface (Fig.
9). On grapefruit, the infected areas tend to flatten out and the
fruit which reaches maturity may show merely large scabby
patches of considerable extent, the entire surface being covered
in severe cases (Fig. 10). Severely attacked fruit may drop
prematurely. Although the scab infections are superficial and
do not affect the interior of the fruit, the fruit when badly in-
fected is unfit for market, while slight attacks lower the grade.






Bulletin 229, Diseases of Citrus in Florida


CAUSE OF SCAB
Scab is caused by a microscopic, parasitic fungus which fruits
on the surfaces of the scabby outgrowths. The fungus was first
isolated by Fawcett in 1906, who incorrectly assumed it to be
Cladosporium citri Massee. It has recently been placed in the
form genus Sphaceloma by Jenkins, who made a critical study
of it and named it Sphaceloma fawcettii in 1925.
The causal fungus lives over the winter period on the attacked









'y


5?. :















Fig. 10.-Grapefruit severely affected with scab.

leaves and twigs, which serve as sources of infection the follow-
ing spring. Winston concluded from cultural data and field ob-
servations that the disease is seldom, if ever, carried over winter
on fruit set in the spring or summer. It is disseminated by
such agencies as wind, rain, dripping of water from dews and
fogs, and by insects. The disseminated spores are capable of
infecting the tissues of susceptible species and varieties of Citrus






Florida Agricultural Experiment Station


only while the growing parts are very young and tender. Ac-
cording to Winston, who has made a careful study of this dis-
ease, the leaves are most susceptible in the early stages of growth
and become entirely resistant by the time they reach a width of
one-half inch. He has also found that young fruits are extremely
susceptible to infection immediately after the falling of the
petals and that they become progressively resistant until they
reach immunity, which, in the case of grapefruit, occurs by the
time they have attained a diameter of about three-quarters of an
inch.
The virulence of the scab disease may vary greatly from year
to year, depending upon the climatic conditions prevailing dur-
ing the stage of growth when the trees are susceptible to attack
and the abundance of the sources of infection at this critical
period. Abundant moisture, occurring simultaneously with the
presence of young, growing tissues of susceptible species or
varieties of Citrus, is essential for the infection and development
of scab. However, Winston found that, under average Florida
conditions, moisture rather than temperature is the important
limiting factor in natural infection. It is not necessary, how-
ever, that the moisture be in the form of rain, since the disease
is commonly quite severe even during springs of minimum rain-
fall in localities where heavy dews or fogs occur during the
critical period of infection or in regions of low hammocks or
flatwoods soils, which are usually damp localities. In the high,
dry, rolling hill sections, scab is a negligible factor during or-
dinary years and only becomes serious during years that are
unusually favorable to its development. In years with rainy
springs, however, this disease is almost certain to be severe on
unsprayed grapefruit and other susceptible citrus fruits in many
sections of the state.
CONTROL OF SCAB
Scab can be controlled satisfactorily and economically in most
cases by the use of bordeaux mixture, provided the spraying be
done with thoroughness at the proper time. However, since
scab infection takes place very quickly under favorable circum-
stances, it is difficult to control this disease in years when fre-
quent rains occur while the fruit is setting. Lime-sulphur is
much less effective against this disease than bordeaux mixture,
but grove conditions sometimes warrant its use, especially where
there is not likely to be a bad outbreak of scab. Since scab at-






Bulletin 229, Diseases of Citrus in Florida


tacks the tissues only while very young and tender, it is neces-
sary that the spraying be done early in the spring if the fruit
is to be protected.
If scab is inclined to be severe it is advisable to make two ap-
plications of bordeaux-oil emulsion. The first of these (3-3-50
Bordeaux mixture plus 1 percent of oil as an emulsion), which
should be made just before the growth starts in the spring, serves
to cover the old scab lesions and thereby materially reduces scab
infections. The second (3-3-50 bordeaux mixture plus 1/2 per-
cent of oil as an emulsion), which should be made during the last
of the bloom when at least two-thirds of the blossoms have fallen,
serves to protect the young fruit against the disease. Generally,
however, commercial control of the disease can be secured with
but the former application. The second application will also
serve to control early melanose.
Where scab infection is not likely to be very heavy, where
melanose is not a factor, and where it is desired to control red
spider or rust mites on late fruit, liquid lime-sulphur should be
used instead of bordeaux-oil emulsion, making the applications
at the same time recommended for bordeaux-oil emulsion. The
lime-sulphur solution should be diluted at the rate of 1 part to
25 or 30 parts of water (3 to 4 gallons per 100) for use before
the new growth starts and at the rate of 1 part to 40 parts of
water (21/ gallons per 100) for use later, based on a concentra-
tion of 320 Baum6. In years when there is an extensive set
of grapefruit from June bloom, spraying for scab control should
be profitable at that time.
Scab may be controlled to a considerable extent by removing
the sources of infection not essential to commercial fruit pro-
duction. The scattered sour orange and rough lemon trees
which frequently occur in groves carry the disease and serve
as centers of infection. Such trees should either be removed
or worked over to commercial Citrus varieties. Sour orange
and rough lemon sprouts from the rootstocks of the grove trees
should be cut off whenever they appear.
Seedbed and other nursery stock prior to budding can be
kept fairly free from scab, with considerable gain in size, by oc-
casional applications of bordeaux-oil emulsion. The frequency
of applications is dependent upon local and seasonal conditions.
It is especially important to keep the new flushes of growth
protected during periods of rainy weather.





Florida Agricultural Experiment Station


CANKER
Caused by Phytomonas citri (Hasse) Com. S. A. B.
Canker is the worst disease of citrus trees that has ever been
introduced into the Gulf States and, had it become well estab-
lished in Florida, it would undoubtedly have caused disastrous
results to the citrus industry. Fortunately, the serious nature
of this disease was early recognized and sufficient sentiment was
created to undertake its complete eradication. Between 1914
and 1931 more than $2,500,000 of state, federal, and private
funds were spent in combatting citrus canker in Florida alone
and the total cost of this disease to the state thus far has been at
least $6,500,000. During this period 257,745 grove trees and
3,093,110 nursery trees were destroyed incidental to the eradi-
cation of this disease, which occurred in 515 properties scattered
through 26 counties. Through the effective control campaign
and systematic inspection of grove properties carried out by
the State Plant Board, it appears that canker has been eradi-
cated in Florida. As a result of this energetic campaign, there
has been no commercial damage to Florida citrus groves by this
disease since 1922, and no infections have been found since 1927.
The apparently complete eradication of this dreaded disease
from Florida, Alabama, and Mississippi is one of the most out-
standing accomplishments in the history of plant disease control.
Constant viligance is necessary, however, to be sure that the
disease does not obtain another foothold.
Canker is widely distributed throughout the citrus-growing
countries of the world but is especially prevalent in the Orient.
It is usually believed to be native to China but has also been
present in Japan for a considerable length of time. This disease
was first introduced into the United States in 1910 or earlier
on trifoliate orange trees shipped from Japan to Texas, Missis-
sippi, Alabama, and Florida. In Florida it was probably first
introduced from shipments of trifoliate orange trees from both
Japan and Texas and also from Satsuma orange trees imported
directly from Japan and planted at Monticello in the spring of
1910. When the disease was first observed in Florida by E. W.
Berger at Monticello on September 30, 1912, it was considered to
be scab, but when found again at Silver Palm in Dade County
in July, 1913, it was realized that a new disease had been dis-
covered. It was not generally recognized until 1914, however.





Bulletin 229, Diseases of Citrus in Florida


that this disease was a serious menace to the citrus industry and
its true cause vas not demonstrated until 1915.
Canker attacks all kinds of citrus trees of commercial impor-
tance in Florida, provided the conditions are favorable for the


iI,~
~1'4 ..~


Fig. 11.-Canker on grapefruit leaves and twig.
development of the disease. The grapefruit is attacked the most
severely, however, and for this reason citrus canker threatened
to cause far more serious damage here than in the other Gulf
States, where the Satsuma orange, a much more resistant spe-
cies, is of greatest commercial importance. The trifoliate orange
is probably next in susceptibility, followed by the limes other
than Tahiti, sweet oranges, American-grown lemon varieties,
and the Satsuma orange. Mandarin oranges, calamondins and
citrons have a very slight degree of susceptibility and are com-
mercially resistant. The kumquat is highly resistant. Inocu-





Florida Agricultural Experiment Station


lation experiments have shown that a large number of species
and varieties, hybrids and wild relatives of Citrus are more or
less susceptible to canker.
SYMPTOMS OF CANKER
Canker may attack any part of the tree above the ground
although the leaves, twigs, young branches, and fruits are most
commonly infected and these parts chiefly while still fairly
young and tender. The distinguishing feature of the disease as
S observed in the field is the characteristic spotting
produced on the foliage, twigs, and fruits.






Fig. 12.-Canker on young twig of trifoliate orange.

On the leaves, canker first appears as small watery, trans-
lucent spots usually of a darker green color than the surround-
ing tissue and with raised, convex surfaces. The spots, which
may appear on either surface of the leaf, usually become evident
on the lower one first. As the disease advances, the surfaces of
the spots become white or grayish and finally rupture, exposing
a light brown, spongy central mass developed in a crater-like
formation of the ruptured epidermis. Such spots usually become
surrounded by a yellowish halo, which persists in very old lesions.
The size, color, and abundance of the lesions vary somewhat on
the different kinds of citrus trees attacked (Fig. 11). For ex-
ample, the lesions are largest on grapefruit leaves, on which they
may be nearly 1/2 inch in diameter, and are usually not quite so
large on leaves of the sweet orange. On the leaves of limes and
lemons, they are much smaller, frequently not being more than
1/8 inch in diameter. Old lesions become brown and corky and
may even appear pinkish or dark-colored on account of becoming
overgrown by saprophytic fungi.
On the Twigs, Branches, and Roots:-Lesions on the twigs
are common on the more susceptible kinds of Citrus, such as the
grapefruit, trifoliate orange, lime, and sweet orange varieties.
The spots on young twigs are like those on the leaves and
fruit (Fig. 12). On the older twigs they are more prominent






Bulletin 229, Diseases of Citrus in Florida


and more or less irregular in shape, especially in the case of old
spots. The spots show the same spongy tissue as those found
on the leaves but acquire a cankerous appearance and the sur-






E1
*


Fig. 1-Canker on stems o graperit
Fig. 13.-Canker on stems of grapefruit


nurs


x

















lery stock


ek.






