• TABLE OF CONTENTS
HIDE
 Front Cover
 Credits
 Table of Contents
 Introduction
 Importance of disease control
 Fungicides
 Spray program
 Diseases caused by parasitic fungi,...
 Diseases caused by algae and lichens...
 Decline














Group Title: Bulletin - University of Florida. Agricultural Experiment Station ; no. 463
Title: Parasitic diseases of citrus in Florida
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00027248/00001
 Material Information
Title: Parasitic diseases of citrus in Florida
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 112 p. : ill. ; 23 cm.
Language: English
Creator: Suit, R. F ( Ross Frisbie ), 1904-
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1949
 Subjects
Subject: Citrus -- Diseases and pests -- Florida   ( lcsh )
Citrus -- Diseases and pests -- Control -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: by R.F. Suit.
General Note: Cover title.
General Note: "A contribution from the Citrus Experiment Station"--T.p.
Funding: Bulletin (University of Florida. Agricultural Experiment Station)
 Record Information
Bibliographic ID: UF00027248
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000925544
oclc - 18254190
notis - AEN6197

Table of Contents
    Front Cover
        Page 1
    Credits
        Page 2
        Page 3
    Table of Contents
        Page 4
    Introduction
        Page 5
    Importance of disease control
        Page 5
        Page 6
        Page 7
    Fungicides
        Page 8
    Spray program
        Page 9
    Diseases caused by parasitic fungi, bacteria and viruses
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
        Page 59
        Page 60
        Page 61
        Page 62
        Page 63
        Page 64
        Page 65
        Page 66
        Page 67
        Page 68
        Page 69
        Page 70
        Page 71
        Page 72
        Page 73
        Page 74
        Page 75
        Page 76
        Page 77
        Page 78
        Page 79
        Page 80
        Page 81
        Page 82
        Page 83
        Page 84
        Page 85
        Page 86
        Page 87
        Page 88
        Page 89
        Page 90
        Page 91
        Page 92
        Page 93
    Diseases caused by algae and lichens and parasitic and epiphytic flowering plants
        Page 94
        Page 95
        Page 96
        Page 97
        Page 98
        Page 99
        Page 100
        Page 101
        Page 102
        Page 103
        Page 104
    Decline
        Page 105
        Page 106
        Page 107
        Page 108
        Page 109
        Page 110
        Page 111
        Page 112
Full Text




Bulletin 463


October, 1949


UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATIONS
HAROLD MOWRY, Director
GAINESVILLE, FLORIDA

(A Contribution From the Citrus Experiment Station)

Parasitic Diseases of Citrus

in Florida

By R. F. SUIT


Fig. 1.-Melanose on grapefruit.

Single copies free to Florida residents upon request to
AGRICULTURAL EXPERIMENT STATION
GAINESVILLE, FLORIDA


V /\, -











BOARD OF CONTROL

J. Thos. Gurney, chairman, Orlando
N. B. Jordan, Quincy
Thos. W. Bryant, Lakeland
J. Henson Markham, Jacksonville
Hollis Rinehart, Miami
W. F. Powers, Secretary, Tallahassee


EXECUTIVE STAFF

J. Hillis Miller, Ph.D., President of the
University3
Harold Mowry, M.S.A., Director
L. O. Gratz, Ph.D., Asst. Dir., Research
W. M. Fifield, M.S., Asst. Dir. Admin.
J. Francis Cooper, M.S.A., Editor'
Clyde Beale, A.B.J., Associate Editor'
W. W. Mosher, Assistant Editor
Ida Keeling Cresap, Librarian
Ruby Newhall, Administrative Manager'
Geo. F. Baughman, M.A., Business Manager3
Claranelle Alderman, Accountant'


MAIN STATION, GAINESVILLE
AGRICULTURAL ENGINEERING
Frazier Rogers, M.S.A., Agr. Engineer'
J. M. Johnson, B.S.A.E., Asso. Agr. Engineer3
J. M. Myers, B.S., Asso. Agr. Engineer
R. E. Choate, B.S.A.E., Asst., Agr. Engineeer'
A. M. Pettis, B.S.A.E., Asst. Agr. Engineer'2

AGRONOMY
Fred H. Hull, Ph.D., Agronomist'
G. E. Ritchey, M.S., Agronomist2
G. B. Killinger, Ph.D., Agronomist3
H. C. Harris, Ph.D., Agronomist3
R. W. Bledsoe, Ph.D., Agronomist
W. A. Carver, Ph.D., Associate
Darrel D. Morey, Ph.D., Associate
Fred A. Clark, B.S., Assistant
M. N. Gist, Collaborator2