44 Florida Agricultural Experiment Station

face membrane completely disappears (Fig. 13). In especially
susceptible varieties, canker has been found on the bark of ma-
ture branches as large as six inches in diameter and branches
from two to three inches in diameter are quite commonly infect-
ed. While underground roots of citrus trees have been repeat-
edly inoculated, with positive results, canker has never been ob-
served occurring naturally on such roots despite numerous exam-
inations of roots of badly diseased trees that were dug in heavily
infected
s groves. How-
ever, the dis-
ease has been
.' -, found on roots
Fg. 1o f grapefruit
h trees exposed
al 1 above the sur-
ove t face of the
ground.
ciad wit On the Fruit:
Individual
IN canker lesions
on t he fruit
t Cr have much the
Ss a m e appear-
ance as on the
leaves except
Fig. 14.-Canker on grapefruit. h e yl
halo is usually
absent and the crater-like appearance is usually more notice-
able (Fig. 14). The spongy, rough eruptions may be scattered
over the surface or several may occur together, forming an ir-
regular, scurfy or scabby mass. Gumming is sometimes asso-
ciated with the spots formed on the fruits. The disease, which
does not penetrate deeply into the rind, does not cause a decay
of the fruit directly but opens the way for fungi to enter and
cause infected fruits to rot.
Distinguishing Canker from Other Diseases:-Citrus diseases
with which canker may be confused are scab, scaly bark or nail-
head rust, and anthracnose on Mexican or Key limes. It can be
distinguished readily from any of these diseases, however, by
noting the following points:-





Bulletin 229, Diseases of Citrus in Florida


1. While canker is likely to be confused with scab only in the
early stages, it differs from scab in the typically round spots
produced and the fact that the spots generally penetrate through
the leaf tissue, look approximately the same on both sides and
develop a watery, translucent zone, or later a yellowish halo,
around them. It does not distort the leaves and there are no
wart-like projections as in scab. Moreover, canker may occur
on quite old bark, while scab does not.
2. It differs from scaly bark or nail-head rust in the size of the
spots, which are much smaller than those of scaly bark. It also
differs in the spongy nature and crater-like appearance of the
spots, scaly bark spots being hard and glazed and rather smooth.
Furthermore, canker is common on grapefruit while scaly bark
is of rare occurrence.
3. It differs from anthracnose on the Mexican or Key limes
in that it is not confined to the fruit but occurs also on leaves,
young shoots, and twigs. Canker on the fruit, however, re-
sembles anthracnose on limes more closely than any other dis-
ease but canker spots do not result in sunken places, distortion
and splitting of the fruit as is often the case with anthracnose
on limes. Moreover, canker lesions do not develop the thick,
corky, mound-like outgrowths generally characterizing anthrac-
nose on limes.
CAUSE OF CANKER
Citrus canker is a bacterial disease caused by Phytomonas
citri (Hasse) Com. S. A. B., a short, rod-shaped, motile organ-
ism that can be seen only with a good compound microscope.
These bacteria are formed in countless numbers in the canker
spots and are exuded in masses whenever the spots become mois-
tened by rain or dew. Drops of water contaminated with bac-
teria spread the disease and when the organism comes into con-
tact with susceptible citrus tissue, especially the tender foliage,
twigs, and young fruit, new canker spots develop readily. In
the older bark on the branches and trunks the bacteria probably
gain entrance through small wounds. The disease may be spread
some distance by wind and driving rains and any animal or in-
sect coming into contact with the moistened foliage of an infected
tree is apt to spread the disease to other trees. The chief agent
in the dissemination of canker, however, has been man, the dis-
ease having been brought to America on nursery trees and spread
through Florida on budwood and nursery trees.





Florida Agricultural Experiment Station


The disease often may remain inactive for a considerable per-
iod of time after infections have taken place before developing
the usual symptoms and evidently can be carried over long per-
iods in lesions on older bark. Lee, Fulton, and Loucks have each
shown that the canker organism in grove soils cannot compete
with the soil organisms occurring in such soils. From these and
other investigations that have been made in regard to the longev-
ity of the citrus canker organism in soils, it appears that most
agricultural soils cannot long retain the possibility of dissem-
inating it.
CONTROL OF CANKER
The prompt and complete destruction of all canker-infected
trees is the only safe and practical method that has yet been
found for checking the disease in Florida. This can be done to
best advantage by burning them in place with blasts of flame
from the nozzle of a small spraying outfit filled with kerosene
or other suitable fuel oil. Any citrus canker, or suspected cases
of this disease, should be reported promptly to the State Plant
Board at Gainesville, who will attend to the eradication of the
disease.
FOOT ROT
Caused by Phytophthora parasitica Dastur
Foot rot, collar rot, or mal di gomma to use the well-known
Italian name, is a gum disease which is probably more wide-
spread than any other citrus malady, being known to occur in
virtually all countries where citrus trees are cultivated. This
disease seems to have been noted in Florida first about 1876,
although it evidently did not become an important disease in
this state until after 1880. While foot rot has been a destructive
disease in Florida citrus groves for many years, it does not ap-
pear to be as severe as in some foreign countries and, as a rule,
does not attack the trees until they are quite old and have been
bearing fruit for several years. This disease is at present wide-
ly distributed throughout the citrus sections of the state and still
causes considerable loss in certain sections. In some cases the
majority of the trees in moderately old sweet seedling orange
groves may develop foot rot more or less simultaneously and
groves that were highly productive for several years may die
out within a short time. Foot rot, however, is by no means so






Bulletin 229, Diseases of Citrus in Florida


important a factor in citrus culture as formerly because its
nature and means of control are now understood and because of
the fact that sweet seedling orange groves are no longer planted
and no longer constitute such a large proportion of the total
citrus acreage.
In Florida, foot rot has been confined largely to the old sweet
seedling orange groves and to trees budded on this and other
especially susceptible stocks. Even where resistant stocks are
used, however, the disease may occur above the bud union on
low-budded trees. Sour orange and bitter-sweet orange stocks
are highly resistant to the disease but not immune. The order
of resistance of citrus stocks to foot rot in Florida appears to be
about as follows: sour and bitter-sweet orange, Cleopatra man-
darin, rough lemon, trifoliate orange, grapefruit, lemon, and
sweet orange. The first four of these stocks may be classed
as decidedly resistant while the last two, together with the citron,
which is not used as a stock in Florida, are very subject to the
disease.
SYMPTOMS OF FOOT ROT
Foot rot is a type of gum disease which affects the bark of the
crown roots and base of the trunk, usually at or just below the
surface of the soil. The first symptom of the disease is a small
area of decayed bark, from which there is a slight exudation of
gum, generally in small drops. The decayed bark at first has a
watersoaked appearance and a watery gum is sometimes found
beneath it. Both the diseased bark and the underlying wood fre-
quently have an ill-smelling, fermented odor. As the disease
progresses, gum exudes from other portions of the bark, which
in turn die. The exudation of gum accompanying the early
stage of this disease is rarely abundant. It develops most abun-
dantly in late spring and early summer and the exuded gum may
be washed away by rains.
Foot rot may manifest itself at one or more places on the
trunk. The patches of bark which first become diseased are
irregular in shape and variable in size. They later dry out and
sink below the level of the healthy bark and are thus sharply
demarked (Fig. 15). The bark remaining alive in advance of
the lesions often develops a callus formation which tends to de-
limit the disease and check its further spread, especially in an
upward direction. Following this, the old patches of diseased






Florida Agricultural Experiment Station


Fig. 15.-Foot rot lesions at base of grapefruit tree on sweet seedling
orange stock, showing sunken, dead areas of bark (outlined with chalk).
The light colored patches on the bark are lichens.






Bulletin 229, Diseases of Citrus in Florida


bark crack loose and slough off. In some cases, the activity of
the disease may appear to be checked for a time but usually is
resumed later. As a rule, the disease continues to spread on
the main crown roots (Fig. 16) and laterally around the base
of the trunk until the tree becomes more or less completely
girdled. The destruction of the bark on the trunk, as a rule,
rarely extends more than 12 to
15 inches above the ground. The
height of the bark lesions above
the ground, however, is by no
means always an index to the
progress of the disease for, since
infection usually starts at some
point beneath the soil, it often
happens that a large part of the
root system may be affected be-
fore there are any visible indi-
cations of the trouble above
ground, other than perhaps the
decline of the top of the tree.
Thus, the root system may be
seriously injured before the
grower is aware that anything is
wrong with the tree.
After foot rot has progressed
to the point where the root sys-
tem or the trunk becomes par- Fig. 16.-Foot rot, showing in-
tially girdled the effect of the fected root with diseased bark
cut away.
disease manifests itself by a
characteristic and usually rapid decline of the tree. The foliage
becomes light-yellow and scanty and the leaves subsequently
developed are much smaller than normal ones and frequently
present a dull, hard appearance. Declining trees have the ap-
pearance of suffering from lack of nourishment, which is indeed
the case on account of the partial girdling. Such trees may con-
tinue to bear a fair amount of fruit for a time and occasionally,
just before dying, may produce a large crop of fruit. The fruit,
as a rule, becomes small in the late stages of decline. In well-
advanced cases of the disease the leaves often become yellow,
especially along the midribs, and exhibit the characteristic symp-
toms of acute starvation. The twigs and smaller branches die





Florida Agricultural Experiment Station


first and the larger branches succumb gradually as the disease
progresses, thus giving the trees a ragged appearance (Fig. 17).


A4
4~ L)


/ oi


Fig. 17.-Orange tree nearly dead from girdling by foot rot.

When the top becomes half dead, numerous watersprouts fre-
quently develop from the branches. It is often characteristic of
trees attacked by foot rot to die irregularly, one side going first.
During dry periods, the foliage of trees which have been severely





Bulletin 229, Diseases of Citrus in Florida


injured by foot rot frequently exhibit a pronounced wilting and
curling.
The rapidity with which trees attacked by foot rot die is
quite variable. In severe cases, the trees may die within a year
or two from the time the disease becomes apparent. Usually,


Fig. 18.-Advanced case of foot rot, in which the tree broke off long be-
fore it died. The taproot and most of the lateral roots have been dead
a long time.
however, it may be several years before the attacked trees finally
succumb. The kind of rootstock and the location of the attacked
tree have a considerable bearing on the severity of the disease.
In some instances, a number of different wood-rotting fungi
invade the exposed wood of foot rot lesions, in which case the
attacked trees may break off at or below the ground line as a
result of decay (Fig. 18) quite a while before becoming com-
pletely girdled.
CAUSE OF FOOT ROT
Foot rot has been studied extensively by many investigators
and a number of diverse causes have been assigned in the past.





Florida Agricultural Experiment Station


Because of its nature and manner of spread, most of the early
investigators were led to consider this disease to be of para-
sitic origin, but no specific organism was definitely proved to be
the cause until a few years ago.
In 1914 a fungus was isolated from a foot rot lesion on a
grapefruit tree at Palmetto, Florida, by Fawcett. This fungus,
which later was identified as Phytophthora terrestria by Sherb-
akoff, was recognized at the time as similar to the one causing
brown-rot gummosis in California. Fawcett had previously
isolated the same organism from gummosis lesions at the junc-
tion of the root and trunk of an orange tree in California in 1912.
He later states that foot rot, under California conditions, can
not be distinguished from certain phases of brown-rot gummosis,
except by means of culture tests for the isolation of the causal
organisms.
In 1916, the same fungus was isolated from an active case of
foot rot by Stevens, who was then with this Experiment Station,
and, a little later, he isolated it from other cases of foot rot in
other localities. A rather extended survey was then made of
the foot rot groves throughout the state and the same fungus
was repeatedly isolated from active cases of foot rot, from trees
in widely separated localities, covering the principal part of the
citrus belt. This fungus was inoculated into the bark of healthy
citrus trees and, in several cases, diseased areas developed that
were typical of foot rot lesions found under natural conditions.
Although this fungus was identified as Phytophthora terrestria
Sherb., it is now considered by most authorities, including Sherb-
akoff, to be a form of the species, Phytophthora parasitica Das-
tur. This fungus belongs to a group which contains some of
our most destructive plant parasites and attacks a number of dif-
ferent plants in Florida and the tropics. It may be properly con-
sidered a soil fungus since it can grow and perpetuate itself in
the soil for a long period. It is most active and develops most
rapidly in low-lying soils and in moist, shady situations.
By no means all the trouble ascribed to foot rot in Florida,
however, is due to that specific trouble. Recent investigations
by the senior author have demonstrated that a very similar
trouble heretofore undistinguished from foot rot is due to Clito-
cybe mushroom root rot (see page 56).