ANIMAL INDUSTRY
R. B. Becker, Ph.D., Dairy Husbandman'
E. L. Fouts, Ph.D., Dairy Technologist'
D. A. Sanders, D.V.M., Veterinarian
M. W. Emmel, D.V.M. Veterinarian3
L. E. Swanson, D.V.M., Parasitologist
N. R. Mehrhof, M.Agr., Paultry Husb.3
G. K. Davis, Ph.D., Animal Nutritionist3
R. S. Glasscock, Ph.D., An. Husbandman'
P. T. Dix Arnold, M.S.A., Asst. Dairy Husb.3
L. E. Mull, M.S., Asst. in Dairy Tech.
Katherine Boney, B.S., Asst. Chem.
J. C. Driggers, Ph.D., Asst. Paultry Hush.3
Glenn Van Ness, D.V.M., Asso. Poultry
Pathologist
S. John Folks, B.S.A., Asst. An. Husb.'
W. A. Krienke, M.S., Asso. in Dairy Mfs.'
S. P. Marshall, Ph.D., Asso. Dairy Husb.'
C. F. Simpson, D.V.M., Asso. Veterinarian


ECONOMICS, AGRICULTURAL
C. V. Noble, Ph.D., Agri. Economist 3
R. E. L. Greene, Ph.D., Agri. Economist
Zach Savage, M.S.A., Associate
A. H. Spurlock, M.S.A., Associate
D. E. Alleger, M.S., Associate
D. L. Brooke, M.S.A., Associate
M. R. Godwin, Ph.D., Associate
H. W. Little, M.S., Assistant
Tallmadge Bergen, B.S., Asst.
Orlando, Florida (Cooperative USDA)
G. Norman Rose, B.S., Asso. Agr. Economist
J. C. Townsend, Jr., B.S.A., Agr. Statistician'
J. B. Owens, B.S.A., Agr. Statistician2
J. F. Steffens, Jr., B.S.A., Agr. Statistician2

ECONOMIC, HOME
Ouida D. Abbott, Ph.D., Home Econ.'
R. B. French, Ph.D., Biochemist

ENTOMOLOGY
A. N. Tissot, Ph.D., Entomologist'
L. C. Kuitert, Ph.D., Assistant
H. E. Bratley, M.S.A., Assistant

HORTICULTURE
G. H. Blackmon, M.S.A., Horticulturist'
F. S. Jamison, Ph.D., Horticulturist2
H. M. Reed, B.S., Chem., Veg. Processing
R. A. Dennison, Ph.D., Asso. Hort.
R. K. Showalter, M.S., Asso. Hort.
Albert P. Lorz, PhD., Horticulturist
R. H. Sharpe, M.S., Asso. Hort.
F. S. Lagasse, Ph.D., Asso. Hort.2
R. J. Wilmot, M.S.A., Asst. Hort.
R. D. Dickey, M.S.A., Asst. Hort.
Victor F. Nettles, M.S.A., Asst. Hort.4
L. H. Halsey, B.S.A., Asst. Hort.

PLANT PATHHOLOGY
W. B. Tisdale, Ph.D., Plant Pathologist'
Phares Decker, Ph.D., Plant Pathologist
Erdman West, M.S., Mycologist and Botanist
Howard N. Miller, Ph.D., Asso. Plant Path.
Lillian E. Arnold, M.S., Asst. Botanist

SOILS
F. B. Smith, Ph.D., Microbiologist 3
Gaylord M. Volk, Ph.D., Chemist
J. R. Henderson, M.S.A., Soil Technologist'
J. R. Neller, Ph.D., Soils Chemist
Nathan Gammon, Jr., Ph.D., Soils Chemist
C. E. Bell, Ph.D., Associate Chemist
R. A. Carrigan, Ph.D., Asso. Biochemist'
H. W. Winsor, B.S.A., Assistant Chemist
Geo. D. Thornton, Ph.D., Asso. Microbiologist'
R. E. Caldwell, M.S.A., Asst. Chemist'
Ralph G. Leighty, B.S., Asso. Soil Surveyor
V. W. Cyzycki, B.S., Asst. Soil Surveyor
R. B. Forbes, M.S., Asst. Soils Chemist
W. L. Pritchett, M.S., Asst. Chemist4
Jean Beem, B.S.A., Asst. Soil Surveyor
Walter J. Friedmann, M.S.A., Asst.
Biochemist
O. E. Cruz, B.S.A., Asst. Soil Surveyor

'Head of Department.
SIn cooperation with U. S.
3 Cooperative, other divisions, U. of F.
4 On leave.