Bulletin 229, Diseases of Citrus in Florida


CONTROL OF FOOT ROT
There are two general methods for the control of foot rot,
namely preventive safeguards in connection with the main-
tenance of the grove, and remedial measures after the trouble
occurs. The most effective way of avoiding foot rot is the use
of resistant rootstocks for budding in cases where these stocks
can be used to advantage, provided the trees are not budded too
low. Additional methods of prevention comprise the selection of
well-drained land or providing adequate drainage before plant-
ing, and the avoidance of too close planting, too low budding,
too deep setting, allowing the soil about the bases of the trees
to become too wet in irrigation, scarring the roots and bases
of the trunks in cultivation or in the old practice of cattle-pen-
ning, and the piling of dirt, wood, weeds, or trash about the
bases of the trees for any great length of time. The Porto Rican
practice of planting trees on mounds so that the crown roots
eventually become exposed above the surface of the soil as it
levels down should prove valuable in Florida for the prevention
of foot rot on sweet seedling orange trees and trees on this
and other susceptible rootstocks. This practice requires that
the mounds be kept free from weeds and mulched to maintain
the supply of soil moisture while the trees are young. It also
requires that implements of cultivation be kept away from the
bases of the trees and that close weeding be done by hand as the
trees become older, not to injure the roots.
In the treatment of trees attacked by foot rot, since the disease
occurs chiefly on the root-crown and the adjoining part of the
crown roots and taproots, it is first necessary to clear away
the top soil in a radius of about 3 feet around the tree in order
to determine fully the extent of the trouble under the ground
(Fig. 19). This should be done carefully so as not to scar the
roots. Where a large number of trees are to be treated, this
tedious job can be done to best advantage by washing the soil
away with water, using one of the leads of hose from a power
spraying outfit after disconnecting the spray nozzle or else re-
placing the regular disk by one with a large hole. If the dis-
ease is found to have developed so extensively as to practically
girdle the tree it is useless to attempt to treat it by surgical
methods. When the disease has not advanced too far, it should
be treated by cutting out all areas of diseased bark, using a
sharp knife, chisel, or gouge, as may be necessary. All dead





Florida Agricultural Experiment Station


roots should be cut off flush with the root-crown and all diseased
bark and roots cut out should be collected and burned to check
further spread of the causal organism. All wounds and exposed
wood surfaces should be treated with some good, safe disinfect-
ant, such as bordeaux paste or lime-sulphur paste. The disin-
fectant may be applied with a brush or, if made sufficiently






















Fig. 19.-Foot rot on sweet orange tree budded low on sour orange stock,
showing method of removing dirt for treating diseased areas. This
tree should have been treated, however, before it became so nearly
girdled.

thin, with a small spraying outfit. All exposed wood surfaces
should be painted later with a good waterproof wound dressing
to prevent the entrance of wood-boring insects and decay-produc-
ing organisms. It is advisable to leave the root-crowns of treated
trees exposed for several weeks, months, or even indefinitely,
provided there is no danger from freezing. This aeration will
allow the bark of the root-crown and the soil about it to dry out,
which will assist in killing out the causal organism and in re-
tarding its subsequent development. Frequently, the mere re-
moval of the soil from immediately around the base of the trunk,
leaving the crown roots exposed, will suffice to check the further
development of foot rot. The exposure method, however, is not





Bulletin 229, Diseases of Citrus in Florida


applicable to trees on heavy clay soils, since the water accumu-
lating in the excavations during rainy weather would be fatal to
the tree.
Trees which have not become weakened too seriously from
foot rot often can be saved merely by planting three or four
::;.sm ma nmr a3.mn .: W V. A NlMI,


Fig. 20.-New roots developed from the callus above the girdled base after
this tree, which had been nearly girdled by foot rot, had been banked
for about two years.
sour orange or other resistant trees about the base of each at-
tacked tree and later inarching the tops into the trunk above the
partially girdled place. However, this is not an easy task for
the average grower and, even if successful, is slow in assisting
the tree. Even trees which have become considerably weakened
by foot rot may be saved and rejuvenated by banking to a height
of a foot or 18 inches with soil free from trash, after the foot
rot lesions have been treated or disinfected. This method in-






Florida Agricultural Experiment Station


duces the development of a new root system, within from three
to five years, from the callus formed at the lower limit of the
living bark just above the nearly girdled base (Fig. 20). A
considerable number of instances have been noted where trees
severely weakened from the ravages of foot rot were restored to
health and productivity by treating in this way.
Growers should inspect their groves semi-annually in order to
detect cases of foot rot and other bark diseases so that the trees
may be treated before the disease progresses to such a point
that it threatens the life of the tree. An inspection of the bases
of the trees will usually reveal the presence of foot rot long
before it becomes evident by the way the top- have died back.
CLITOCYBE MUSHROOM ROOT ROT
Caused by Clitocybe tabescens (Scop.) Bres.
Clitocybe mushroom root rot, which is known to attack fruit
trees, grapevines, forest, shade, and ornamental trees, and shrubs,
occurs in a number of the southern states ranging from Florida
and North Carolina west to Missouri, Oklahoma, and Texas.
Numerous cases of this disease have been recorded by the senior
author during the past eight years, on a large variety of woody
plants from various parts of Florida, where it has been known
to occur since 1902. However, it was not until the end of 1929
that it was learned that citrus trees also are attacked.
To date approximately 150 citrus trees, located in several
groves at a few points in Polk County and in one on Merritt's
Island, have been found to be attacked by this root rot fungus.
The attacked trees, ranging in age from 9 to 20 years, were on
rough lemon rootstock and on well-drained sandy land in all
cases. They comprise mostly grapefruit trees, with some orange
and a few tangerine trees. These ranged from individual trees
to more or less grouped trees and even areas of grove where a
considerable number of the trees were attacked. The extent to
which this trouble has been found to occur in a few fairly young
groves where the trees appear quite healthy above ground in-
dicates that it is a problem for serious consideration. In one
locality in Polk County where mushroom root rot was found a
careful survey of the scattered groves under the same manage-
ment led to the finding of a total of 70 attacked trees out of a
total of forty odd thousand, which is equivalent to slightly more
than one acre of attacked trees out of about 700. In the same





Bulletin 229, Diseases of Citrus in Florida


locality an area of about a half-acre had been located previously
in a grove under different management where several trees were
attacked by mushroom root rot and occasional trees had been
dying during the last few years from a trouble thought to be
foot rot.
From the widespread distribution of the Clitocybe root rot
fungus throughout a large part of the state and the numerous
records obtained of its occurrence on citrus trees during 1930
and the early part of 1931 it is undoubtedly likely that this dis-
ease will eventually be found to be of fairly widespread and com-
mon occurrence on citrus trees, especially throughout the central
portion of the peninsula where the rough lemon rootstock is
universally used and oak trees are prevalent. It is quite evident
that this mushroom root rot of citrus trees is no new trouble
but that it simply has not been distinguished heretofore from
foot rot, which it closely resembles in the behavior of the at-
tacked trees.

SYMPTOMS OF MUSHROOM ROOT ROT
The symptoms exhibited by the attacked trees vary greatly
with the age and progress of the disease. In the 70 attacked trees
treated in one locality in Polk County the disease, with but two
exceptions, had not attained sufficient headway to cause a decline
of the tops and these trees, with the exceptions noted, appeared
as good and as productive as the adjoining trees which were not
attacked. With but three or four exceptions where trees were
known to have been treated previously for dead roots, the at-
tacked trees in this locality were located solely by finding either
fresh or old clusters of the mushrooms which had fruited at the
bases. Upon excavating the soil under the bases of these trees,
however, to examine the root system and prepare them for treat-
ment, a number of the lateral roots were found to be dead in all
cases, these often being dead clear back to the root crown and
with the terminal portions disintegrated. In all but four of
these trees the taproots were also dead. In some cases an area
of bark was found to be dead at one side of the base of the trunk
(Fig. 21, left) and a slight exudation of gum occurred just above.
These areas of dead bark frequently marked the point where a
large lateral root had been killed back to the trunk and usually
were the places at which a cluster of the fruiting bodies of the
fungus had developed. (Fig. 21, right).






Florida Agricultural Experiment Station


It was equally surprising to find that oak and occasionally
other hardwood roots extensively invaded by the mycelium of
the Clitocybe root rot fungus occurred under each of these citrus
trees and that some of these infected roots were in contact with
one or more roots of the citrus trees in apparently all cases.


Fig. 21.-Grapefruit trees attacked by Clitocybe mushroom root rot, show-
ing bark lesion at base on left and a partially dried cluster of fruiting
bodies developed at base on right.
The inference is that the oak and other hardwood roots which
were left in the land when it was cleared became infected by the
Clitocybe root rot fungus and that these roots served to transmit
the fungus to the roots of the citrus trees. It was also observed
in several instances that where an infected citrus root crossed
or otherwise came into contact with other citrus roots, the root
rot fungus was transmitted to these roots. The majority of the
attacked roots of the citrus trees, as well as the old oak roots,
exhibited an abundant development of the mycelium of this
root rot fungus except in those cases where they were fairly well
decayed or the mycelium had run its course.
Unlike the development of Clitocybe root rot observed in most
other attacked trees, in citrus trees the mycelium rarely seems to
develop very far above the ground line. The mycelium, or vege-






Bulletin 229, Diseases of Citrus in Florida


tative part of the fungus, varies from thin filmy white wefts
with a radiating marginal growth to whitish, creamy, or old
chamois-colored papery layers of extensive development between
the bark and the wood and also permeating the bark. Freshly
dug roots in which the mycelium of the root rot fungus is still
active have a pronounced mushroom or fungous odor. The fungus
frequently causes a whitish decay of the old attacked roots. In
a few instances the lower divisions of the taproots were still alive
and in several instances the distal portions of many of the more
recently attacked lateral roots still remained alive. The majority
of the root infections quite obviously started at points adjacent
to the base of the tree, the disease spreading quickly to other
roots. When the roots become girdled the ends or distal por-
tions soon die regardless of whether they are invaded by the
fungus. In several cases narrow areas of dead bark varying
from a few inches to a foot or more in length extended along the
under side or other portion of the main lateral roots. In some
instances such areas of bark infection had become delimited
by the formation of a callus around the periphery, in which case
the mycelium of the fungus appears to die and the dead bark
disintegrates and sloughs off.
By the time the root rot fungus has invaded a portion of the
root crown and sufficient of the roots to interfere seriously with
the support of the top of the tree, its decline follows rapidly.
The foliage on the most severely affected branches becomes pallid
and small and the leaves bleach out along the midribs as is in-
variably the case with acute starvation induced by any form of
girdling of citrus trees. The most seriously affected branches
die back rapidly and the fruit produced on these devitalized
branches is usually small and frequently drops before reaching
maturity, due to the rapid decline of the top of the tree. By the
time the top of the tree has declined considerably basal lesions
of dead bark, similar to those developed in foot rot, usually de-
velop at the base and the bark may crack and gum more or less
freely. Dead lateral roots may be evident in many cases, even
without scraping away the soil to any particular extent. Such
was the appearance of the disease in a number of the trees at-
tacked by mushroom root rot in the grove on Merritt's Island,
where a few trees declined rapidly. No infected oak or other
foreign roots were found under the 8 trees treated in this grove,
although a couple of well rotted pieces of oak root were found






Florida Agricultural Experiment Station


under one tree and the infected area in the corner of this grove
adjoined a woods in which oak trees were numerous. With the
exception of one tree with a twin taproot in which one half was
dead, the main portions of the taproots on these trees were alive.
CAUSE OF MUSHROOM ROOT ROT
The organism causing this root rot is one of the gill fungi,
Clitocybe tabescens (Scop.) Bres., popularly termed mushrooms
or toadstools, which occur in clusters comprising several indi-
viduals developed from a common base, usually at the base of
the attacked tree (Fig. 22). This root rot closely resembles in
many respects that caused by the well-known honey agaric or
oak root fungus (Armillaria mellea), especially in regard to
habit of growth, the production of rhizomorphs, appearance of
the mushroom-like fruiting bodies, and prevalence on land where
oak trees have occurred.
The Clitocybe root rot fungus is propagated by means of the
myriads of spores developed on the gills of the fruiting bodies,
by roots of trees coming into contact with those of infected trees
or old infected roots that may be in the soil, and by means of the
whitish to tan-colored or brownish fungous strands or rhizo-
morphs which are developed under certain conditions. The
latter develop readily in artificial cultures of the fungus and fur-
nish one of the chief means of identifying the organism before
the mushroom-like fruiting bodies are produced. The rhizo-
morphs are very inconspicuous and have been observed only oc-
casionally in nature.
Since the fungus can penetrate roots through uninjured bark
it must be regarded as an active parasite of living trees, which is
readily capable of spreading to adjoining trees, especially where
closely planted. It flourishes also as a saprophyte in old stumps
and roots of various trees, especially oaks. When the fungus
has invaded the roots and root crown of an attacked tree suffi-
ciently to curtail the life processes a more or less rapid decline
of the top of the tree follows.
The clusters of the mushroom-like fruiting bodies are pro-
duced after the fungus has attained a certain stage of develop-
ment, provided the moisture and temperature conditions are
favorable. The season of their appearance in Florida is chiefly
from October to early December. They are sufficiently dis-
tinctive so that they should not be confused with other mush-






Bulletin 229, Diseases of Citrus in Florida


rooms or toadstool-like fungi which may occur around trees oc-
casionally but be without any pathological significance. When
fully developed, the caps are convex in shape, tan-colored, and
range from 2 to 31/> inches in diameter as a rule (Fig. 22). The


Fig. 22.-Cluster of the mushrooms or toad-stools of the Clitocybe
root rot fungus.
clusters attain their full development within a few days under
favorable conditions and decay very quickly in warm wet
weather. However, if dry weather follows their development,
especially during the fall or early winter, they may dry up and
remain recognizable for two to three months, and often much
longer, thus marking the tree as undoubtedly one attacked by
root rot (Fig. 21, right). In the absence of the fruiting bodies
of the fungus the disease appears to be distinguishable from foot
rot only by finding the characteristic mycelial growth of the fun-
gus between the bark and the wood of the roots, which is best
done in those below the surface of the ground, where better mois-
ture conditions prevail.





Florida Agricultural Experiment Station


CONTROL OF MUSHROOM ROOT ROT
It is far easier to take steps to prevent the occurrence of mush-
room root rot than it is to control the disease after it develops in
the trees. In clearing timbered land for grove sites, with the
exception of low hammock land where it would be economically
impossible, it is highly desirable to remove all roots as thorough-
ly as is practical and to prepare the land by deep plowing,
which will bring to the surface roots which have been overlooked
in clearing. This is especially important where oak trees occur
on the land.
Unfortunately, no work has been done in regard to the pos-
sbility of controlling Clitocybe root rot although more than 125
citrus trees have been treated recently in Florida to determine
this point. However, considerable valuable experimental work
has been done in attempting to control the closely related Armil-
laria root rot. From the known facts it would appear that there
is but little hope of saving trees attacked by either of these
mushroom root rot fungi without careful surgical work and aera-
tion of the root crowns and crown roots, which calls for rather
tedious and painstaking work. The mycelium of these root-
rot fungi is very sensitive to drying out, and the removal of the
soil from under the bases of the trees so as to leave the root
crowns and crown roots exposed for aeration and drying has
proved to be very beneficial in checking the development of these
root-rot fungi. Considerable experimental work has been done
in attempting to control Armillaria and other root-rot fungi by
various chemical treatments of the soil, both in this country and
abroad, but without sufficiently consistent success to justify any
recommendations.
When trees in the grove are attacked by Clitocybe root rot
the control measures will be determined to a large extent by the
local circumstances. Two main problems are presented: (1)
the prevention of the spread of the disease to adjoining unat-
tacked trees and (2) the treatment of the trees already attacked.
Probably the most effective method of preventing the spread
of the disease when the attacked trees occur in a group is to
isolate them by digging a narrow trench of sufficient depth to
extend below all the communicating roots and cutting out all roots
crossing from either side. The dirt removed should be thrown
back on the inside of the area. A depth of from two to three feet
will probably suffice ordinarily under Florida conditions.






Bulletin 229, Diseases of Citrus in Florida


In order to be effective, remedial measures must be applied
before the disease has progressed to such an extent as to kill a
large percentage of the roots and girdle the root crown since,
after the disease has progressed to this extent, the death of the
tree is inevitable. Unfortunately, this extent of development
of the trouble usually is required before the symptoms of decline
develop in the top of the attacked tree to warn the casual ob-


~~-I





IxJ


-
r ..
--Wb
3-4:
.J-.. -' 7-~ T


Fig. 23.-Grapefruit tree treated for Clitocybe mushroom root rot,
showing the dead roots and diseased areas of bark cut out.
server that something is wrong with the tree. However, attack-
ed trees often can be located quite a while before the tops of the
trees have declined to any considerable extent by finding the
clusters of the mushroom-like fruiting bodies of the root-rot
fungus or sometimes only a slight gumming area of bark at the
bases of the trees.
Unless the top of the tree has declined considerably, the extent
to which the disease has attacked the root system can be de-
termined only by removing the soil sufficiently from around and
under the root crown and crown roots. This requires removing
.a basin of soil with a radius of from two to three feet from the
base of the tree, working carefully so as not to injure the roots.
Unless facilities are available for washing, the soil should first





Florida Agricultural Experiment Station


be worked away from the root crown with a trowel and a pointed
stick until a shovel can be used. One half of a posthole digger
makes an excellent implement to remove the soil under the root
crown when the lateral roots lie close together.
If it is practicable to treat the tree, all dead roots should then
be cut out, using a mallet and chisel or gouge to trim them back
to the root crown (Fig. 23). Special effort should be made to
remove all oak or other foreign tree roots that may be encoun-
tered under the citrus trees being treated. All dead roots of
either citrus or other trees which are cut out should be followed
out to the ends and removed in their entirety. All bark lesions
that may occur on the base of the trunk, root crown, lateral roots,
or taproot in case the latter is not entirely dead, should have the
dead or infected bark cut out back to healthy bark and cleaned
off down to the wood. Particular care should be taken to exam-
ine for lesions under the lateral roots, especially where they leave
the root crown. Where lesions have ceased activity and have
become limited by a callus formation at their margins it will
suffice merely to scrape off the dead bark down to the wood. All
pieces and chips of wood and bark cut out should be collected
in a container and piled at some convenient point for burning
later. A piece of heavy paper placed under the tree will prove
helpful in collecting the chips and small pieces of wood and bark
cut out, which, if allowed to remain, may act as sources of in-
fection. All trees not worth treating should be dug up, care be-
ing taken to remove and burn all roots. Where attacked trees
are removed, it is advisable to leave the excavations open for
a few months before replanting.
After completing the surgical treatment of the tree the root
crown should be left exposed for aeration and drying for a
few clear days, after which the treated areas should be brushed
clean of sand and inspected for any bark infections that may
have been overlooked, and all exposed wood surfaces should then
be painted with a durable waterproof wound dressing. After
this has dried the excavation may be filled in completely or the
immediate root crown may be left exposed indefinitely if prefer-
red. If the tops of the trees have died back they should be de-
horned proportionately and pruned of all dead wood. The lo-
cation of the treated trees should be recorded and they should
be inspected periodically to detect any subsequent development
of the root-rot fungus.






Bulletin 229, Diseases of Citrus in Florida


DIPLODIA COLLAR ROT AND OTHER BASAL BARK DISEASES
Caused by Diplodia natalensis Evans, and other fungi
Instances have been noted occasionally in Florida citrus groves
where the bark at the bases of young to moderately young trees
dies rapidly, as a result of which the trees suddenly become
girdled. A slight amount of gumming usually accompanies this
collar-rot type of girdling at the margins of the lesions and Dip-
lodia has been found associated with the early stages of this
trouble in various localities in the central portion of the pen-
insular section of the state. Diplodin has been found associated
with this type of trouble also in the Satsuma district of West
Florida. Young trees seem to be attacked most frequently. In
many cases only a single tree may be affected while, in others,
a group of from a few to several trees may be involved. Un-
fortunately, this trouble is rarely observed by growers until the
trees have become more or less completely girdled. When ex-
amined at this stage, the entire root systems of young trees are
often found to be more or less dead and with the bark so far dis-
integrated that no pathogenic organism is in evidence. Grow-
ers often attribute this trouble to foot rot or to lightning injury.
Further observation and study will be required before the exact
cause is demonstrated conclusively.
Other basal bark diseases of older citrus trees have also been
encountered from time to time in Florida groves. In some cases
these appear to be caused by a form of gummosis while in other
cases the trouble is an inconspicuous dying of the bark with
little or no gum formation. These diseases have one point in
common, however, in that they frequently girdle trees rapidly
and may result in considerable loss unless detected and treated
in time. Unfortunately, but little is known concerning them.
Trees attacked by the collar rot type of girdling or other basal
bark diseases should be treated promptly if discovered before
they are too seriously girdled. The diseased bark should be cut
out to a point slightly in advance of the discoloration on the
underlying wood. After this bark has been removed down to the
underlying wood the treated place should be painted with some
good disinfectant such as lime-sulphur solution. When the wood
has again dried, all exposed surfaces should be painted with a
durable waterproof wound dressing. Care should be taken at all
times to prevent the occurrence of bark wounds and to treat






Florida Agricultural Experiment Station


them properly when they occur, since these bark attacking fungi
often gain entrance through injured places.
GUMMOSIS
Caused by various fungi, chemical injuries, and
physiological stimuli
Gummosis has been regarded as an extremely important dis-
ease in virtually all citrus-growing countries of the world, be-
ing mentioned as early as 1646 in the Latin work on Citrus by
Ferrarius. This disease, in its various forms, .has caused wide-
spread destruction in many of the older citrus-growing coun-
tries. The forms of gummosis mentioned in the earlier lit-
erature, however, are not described with sufficient accuracy to
identify each with certainty.
Records place the appearance of gum diseases of Citrus in the
United States at about the year 1875 in California, and 1876 in
Florida. In the former state gummosis was a serious trouble
in nearly every citrus locality by 1878. The horticultural litera-
ture of this period indicates that the discontinuance of the use
of the common lemon, lime, and citron stocks in California was
due to this disease.
Gummosis, today, is one of the more important citrus dis-
eases in Florida. Of the citrus trees commonly cultivated, the
lemon is most susceptible and the sour orange the most resistant.
The grapefruit is also very susceptible, the sweet orange quite
so, while the tangerine is very resistant to true gummosis. Trees
on rough lemon stock appear to gum worse than those on sour
orange stock.
SYMPTOMS OF GUMMOSIS
Gummosis in Florida is chiefly confined to trees that have
reached the age where they are capable of bearing good-sized
crops of fruit. A form of this disease which commonly attacks
cultivated lemon trees as soon as they attain good bearing size
is characterized by the disease starting just above the bud union,
from which point it spreads up the trunk and involves the basal
portions of the main limbs (Fig. 24). In the grapefruit and
orange trees, however, the disease rarely starts at the bud union
but usually at various points higher up on the trunk or on the
larger branches. In addition to these forms of gummosis, minor
forms of gumming may occur on different parts of trees in con-





Bulletin 229, Diseases of Citrus in Florida


nection with various injuries and also with certain diseases at-
tacking twigs and small branches.
The first symptoms of gummosis are the development of one
or more slight cracks
in the bark and the
oozing out of a pale-
colored liquid g u m
which usually runs
down the bark in
streak s, forming
"tears". Later, the
exuded gum hardens
and darkens upon ex-
posure to the air and
collects in I a r g e
quantities at the
cracked places in the
bark, often becoming
so abundant as to
completely obscure
the cracks in the
bark (Fig. 25). The
gumming areas may
be confined to one
side of a trunk or
limb or may extend
entirely ar o u n d
them. The gum ex-
udes in the greatest
abundance d u r i n g
the period of active
growth in the spring
and early summer
and frequently ceases
entirely in the fall
and winter.
I f t h e exuded Fig. 24.-Severe case of gummosis on lemon
masses of gum be cut trunk.
off and the outer, dark part of the bark scraped away over the
gumming portion of the trunk or limb the various stages in the
development of the disease can be seen (Fig. 26). The most re-