BRANCH STATIONS

NORTH FLORIDA STATION, QUINCY

J. D. Warner, M.S., Vice-Director in Charge
R. R. Kincaid, Ph.D., Plant Pathologist
W. H. Chapman, M. S., Asso. Agron.
L. G. Thompson, Ph.D., Soils Chemist
Frank S. Baker, Jr., B.S., Asst. An. Hush.
W. C. Rhoads, M.S., Entomologist
Mobile Unit, Monticello
R. W. Wallace, B.S., Associate Agronomist
Mobile Unit, Marianna
R. W. Lipscomb, M.S., Associate Agronomist
Mobile Unit, Chipley
J. B. White, B.S.A., Associate Agronomist
Mobile Unit, DeFuniak Springs
R. L. Smith, M.S., Associate Agronomist

CITRUS STATION, LAKE ALFRED
A. F. Camp, Ph.D., Vice-Director in Charge
W. L. Thompson, B.S., Entomologist
J. T. Griffiths, Ph.D., Asso. Entomologist
R. F. Suit, Ph.D., Plant Pathologist
E. P. Ducharme, M.S., Asso. Plant Patho.4
R. K. Voorhees, Ph.D., Asso. Horticulturist
C. R. Stearns, Jr., B.S.A., Asso. Chemist
J. W. Sites, M.S.A., Horticulturist
II. O. Sterling, B.S., Asst. Horticulturist
J. A. Granger, B.S.A., Horticulturist
H. J. Reitz, Ph.D., Horticulturist
Francine Fisher, M.S., Asst. Plant Path.
I. W. Wander, Ph.D., Soils Chemist.
A. E. Wilson, B.S.A., Asso. Biochemist
J. W. Kesterson, M.S., Asso. Chemist
R. N. Hendrickson, B.S., Asst. Chemist
J. C. Bowers, B.S., Asst. Chemist
D. S. Prosser, Jr., B.S., Asst. Horticulturist
R. W. Olsen, B.S., Biochemist
F. W. Wenzel, Jr., Ph.D., Supervisory Chem
Alvin H Rouse, M.S., Asso. Chemist
L. W. Fayville, Ph.D., Asst. Chemist
L. C. Knorr, Ph.D., Asso. Histologist

EVERGLADES STATION, BELLE GLADE
R. V. Allison, Ph.D., Vice-Director in Charge
F. D. Stevens, B.S., Sugarcane Agronomist
Thomas Bregger, Ph.D., Sugarcane
Physiologist
J. W. Randolph, M.S., Agricultural Engineer
W. T. Forsee, Jr., Ph.D., Chemist
R. W. Kidder, M.S., Asso. Animal Hush.
T. C. Erwin, Assistant Chemist
Roy A. Bair, Ph.D., Agronomist
C. C. Seale, Asso. Agronomist
N. C. Hayslip, B.S.A., Asso. Entomologist
E. H. Wolf, Ph.D., Asst. Horticulturist
W. H. Thames, M.S., Asst. Entomologist
W. N. Stoner, Ph.D., Asst. Plant Path.
W. A. Hills, M.S., Asso. Horticulturist
W. G. Genung, B.S.A., Asst. Entomologist
Daniel W. Beardsley, B.S., Asst. An. Hush.


SUB-TROPICAL STATION, HOMESTEAD

Geo. D. Ruehle, Ph.D., Vice-Dir. in Charge
D. O. Wolfenbarger, Ph.D., Entomologist
Francis B. Lincoln, Ph.D., Horticulturist
Robt. A. Conover, Ph.D., Asso. Plant Path.
R. W. Harkness, Ph.D., Asst. Chemist
Milton Cobin, B.S., Asso. Horticulturist

W. CENT. FLA. STATION, BROOKSVILLE

William Jackson, B.S.A., Animal Husband-
man in Charge2

RANGE CATTLE STATION, ONA

W. G. Kirk, Ph.D., Vice-Director in Charge
E. M. Hodges, Ph.D., Agronomist
D. W. Jones, B.S., Asst. Soil Technologist
E. M. Kelly, B.S.A., Asst. An. Hush.