Florida Agricultural Experiment Station


cently formed cracks in the bark will very likely be found to be
free from discoloration, while in those of longer standing the
bark will be found to exhibit a dark color on either side but not
necessarily extending into the cambium. In still older cracks the
discolored area will have enlarged and the bark may be dead
down to the wood. In some cases the dead patch of bark will be
found to be loose and springy, due to the accumulation of liquid
g u m beneath i t.
This is the result of
v t t h e cambium, o r
tios growing layer, form-
Siing elements which
degenerate into gum
instead of forming
normal woo d ele-
ments. With the con-
tinued development
of the disease, addi-
tional cracked, gum-
ming areas appear in
the bark and, eventu-
ally, extensive areas
become involved.
In some cases
where the gumming
occurs i n isolated
spots the gum flow
may become checked
a n d the ruptured
places in the bark
h e al over. In such
cases, new bark
forms under the old
which dries out,
hardens, and is
Fig. 25.-Close-up view of portion of grape- forced up in irregu-
fruit trunk affected with gummosis just lar partially detached
above the bud union. Note copious exuda-
tion of gum down center of trunk; also note scales o r narrow
cracks in bark at left (indicated by arrows) strips. This exposes
that probably will exude gum later.
lig h t colored to
brownish, resinous-looking scars to which the loosened bark re-






Bulletin 229, Diseases of Citrus in Florida


mains attached at the margin for a time. In some cases the dis-
ease may remain quiescent for a time and later break out anew
beyond the edge of the old area. In the well advanced cases of
this type of the disease the bark acquires a scaly, ulcerated ap-
pearance which may involve considerable portions of the trunk
or branch (Fig. 27). Gum may exude from the gumming areas
more or less continuously or the gumming may cease for a time


and much of the old
exuded gum may be
washed a wa y by
rains. In the latter
case there is left only
a scaly appearance
which may be mis-
taken for psorosis by
those not especially
familiar with these
respective diseases.
I n gummosis, how-
ever, the lesions rare-
ly extend around the
trunk or limb in
band-like fashion, as
is commonly the case
with psorosis. In
severe, or long-stand-
ing, cases of gummo-
sis the bark over por-
tions of the affected
area dies to the wood
and the wood itself
will be found to have
been invaded and
discolored. When this
stage is reached, the
affected limb or trunk


Fig. 26.-Portion of grapefruit trunk with early
stage of gummosis (just above bud union)
treated by scraping off the outer, dark part
of the bark over the diseased portion. Note
cracks in the bark from which gum has been
S.
ii















Fig. 26.--Portion of grapefruit trunk with early
stage of gummosis (just above bud union)
treated by scraping off the outer, dark part
of the bark over the diseased portion. Note
cracks in the bark from which gum has been
exuding, and the discoloration of the bark
starting about the older cracks above.


may die from girdling.


Cross sections of trunks or branches on which gummosis has
been developing for a number of years invariably show a series
of lines of compactly grouped gum canals in the outer portion
of the stem, as in psorosis (Fig. 31), scaly bark and other citrus
diseases characterized by gum formation. These lines of gum






























































Fig. 27.-Portion of grapefruit trunk with old, inactive case of gummosis.
Note the numerous cracks and gum pockets in the bark that were
formerly gumming actively but are now healing up and sloughing off
the dead bark.






Bulletin 229, Diseases of Citrus in Florida


canals may be short, variously interrupted, or extend continu-
ously around the trunk or branch. They often alternate with
normally developed wood accretions.
Considerable variation occurs in the severity of gummosis
and the rapidity with which it causes the tree to decline. In
some cases the disease may be present for a number of years
without seriously affecting the tree, while in other cases it de-
velops rapidly and kills large areas of bark within a short time.

CAUSE OF GUMMOSIS
Citrus trees, in common with stone fruit trees and a number
of others, are characterized by a strong tendency to gum. The
formation of gum may be induced by various fungi, by chem-
ical, mechanical, and insect injuries, and by certain physiologi-
cal stimulations. Although minor forms of gumming may be
induced by a number of citrus diseases, both parasitic and non-
parasitic, and by certain chemical and mechanical injuries, there
is a characteristically severe and often chronic form of gumming
of uncertain cause on large bearing trees that is commonly term-
ed gummosiss."
The various citrus diseases characterized by the formation of
gum cannot be diagnosed by the occurrence of the gum itself,
since its nature and appearance are essentially the same regard-
less of the influence that induced its formation. The process of
gum formation is a pathological condition in which the cell walls
of the wood-forming tissue, in certain more or less localized zones
of special cell formation, degenerate either partially or entirely.
This transformation is a hydrolysis process, apparently result-
ing from enzyme action, which may change the walls of entire
cells into gum while they are still in the developmental stage.
When gum forms in sufficient quantity to rupture the bark by
the internal pressure exerted the gum exudes and runs down the
trunk. In a sense, the gum is merely a by-product or secondary
feature of an abnormal condition or disease. It will thus be
seen that it is not the exudation of gum but rather the death
of the more or less large areas of bark that precedes, accom-
panies, or follows this gum formation that is most menacing
to the life of the tree.
Fawcett, in California, has shown recently that environment-
al conditions cannot, by themselves, initiate all the severe forms
of gummosis in Citrus earlier attributed to them, although many






Florida Agricultural Experiment Station


of these factors are bound to play an important role as con-
tributing conditions which favor infection of the trees by the
causal parasites. He has demonstrated that at least six types
of gummosis in California, including the most common and
destructive forms, are caused by parasitic organisms.
In Florida, however, but little is known concerning the cause
of gummosis. While some forms of it unquestionably are caused
by the attack of certain parasitic organisms, others appear to be
initiated by totally different causes and to be aggravated by the
subsequent invasion of the tissues by various organisms. The
relation of a fungus, apparently identical with Diplodia natalen-
sis, to the gumming and death of citrus twigs and branches in
Florida was reported by Fawcett and Burger in 1911. This or-
ganism was isolated from a number of gumming areas on citrus
trees and was shown by inoculation on healthy trees to produce
copious gumming. In 1913, Fawcett reported having repeatedly
isolated this fungus from the discolored wood at different depths
under gumming areas on trees in widely separated localities.
He also showed that certain other fungi, when inoculated into
citrus trees, will produce some gumming, but he found none that
equalled Diplodia in the amount of gum produced.
Stevens, who later worked on gummosis in Florida for a num-
ber of years, isolated and studied a number of fungi and bacteria
from gumming areas of citrus trees and found Phomopsis citri
to occur with the greatest frequency. However, his inoculation
experiments with this organism, Diplodia natalensis, and tissues
from gumming areas gave negative results. Gumming was in-
duced in a number of cases but the wounds soon healed with but
little gumming of the tissue, especially when the inoculations
were made on young trees. Inoculations on old trees gummed
more freely and much larger areas of tissue were killed in most
cases. However, typical gummosis areas did not develop from
any of these inoculations. Therefore, Stevens questioned whether
any organism is directly responsible for the initial stage of
gummosis. However, he concluded that, in the later stages of
the disease after the cracks had formed and enlarged in the bark,
Diplodia, Phomopsis, and other fungi probably were responsible
for the gradual enlargement and aggravated condition of gum-
mosis areas. While it is evident that such organisms do fre-
quently invade gummosis lesions and aggravate the trouble, the
senior writer has observed a number of instances where active






Bulletin 229, Diseases of Citrus in Florida


cases of gummosis started in improperly made or unprotected
pruning wounds on healthy trees within less than a year from
the time they were made. This would indicate that the infection
of wounds by certain organisms undoubtedly may initiate the
development of at least some forms of gummosis in Florida.
On the other hand, a severe outbreak of gummosis was recently
observed on a large number of vigorously growing young grape-
fruit trees which had been given a heavy application of nitrog-
enous fertilizer. The disease had broken out at numerous points
on the trunks, larger branches, and even on the twigs, but healed
up in a few months in many cases. It was evident that in this
case no organism was responsible for initiating the trouble but
rather some excessively strong physiological stimulus which re-
sulted in the formation of a large amount of gum instead of
normal wood elements by the growing tissues. It is of interest
in this connection to state that the effect of various chemical
stimuli in inducing gum formation in citrus trees is well known.
Excessive concentrations of copper sulphate in the soil, or when
applied to the trees as injections, will induce copious gum forma-
tion. In some cases fumigation with hydrocyanic acid gas and
the spilling of cyanide or its products on the roots will also in-
duce gum formation. It is quite evident that further investiga-
tion of the gummosis problem in Florida is required, particularly
to demonstrate the different types of gummosis that may occur
and the extent to which parasitic organisms or other agencies
are responsible for each.
CONTROL OF GUMMOSIS
It is believed that gummosis can be prevented to a large extent
by maintaining the trees in a vigorous condition and protecting
them, as far as possible, from shocks or weakening influences
occasioned by severe storms, water injury, drought, starvation,
cold, or other adverse influences. Making pruning wounds prop-
erly and protecting the wounds by applying a disinfecting water-
proof dressing will also do much to minimize the development of
gummosis.
Trees attacked by gummosis should not be neglected with the
hope that they will eventually recover. Although slight cases of
gummosis may do little injury and small spots may heal without
attention, in the majority of cases the gumming becomes chronic
and spreads to such an extent as to threaten the life of the tree.
All diseased trees should be located and treated promptly. Gum-





Florida Agricultural Experiment Station


mosis can be cured fairly readily in most cases by proper treat-
ment, provided the disease has not progressed beyond the inter-
mediate stages. However, the ease of treatment and likelihood
of cure are decreased greatly when the disease has been allowed
to go unchecked for several years.
The bark-scraping method recommended for the treatment
of psorosis has been found equally effective for the treatment
of the various forms of gummosis so commonly seen on older
bearing trees in Florida. Since this method of treatment is
so similar for the two diseases, the reader is referred to the sec-
tion on the control of psorosis, where a detailed account of the
treatment is given in connection with the control of that dis-
ease (see page 80). The bark-scraping method here recom-
mended has proved vastly superior to the old method heretofore
used, which consisted merely in scraping off the loose, scaling
bark and masses of exuded gum, cutting out the gumming areas
of bark down to the wood and applying some disinfectant, usual-
ly carbolineum. Aside from its inefficiency and likelihood of in-
juring the trees with the carbolineum, such a method resulted
in unsightly wounds which required years to heal and paved the
way for the entrance of wood-decaying fungi.
PSOROSIS
Cause unknown but undoubtedly an organism
Psorosis* was first described briefly from Florida by Swingle
and Webber in 1896. In 1908 this disease became known in
California under the name "scaly bark." However, this name
is reserved for an entirely different disease in Florida. Psoro-
sis is generally understood by Florida growers, but erroneously
so, to be a form of gummosis. This disease, or a similar one,
has been reported in nearly every important citrus growing
country of the world. The available evidence led Lee to conclude
that psorosis originated in the Orient and has been distributed
to other citrus-growing countries with the spread of Citrus va-
rieties and species.
Psorosis occurs throughout the citrus-producing section of
Florida, affecting tangerine, orange, and grapefruit trees to a
serious extent. Tangerines are extremely susceptible to psorosis
but seem to withstand the ravages of the disease longer than
*This name is from the Greek word meaning ulcer. It is pronounced
so-ro'-sis.







Bulletin 229, Diseases of Citrus in Florida


either oranges or grapefruit. Of the oranges, the Valencia ap-
pears to be most severely affected. The sour orange is highly
resistant, although not immune, to the disease. In many groves
as high as 15-25 percent of the orange trees and often 50 per-
cent or more of the tangerine trees are affected by psorosis. Oc-
casional instances occur where these figures run much higher.

SYMPTOMS OF PSOROSIS
Psorosis is a disease affecting the bark of the trunks, main
limbs, branches and,
occasionally, e v e n
large twigs. Although
psorosis is generally
classed as a gum dis-
ease, gu m exudes
only during the act-
ive growing season
of the tree. As a rule,
the exudation of gum
is very small and
often may be entirely
lacking. The frequent
slowness of the de-
velopment of the dis- 4'1
ease, its inconspicu-
ous appearance and
lack of effect upon
the foliage i n t h e
earlier stages, fre-
quently prevent it
from being noticed
by growers until it
has become well ad- Fig. 28.-Early stage of psorosis started about
vanced. an untreated pruning wound on a tangerine
limb, showing the superficial scaling of the
Psorosis usually be- bark in small pieces.
gins as an inconspic-
uous spot in which the bark dies and becomes infiltrated with
gum. The first conspicuous symptoms of the disease are the
cracking and scaling off of the outer portions of the bark from
around the small initial diseased spot. In this stage the inner
layer of bark next to the cambium is still alive, active and free






Florida Agricultural Experiment Station


from discoloration, except for a slight yellowish tint. As the
disease progresses, which often occurs very slowly at first, the
original scaly spot enlarges from year to year and is accompanied
by additional bark scaling. Even in the intermediate stages,
psorosis may completely encircle the trunk or limb without caus-
ing serious injury to the underlying wood.
Up to a certain point in the development of psorosis, new
bark forms under the
outer bark that loos-
ens and scales off
but, instead of re-
maining healthy, the
disease breaks o u t
anew in this bark
and the scaling pro-
cess continues. The
bark within the af-
fected area thus be-
S comes considerably
thickened, rough and
brownish, giving this
area a more or less
swollen and distinct-
ly ulcerated appear-
ance. In this stage
the lesions are very
conspicuous because
of the pronounced
.. breaking up of the
i.t outer bark into large
flakes, sca 1 e s, or
shaggy strips which
curl away from the
trunks or branches
as they slough off
(Fig. 29). After the
lesions encircle the
trunk or branch they
Fig. 29.-Psorosis on orange tree, showing well continue to increase
advanced stage in which the disease has
spread entirely around the divisions of the in l e n g t h. Scaly
trunk. bands are thus form-
ed which vary in length from a few inches on the smaller






Bulletin 229, Diseases of Citrus in Florida


branches to from two to three, or even more, feet on the larger
branches, main limbs and
trunks. In long-standing
cases of psorosis, the dis-
ease frequently involves
the entire main trunk
above the bud union and
also many of the main
limbs. Independent le-
sions may occur farther
up on the limbs and even
on the smaller branches
of the tree (Fig. 30).
In the well advanced
stages of the disease, in-
ternal gum formation
begins in the cambium
and embryonic wood
cells and a yellowish dis-
coloration of the inner
bark may appear, due to
the gum infiltrating the
tissues. Gum may ap-
pear on the surface of
the bark, especially dur-
ing the active growing
period of the tree, but its
exudation is never a s
copious as in gummosis,
usually being reduced to
scattered drops at either
end of the lesion. A
cross-section of an at-
tacked trunk or branch
in the late intermediate
stage of the disease may
not show any discolora-
tion except dark brown
lines of closely crowded
gum canals in the outer
Fig. 30.-Psorosis lesion on orange
layers of the wood (Fig. branch which is virtually girdled.






Florida Agricultural Experiment Station


31), as described in this connection under gummosis (See page
69). It is only in the late stages of psorosis, after the disease
has been present for probably five or more years, that the wood
begins to be seriously injured. This starts as a drab discolora-
tion just under the diseased bark. Almost the entire interior,
with the exception of a narrow layer of wood just beneath the


Fig. 31.-Cross-section of orange branch on which psorosis has been
active for the past few years. Note the dark concentric lines in the
outer layers of growth, which consist of compact rows of gum canals.
(Natural size.)
cambium or growing layer may, in certain cases, become dis-
colored in this way but remains firm and hard unless wood-de-
caying fungi invade the dead tissue.
After the inner bark tissue becomes involved the growth of
the tree is reduced and the cambium may be killed in scattered
spots or even over large areas. This results in the decline of
the top of the tree in so far as it was dependent upon the part





Bulletin 229, Diseases of Citrus in Florida


that had been attacked. -The leaves of this part of the top be-
come reduced in size and number. Twigs die back, followed by
the death of the smaller branches, and this part of the tree


Fig. 32.-Orange tree dying on one side from psorosis. Two of the three
main limbs have been girdled by an old lesion occurring at the junc-
tion of the limbs.

ceases to be productive. When the cambium becomes injured
to such an extent that the attacked limb or trunk is more or less
completely girdled, this part of the tree dies (Fig. 32).





Florida Agricultural Experiment Station


CAUSE OF PSOROSIS
Nothing definite is known concerning the cause of psorosis.
The nature of this disease, its spread on the tree and to other
trees in the grove, and the different degrees of susceptibility ex-
hibited by different kinds of citrus trees strongly indicate that
it is caused by an organism. Fawcett, working in California, has
isolated many kinds of organisms from first-stage lesions but
none have produced the disease when inoculated into citrus trees.
He states that, in certain cases, it has been possible to transmit
the disease to healthy trees by inoculations with bits of tissue
from diseased bark but that most of the attempts have failed.
In one of the successful cases about two years elapsed before the
disease became visible. Efforts are also being made in Florida
to isolate the causal organism from psorosis lesions.
Observations, in both California and Florida, indicate that
wounds or injuries of various kinds may pave the way for the
development of psorosis. This disease has been observed in sev-
eral cases to start on the shoulder of the callus formation about
old pruning cuts. In many cases, however, the disease starts
in branch crotches and at other places where no bark injury is
apparent. Although primarily a disease of bearing trees, at
least from 10 to 12 years old, psorosis occasionally occurs on
much younger trees in Florida. A severe outbreak of this dis-
ease was recently observed on tangerine nursery stock in a nurs-
ery in a densely wooded, low hammock, where especially favor-
able conditions existed for disease development.

CONTROL OF PSOROSIS
Although the cause of psorosis is not known, a bark-scraping
method of treatment, which is described later, has been found
very effective for the control of this disease. Fawcett has con-
clusively demonstrated the effectiveness of this method of treat-
ment for the control of psorosis in California when the disease
has not progressed to the late stages. His experiments have
shown the advisability of thoroughly scraping not only the area
visibly affected but also well beyond this area to head off the
advance of the disease in bark not yet visibly affected. He has
tested a large number of disinfectants, both with and without
scraping the outer bark. Almost without exception, the diseased
trees on which the bark was carefully scraped,showed the best





Bulletin 229, Diseases of Citrus in Florida


recovery. Unscraped lesions continued to increase in size and
at the same rate whether disinfected or not.
The bark-scraping method for the control of psorosis has been
tried out extensively in Florida by the senior author and has
given excellent results. Since gummosis and psorosis usually
have not yielded to the haphazard methods of treatment formerly
used by growers, many growers simply have become resigned to
the occurrence of these bark diseases and consider it useless to
attempt to control them. Like gummosis, however, psorosis can
be cured fairly readily by the proper method of treatment, if it
has not progressed beyond the intermediate stages, although
the ease of treatment and likelihood of cure are decreased when
the disease has been allowed to go unchecked for several years.
When psorosis has progressed to such an extent as to involve
more than one-half of the circumference of the trunk or branch
and has begun to involve the cambium but has not seriously in-
jured the wood, the disease may still be checked and frequently
cured by careful scraping, although the cure is more uncertain.
Where the disease has been present for a number of years, or
until the greater part of the bark of the trunk or limb is af-
fected and the wood underneath is killed and discolored, there is
little hope for a permanent recovery. If the bark on the trunk
is badly killed or the top of the tree as a whole appears stunted
or unproductive, the tree should be removed.
Since the bark-scraping method of treatment applied equally
well to both gummosis and psorosis and since Florida growers
usually do not distinguish between these two bark diseases, the
method of treatment for both these diseases is here described.
The principle governing the treatment of these bark diseases by
this method is the elimination of the diseased outer bark tissue
without interfering with the production of new, healthy bark
from the cambium, or growing layer. This is accomplished by
scraping off the outermost, diseased layer of bark and applying
some good disinfectant paste or wash, such as lime-sulphur paste
or wash or bordeaux paste, as soon as the bark lesions on each
tree have been thoroughly scraped. This allows the remaining
outer, corky layers of bark to dry out and checks the development
of the disease. This part of the bark will. slough off later in
flakes or scales of variable size. The slight exudation of gum
from season checks that develop in the freshly scraped bark sur-
faces is no indication that the treatment will not be successful,





Florida Agricultural Experiment Station


as this is bound to occur as a result of the liberation of the gum
already formed under the bark.
In treating trees attacked by either gummosis or psorosis,
first cut off all dead branches and all those badly weakened by
the disease. Next cut off all gum masses that may be present
and remove all loose, scaling bark. Then carefully scrape off the
dark, outer bark over all the affected parts of the trunk or limb
to be treated. The scraping should be just deep enough to remove
almost all the green layer that is found just beneath the outer-
most dark-colored, corky layer and should not extend deeper
than about a third of the way through the bark; otherwise the
bark is likely to die. It is also extremely important to scrape
the bark for a distance of from at least 4 to 6 inches above and
below and from 3 to 4 inches laterally beyond the margin of the
affected bark in case the lesion does not extend more or less com-
pletely around the trunk or branch. The scraping of the bark
in advance of the lesion can be done more lightly.
Discolored areas that appear in the bark about the gumming
cracks when the bark is scraped, in the case of gummosis, should
be disregarded unless the bark is dead clear down to the wood
or has been loosened by gum formation beneath it, in which case
the dead area of bark may be cut out. The latter treatment in
the case of gum pockets, however, is by no means necessary since
the loosened bark will eventually be sloughed off after the scrap-
ing treatment by the callus formation developed from the margin
of the living bark. In cutting out gum pockets the excisions
should be narrowed or slightly pointed at the upper and lower
ends in order to secure the most rapid healing.
After the bark has been thoroughly scraped as directed, the
scraped area should be painted with some good, safe disinfect-
ant paste or wash, as discussed later. All areas of exposed wood
should then be painted with some good waterproof wound dress-
ing to prevent the entrance of wood-boring insects and decay-
producing fungi. The application of a fungicidal paste or wash
to the freshly scraped bark is of value in checking the develop-
ment of the organism causing the disease, in preventing subse-
quent infection, and in stimulating the drying up and sloughing
off of the outer layer of scraped bark. Of the various disin-
fectants thus far tested, the lime-sulphur or lime and sulphur
compounds appear to be the most effective in stimulating the
bark sloughing process, while thick carbolineums, paints and







Bulletin 229, Diseases of Citrus in Florida


other preparations that exclude the air retard it. Carbolineums
are not recommended for the treatment of either psorosis or
gummosis because of their variable nature and the tendency of
some to be injurious. None of those tried have proved to be
any more effective than a number of much cheaper preparations,
such as lime-sulphur pastes and washes or bordeaux paste, all
of which are much easier to obtain and are perfectly safe to use.
Within a period of from three to six months from the date of
treatment, depending upon the fungicide used and the kind of
tree, the vigor of the tree and the time of the year it was treated,
the outer corky portion of the scraped bark will crack loose and
slough off, exposing the new and usually healthy bark. Bark
treated during the spring months appears to recover most
rapidly.
All trees treated for either gummosis or psorosis should be
inspected at intervals of a few months, especially during late
spring and early summer, when these bark diseases develop most
rapidly. In case the bark was not scraped sufficiently far in
advance of the disease to check its development or if new in-
fections develop, an additional area should be scraped and treated
as before. Growers cannot expect an infallible cure in advanced
cases of either gummosis or psorosis by a single treatment, since
a second and sometimes even a third, supplementary treatment
may be required. In order to be effective, the method of treat-
ment recommended must be administered carefully, thoroughly,
and intelligently. The importance of scraping the bark well
in advance of the margins of the lesions and of the follow-up
treatments where the development of the disease has not been
checked cannot be too strongly emphasized.
Tools-A number of different types of scraping implements
may be used for scraping bark attacked by psorosis or gummosis.
The most important feature of such tools, in addition to con-
venience and efficiency for the work, is that they have a sharp
scraping edge. A box scraper and a farrier's knife have been
found to be the most useful implements thus far tried. The
former is excellent for heavy work; the latter, with the end
curved slightly, is fine for work in crotches and for gouging out
small diseased areas or gum pockets in the bark. The box
scraper comprises a reversible, triangular-edged scraper mounted
in a handle with a slightly curved shank. If desired, one or more
of the corners of the scraping blade may be ground off round






Florida Agricultural Experiment Station


to enable one to scrape in crotches and other restricted places.
In scraping old psorosis lesions, where a scraper jumps over the
hard, gum-infiltrated spots in the bark, a spokeshave sometimes
proves useful.
SCALY BARK OR NAIL-HEAD RUST
Causal organism not definitely known
Scaly bark, or nail-head rust, as the disease on the fruit is
termed in Florida, is a disease chiefly confined to the sweet
orange. This disease has been termed leprosis by Fawcett and
Lee. It was formerly thought to occur only in Florida but re-
cently it has been found in southern China, the Philippines, and
Mexico. Its present distribution in Florida is limited to a few
of the citrus-growing sections of the state. While scaly bark
was first observed in the Pinellas peninsula about 1860, it is re-
ported to have spread very slowly and almost imperceptibly for
several years and to have been known for at least 30 or 40 years
before serious damage was reported. Scattering cases have
also been found in the central part of the slate and in a few
localities along the central portion of the East Coast.
Scaly bark has caused serious damage in some of the older
sweet seedling orange groves of Florida. It has been estimated
by Fawcett that the annual loss of fruit alone from spotting as
a result of this disease in the Pinellas peninsula has amounted
to between 20,000 and 30,000 boxes, the loss from prematurely
dropped fruit in badly infected groves in this section being
estimated at from 35 to 75 percent of the total crop. In ad-
dition to the fruit loss, there is a constant weakening of the
trees from the scabbing and scaling of the bark and the dying
back of the attacked twigs and branches.
While scaly bark is confined almost entirely to the sweet
orange, it occasionally attacks other kinds of Citrus. It has been
found to occur on rough lemon trees in old neglected nurseries
and groves and also occurs on the sour orange. A very light in-
fection may be observed occasionally on grapefruit, tangerine,
and mandarin orange trees. The latter, however, are very re-
sistant to scaly bark, even when growing in close proximity to
badly diseased sweet orange trees.
Scaly bark is a peculiar disease which is as yet but poorly
understood. It has been suggested that moisture at the right
season for infection is an important factor in the severity of this






Bulletin 229, Diseases of Citrus in Florida


disease, since virtually all known cases of scaly bark occur in
damp localities near bays, lakes, rivers or swampy hammocks.
Strange to say, in many groves where the disease was known
to have been very
severe i n former
years, it has decreas-
ed greatly within the
past few years with-
out any special steps
being taken to com-
bat it. This marked
decrease in the prev-
alence and severity
of scaly bark within
late years is believed
to be due to the in-
crease in the use of
fungicidal spra y s
and to the improve-
ment in grove man-
agement. At pres-
ent the disease seems
to be most virulent
in badly neglected or
abandoned groves.
In 1925, grove in-
spections of 10,000
citrus trees that had
been grown in scaly
bark territory and
set out at various
points along the mid-
dle East Coast show-
ed no scaly bark in-
fections. This a n d
other investigations
indicated that scaly
bark should no long-
er be classed as a dis- Fig. 33.-Scaly bark spots on sweet seedling
ease of such impor- orange twigs, showing early stages of the
disease. The youngest stages show as slight
tance as to justify yellowish spots. (Twice natural size.)






Florida Agricultural Experiment Station


the former drastic quarantine measures adopted by the State
Inspector of Nursery Stock in 1911 and the State Plant Board
in 1915, which prohibited the shipment of citrus stock grown
in areas declared to be scaly bark territory, to points outside
of such areas. Accordingly, these were greatly modified in 1925.

SYMPTOMS OF SCALY BARK
Scaly bark is primarily a disease of the bark and of the rind
of the fruit. The greatest injury is done to the twigs and young-
er branches up to about one-half inch in diameter, and to the
fruit. The symptoms of the disease are very distinctive and
characteristic, especially in the early stages.
On the Twigs and Branches.-In their initial stages, the young
spots appear on the surface of the green bark as inconspicuous
greenish-yellow spots which have a somewhat translucent ap-
pearance (Fig. 33). According to Fawcett, twig infections may
occur at any time of the year but the greatest number appear
to occur between about June 1 and December 1. He found that
the spots rarely occur on twigs which are less than six months
old and that they are more commonly observed in the bark of
growth from nine months to a year old. On twigs of this age
the spots, in the course of eight or ten months, become from one-
sixth to one-half inch in diameter, raised above the surface of
the surrounding bark, rusty in color, and with well defined mar-
gins. As the spots grow older the bark becomes glazed, then
brittle, develops longitudinal cracks, and finally breaks into
small flakes and scales (Fig. 33). A zonate effect is often seen
in the more advanced stages. The spots may be isolated and
distinct or, in severe cases, they may increase in number until
several join together, forming large patches of rusty or red-
dish-brown, scaly bark (Fig. 34) ; hence the name of this disease.
The branch is rarely killed the first year. During the second
year additional spots may form between the old ones and this
may continue for several years until the bark becomes decidedly
rough and scaly and the branch is finally killed as a result of
girdling. The watersprouts, which develop abundantly when
badly attacked trees are cut back, are especially susceptible to
infection.
On the larger branches and trunks, where the disease is of
long standing, the original spots are no longer distinct and the
surface of the affected bark becomes broken into thin flakes or





Bulletin 229, Diseases of Citrus in Florida


scales. Ultimately the trunks and larger branches acquire an
extremely rough and shaggy appearance (Fig. 35). Pieces of
bark one-half inch or more in size scale loose, sometimes over
wl ll-lli M IVIM W


Fig. 34.-Scaly bark on watersprouts of sweet seedling orange tree, show-
ing intermediate stages of the disease. (6/7 natural size.)






88 Florida Agricultutral Experiment Station


















i




I ,.




































Fig. 35.-Advanced stage of scaly bark on sweet seedling orange tree,
showing roughened bark on the branches and nail-head rust spots
on the fruit.






Bulletin 229, Diseases of Citrus in Florida


areas of considerable size. New bark often forms under the old
as the latter cracks off and only rarely do dead areas on the
trunks or large limbs result from this disease. Where the dis-
ease has been of several years' standing, the branches often
exhibit band-like lesions encircling them, which closely resemble
psorosis.


Fig. 36.-Nail-head rust spots on orange.
In cross sections of old diseased branches on which the bark
has become quite rough and scaly, lines of gum canals, identical
with those shown for psorosis, in Fig. 31, are invariably found in
the outer layers of the wood. These occur intermittently in the
wood tissue formed after the start of the disease. They may
be either partly or entirely continuous around the stem and may
be repeated at varying intervals. In badly diseased branches,
new bark sometimes forms under the old as the latter sloughs






Florida Agricultural Experiment Station


off. Twigs and branches attacked by scaly bark frequently ex-
hibit more or less formation and exudation of gum.
On the fruit, where characteristic dark-colored, sunken spots
develop (Fig. 36), the disease is quite distinctive and is com-
monly termed "nail-head rust". The disease starts as round,
or occasionally ring-like, spots which usually begin to appear in
July and August and may continue to appear until the fruit
ripens. As the fruit approaches maturity, and while it is still
green, the spots become sunken and brown, later becoming dark
brown and hard. Secondary organisms, such as the withertip
fungus, frequently appear to invade the spots and intensify the
injury. The spots on the fruit usually are round and vary in
diameter from one-fiftieth to one-half inch. Fruits spotted by
scaly bark color prematurely and many of them drop before the
picking season. Occasionally there is a slight exudation of gum
from the spots on the fruit. The disease affects only the rind
of the fruit but produces an unsightly and unsalable fruit.
On the leaves:-Spots also occur occasionally on the leaves but
are of no economic importance. They appear as brown blotches
showing on both sides of the leaf. The tissue of the spot be-
comes hard, glazed, and brittle and the spots eventually become
surrounded by a yellowish border.

CAUSE OF SCALY BARK
Fawcett's investigations indicate that a fungus similar to
Cladosporium herbarum Lk., which has been known as C. herb-
arum, var. citricolum, may be responsible for the initial spots
and that Colletotrichum gloeosporioides, which causes wither-
tip, and other fungi may be important secondary agents in the
damage done by the disease in the later stages. Phomopsis citri
and Diplodia natalensis, two fungi which cause stem-end rot, are
also frequently found in the dead twigs and branches that ac-
company this disease under Florida conditions. Considerable
work has been done in Florida recently by West in isolating and
studying the organisms associated with scaly bark lesions. A
number of fungi and bacteria have been obtained but none of
them have reproduced the disease when they were inoculated
into citrus twigs of various ages.
Scaly bark is quite slow in its growth, developing so gradu-
ally into its destructive form that the growers are often unaware
of its importance until it has been present for several years. It





Bulletin 229, Diseases of Citrus in Florida


seems clearly evident that the disease can be spread by picking
crews and in pruning operations.
CONTROL OF SCALY BARK
Experiments by Fawcett and others have demonstrated that
scaly bark can be controlled and that fruit losses from nail-
head rust can be greatly diminished by a combination of pruning
and spraying. In some groves, pruning alone suffices. The
trees should be pruned of all dead wood and, as far as practicable,
of all badly diseased watersprouts and weakened branches be-
fore growth starts in the spring. This treatment alone will prove
beneficial in lessening the sources of infection. All prunings
should be burned and the tools used on the diseased trees should
be disinfected before they are used on trees free from the disease.
After pruning, the trees should be given an application of bor-
deaux-oil emulsion early in February before they bloom. All
twigs and branches, as well as the main limbs, should be spray-
ed thoroughly, since the diseased spots are almost entirely lo-
cated on these parts. A second application of bordeaux-oil
emulsion should be made at some time between one and two
months after the bloom has shed. This treatment will suffice
for the control of scaly bark and nail-head rust under ordinary
conditions. Where the disease is exceptionally severe, however,
a third application of bordeaux-oil emulsion, made two months
after the second, will prove of value in protecting the fruit and
twigs against subsequent infections. In such case it may be ad-
visable to make one or two applications of bordeaux-oil emulsion
during the year following the treatment.
WITHERTIP AND ANTHRACNOSE
Caused by Colletotrichum gloeosporioides Penz. and
Gloeosporium limetticolum Clausen
Withertip and anthracnose have long been regarded as fre-
quent sources of trouble in Florida citrus groves. Although
most attention has been called to these diseases in Florida, es-
pecially in the publication by Rolfs in 1904, the same troubles
have been reported in nearly every citrus-growing country.
Two forms of these diseases occur, namely, one which attacks
the young growing tissues of the Key lime, and the more com-
mon and widespread form which usually attacks only mature,
weakened leaves, twigs, and fruit of oranges, lemons, grapefruit,
and other citrus trees. The latter form of withertip may occur





Florida Agricultural Experiment Station


on trees of any size from nursery trees to old bearing ones. It
rarely develops on trees that have been maintained in a vigorous
condition but is confined largely to trees that have become
weakened or injured as a result of inadequate fertilization, over-
cropping, drought, cold, injury from spraying, and attacks of
insects and other diseases, in which case the weakened twigs and
branches may be attacked and the unhealthy condition of the
trees becomes accentuated. In reality, withertip on citrus trees
other than limes, is largely a secondary trouble. The importance
of withertip and anthracnose have been so overemphasized in
past years that they still constitute a great bugaboo in the minds
of many Florida growers. With the exception of the form on
limes, which is distinct from that on other citrus trees, it is
obvious that these diseases cannot be regarded of as much im-
portance in Florida as they were formerly.
In southern Florida and the West Indies, withertip and an-
thracnose frequently cause serious damage to the buds, blossoms,
young fruits, and shoots of the variety of lime variously known
as the West Indian, Mexican, or Key lime. The only other lime
or Citrus variety that has been observed to be attacked by this
form of withertip and anthracnose is the Dominican Thornless
lime, which supposedly originated as a sport from the West In-
dies. The organism which causes withertip and anthracnose of
the lime is somewhat different from that causing these diseases
of other citrus fruits and is much more severe in its effect, fre-
quently causing serious losses to growers of limes in southern
Florida and parts of the West Indies.

SYMPTOMS
Since there are a number of different phases of this disease,
depending upon the part of the tree attacked, the symptoms
will be discussed under the following three subheadings, namely,
withertip, anthracnose, and russeting and tear-staining.
Withertip:-The term "withertip" is applied to the withering
and dying back of twigs and branches attacked by the fungus
causing this disease. On the various kinds of citrus trees other
than limes, it is the mature twigs and branches that are attacked
by the withertip fungus. The dying back often progresses slow-
ly, with the leaves turning yellow, withering and dropping grad-
ually. However, in some cases, it may be sufficiently rapid to
cause the leaves to wither and dry up before the abscission layer





Bulletin 229, Diseases of Citrus in Florida


has taken place, in which case they remain on the tree, giving
the attacked twigs and branches much the appearance of having
been scorched by fire (Fig. 37). This is soon followed by the
dropping of the fruit on the affected branches and the dying of
many of the twigs and smaller branches, and even good-sized
branches in severe cases of the disease. Ordinarily, after travel-
ing down the twigs and smaller branches a greater or less dis-
tance, a slight exudation of gum occurs and the disease becomes
self-limited, ending at a sharply defined line which separates the
ain A6:If 7 -1*,


Fig. 37.-Outbreak of withertip on orange tree. The foliage of several
branches is drying up as though scorched by fire.
diseased from the healthy tissue. After the diseased wood dies
the characteristic, minute black pustules, or fruiting bodies, of
the fungus develop abundantly (Fig. 38).
Outbreaks of a trouble resembling withertip often develop
during the winter months after the root systems of trees have
been injured by drought or fluctuations of the water table and
run their course within a month or two. The leaves and fruit
may fall from the affected branches but the branches and some
of the twigs often remain green and may develop new leaves




Florida Agricultural Experiment Station


in the spring. The trouble may appear suddenly on a few trees
in the grove and speedily develop on a large number of trees,
both young ones and o!d bearing ones. In such cases it is a ques-
tion whether the trouble should be regarded as withertip or as
merely a natural balancing
of the top of the tree inev-
itably following the reduc-
tion of the root system.
Outbreaks of withertip
often follow prolonged high
winds, even though they
are not sufficiently severe
to cause any apparent dam-
age to the trees. Withertip
also develops abundantly in
trees that have been se-
verely whipped by hurri-
canes, especially when the
tissues of the leaves, twigs,
and branches have been in-
jured by extreme desicca-
tion or by the action of salt
spray. Such injuries, to-
gether with the numerous
abrasions and wounds re-
sulting from winds of hur-
ricane intensity, afford ex-
cellent places for the spores
of the universally present
withertip fungus to gain
entrance. Owing to the
rapid propagation of the
withertip fungus in weak-
ened or injured tissues of
citrus trees, it is difficult to
distinguish between injury
initiated by the withertip
fungus and that initiated
Fig. 38.-Withertip on small lemon by injuries alone, after
twigs, showing the sharp line of de-
markation between the dead and liv- which the fungus has in-
ing wood, and the numerous black vaded the tissues as a sap-
fruiting bodies developing on the
dead wood. rophytic or semi-parasitic






Bulletin 229, Diseases of Citrus in Florida


organism. In such a disease as withertip it is not safe to con-
clude from the mere presence of the organism that it has been
responsible for a given injury.
In the case of limes, on the other hand, the withertip fungus
attacks young shoots, leaves, and fruits while they are still
tender. The infected twigs wither and shrivel at the tips for
a distance of from one to several inches. In some cases they may


Fig. 39.-Anthracnose on grapefruit.
be girdled by an attack farther back, in which case they fall
over and hang lifeless as if attacked by some insect. Young
leaves which are sometimes infected but not entirely killed may
have dead areas on the margins or tips with distortion of the
remaining portions as further growth takes place. The flower
buds, when attacked, turn brown and fall without opening. If
severe infection occurs later as the buds open, the flowers may
drop before the fruit sets.






Florida Agricultural Experiment Station


Anthracnose:-This is the name given to the disease when
it causes a spotting of the fruit. Anthracnose of grapefruit
was first described in 1904 in Florida by Hume, who reported
having first observed it in 1901. Later, Rolfs referred to the
disease as being especially severe during the winters of 1901-02
and 1910-11.
The anthracnose blemishes vary from small specks to spots
ranging from a quarter of an inch or more in diameter. At first,


















I D.







Fig. 40.-Anthracnose blemish developed in thorn puncture on grapefruit.
(1/4 natural size.)

these spots are reddish-brown in color but, when ful'y developed,
they become dark brown to black, circular or nearly so, sunken
below the general level of the surface, dry and hard (Fig. 39),
and occasionally dotted with minute, black fruiting bodies of the
fungus. Ordinarily, only the rind is affected but, in the course
of time, the disease commonly extends through the rind into
the pulp beneath, imparting to the fruit a disagreeable, and
often bitter flavor. These spots, however, may serve as points






Bulletin 229, Diseases of Citrus in Florida


of entrance for other fungi which break down the fruit. Con-
sequently, fruit affected by anthracnose or similar spotting is
worthless for shipping. On very mature, overripe fruit certain
strains of the withertip fungus may produce a soft, pliable de-
cay and dropping in the field. Conspicuous anthracnose lesions
commonly develop about gashed or punctured places in the rind
where the fruit has been swung against thorns by high winds.
Such blemishes are especially common on grapefruit (Fig. 40)





















Fig. 41.-Anthracnose of young Key limes resulting from infection of
fruits developing during the rainy summer period. Note the numer-
ous prominent corky spots, deformation, and splitting of the fruit.
and cause a large amount of fruit to be culled at the packing-
house. Other fungi than the withertip fungus, however, also
commonly contribute to the decay of fruit following such in-
juries.
Spots of the same general nature as anthracnose blemishes
may also be caused by organisms other than the withertip fun-
gus. Such spots also develop frequently following a physiologi-
cal breakdown or pitting of the rind of fruit which has been
held on the trees after it has attained maturity. This trouble
develops abundantly during some years and constitutes an im-
portant source of loss, especially if such fruit is placed in cold
storage. Cold storage spots, now considered to be a physiologi-






Florida Agricultural Experiment Station


cal breakdown of the rind, have been confused with anthracnose
in the past. Thus, a number of spots other than those due to
the withertip fungus unquestionably have been included under
anthracnose in the earlier literature on this disease. The pres-
ence of the withertip fungus in these spots does not constitute
proof that it was responsible for the initiation of the breakdown.
Anthracnose, except in the case of lime fruits, occurs most
often on old or very mature fruits. It is of frequent occurrence
on the Triumph and other early or midseason varieties of grape-
fruit that have been allowed to remain on the trees long after
maturity. The development of anthracnose is greatly stimu-
lated by warm, humid weather conditions. Fruit on poorly
nourished trees or trees of reduced vitality is, as a rule, more
subject to this trouble than fruit on vigorous trees.
The anthracnose of the lime, which is quite distinct from that
on the other citrus fruits, does not depend upon the lowered vi-
tality of the tree, and is much more severe. Infection of the
young fruits, if not so severe as to cause them to drop off, usually
results in corky spots and, frequently, in distortion and splitting
as they increase in size (Fig. 41). Sometimes the corky out-
growths will be sloughed off, leaving shallow or deep depres-
sions, depending upon the severity of the attack.
Russeting and tear-staining:-Staining of the rind of citrus
fruits, commonly known as russeting and tear-staining, is some-
times produced by the spores of the withertip fungus that are
washed down over the fruits in the water-drip from dead twigs
on which the organism is fruiting. These spores germinate on
the surface of the rind and, instead of causing the usual type
of infection which results in anthracnose, merely result in a cer-
tain amount of russeting of the rind. In some cases this russet-
ing develops as a large blotch (Fig. 42), while in others it de-
velops as a tear-stain effect reported by Rolfs in 1904. The ex-
perimental work of Winston, published in 1921, indicates that
most of the tear-stain in Florida commonly attributed to the
withertip fungus is due directly to rust mite effects and can be
prevented by controlling the rust mites. In California, however,
several workers have produced true withertip russeting and
tear-stains by inoculations with pure cultures of certain strains
of the withertip fungus. These stains are similar to but fainter
than rust mite russeting.






Bulletin 229, Diseases of Citrus in Florida


CAUSE OF WITHERTIP
Withertip and the accompanying injuries on citrus trees other
than limes are caused by the fungus Colletotrichum gloeosporioi-
des Penz. Strains of this organism, so similar that they are
not capable of being distinguished from one another, have been
found to have a
world-wide distribu-
tion. Work done in
California by Burger
showed that several
distinct strains of
this fungus exist in
the groves. One is
led to believe that
the amount of injury
to citrus trees and
fruits depends some-
what upon the viru-
lence of the strain of
the fungus as well as
upon the condition of
the tree.
The fruiting
bodies of the wither-
tip fungus develop as
minute, dark, slight-
1 y raised, densely
massed pustules on Fig. 42.-Withertip russeting on grapefruit,
t h e twigs, leaves, caused by spores of the fungus on the dead
twig above washing down over the fruit.
fruits, and dead bark
of citrus trees (Figs. 38 and 43). The fruiting bodies and spores
are often innumerable and may occur on almost any dead twig
throughout the grove. Spores of the fungus occur universally
on the surface of live twigs and leaves of healthy as well as dis-
eased trees.
The withertip fungus, however, with the exception of the
species on lime, rarely attacks growing tissues unless the trees
are in a weakened or run-down condition. Trees weakened from
overbearing, drought, water injury, cold, lack of fertilization,
insect attacks, or almost any other cause are very susceptible
to the disease and it is on trees of this character that withertip






Florida Agricultural Experiment Station


is most abundant and causes the greatest injury. For this
reason, there has been considerable difference of opinion among
investigators and growers as to the part played by the fungus in
any given outbreak of withertip.
































Fig. 43.-Grapefruit leaves attacked by the withertip fungus. These leaves
were turning yellow and falling from the tree. Note the character-
istic fruiting bodies of the fungus, consisting of concentric zones of
minute black pustules formed on the dead areas of the leaves.

In regard to the withertip and anthracnose previously de-
scribed on the lime, which is caused by Gloeosporium limettico-
lum Clausen, there never has been any doubt as to the parasitic
nature of the organism involved, since it attacks young growing
tissue and not the mature tissue.




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