CENTRAL FLORIDA STATION, SANFORD

R. W. Ruprecht, Ph.D., Vice-Dir. in Charge
J. W. Wilson, Sc.D., Entomologist
Ben. F. Whitner, Jr., B.S.A., Asst. Hort.

WEST FLORIDA STATION, MILTON

C. E. Hutton, Ph.D. Agronomist1
H. W. Lundy, B.S.A., Associate Agronomist


FIELD STATIONS

Leesburg

G. K. Parris, Ph.D., Plant Path. in Charge

Plant City

A. N. Brooks, Ph.D., Plant Pathologist

Hastings

A. H. Eddins, Ph.D., Plant Path. in Charge
E. N. McCubbin, Ph.D., Horticulturist

Monticello

A. M. Phillips, B.S., Asso. Entomologist'
John R. Large, M.S., Asso. Plant Path.

Bradenton
J. R. Beckenbach, Ph.D., Hort. in Charge
E. G. Kolsheimer, Ph.D., Entomologist
David G. Kelbert, Asso. Horticulturist
E. L. Spencer, Ph.D., Soils Chemist
Robert O. Magic, Ph.D., Gla.'oli Hort.
J. M. Walter, Ph.D., Plant Pathologist
Donald S. Burgis, M.S.A., Asst. Hort.

Lakeland

Warren O. Johnson, B.S., Meterologist2

i Head or Department.
'2 In cooperation with U. S.
3 Cooperative, other divisions, U. of F.
4 On leave.









CONTENTS
PAGE
INTRODUCTION -- -5---.----- 5
IMPORTANCE OF DISEASE CONTROL --..------- 5
Spraying 6.... ........ ... ... 6
FUNGICIDES 8
SPRAY PROGRAM ---. _...--- ....- ........--...------------ 9
DISEASES CAUSED BY PARASITIC FUNGI AND BACTERIA AND VIRUSES 9
Melanose 9---------
Star Melanose .18
Scab --- ----.-....--- -....-- ..-- .- ..---- ... 18
Canker 26
Foot rot 33
Clitocybe Mushroom Root Rot 42
Diplodia Collar Rot and Root Rot ------- 50
Gummosis 52
/ Ps6rosis -- 54
Scaly Bark or Nail-head Rust ----- -.---- 60
Anthracnose 68
Diplodia Twig and Branch Dieback ---------- 77
Sphaeropsis Knot ------ .. .. .. 79
Heart or Wood Rot 81
Damping-off ------.---- --. .... .... ... .. ... 83
Sooty Mold ..... .... .. .... ..... ....... 85
Felt Fungus -------..---.-..-.-.- ..- ... ... 87
Flyspeck and Sooty Blotch 88
Thread Blight 89
Black Rot ------89
Stylar-end Rot of Tahiti Limes -...----- -- 93
DISEASES CAUSED BY ALGAE AND LICHENS AND PARASITIC AND
EPIPHYTIC FLOWERING PLANTS -- -------- 94
Algae 94
Lichens 98
Dodder ..--------.. ----- 100
Cassytha -----------102
Spanish Moss and Other Air Plants 102
DECLINE .----------- --- ------- ------ 105
SCitrus Nematode -- --. --- ..-- 107
Water Damage .. ...- .---.. ..------ -110











Parasitic Diseases of Citrus in Florida '


By R. F. SUIT2


Introduction
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 importance of the particular diseases
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 diseases and insects
often goes hand in hand.
It is the purpose of this bulletin to present the most recent
information on the citrus diseases that might occur in a grove
so that a grower can obtain a knowledge of various diseases and
employ the best known methods for their control.

Importance of Disease Control
Diseases are not apt to become less numerous in a grove or
to disappear unless proper control measures are undertaken. As
a rule, very little thought or attention is given to a disease until
it is well established in a 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.
Growers will do well to inspect their trees periodically for dis-
eases and conditions that need correcting. 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 threatened. With a moderate

IA condensed revision of Florida Agricultural Experiment Station
Bulletin 229, "Diseases of Citrus in Florida," by Arthur S. Rhoads and
E. F. DeBusk.
2 The writer is indebted to Dr. A. F. Camp for helpful criticisms and
suggestions in the preparation of this bulletin.











Parasitic Diseases of Citrus in Florida '


By R. F. SUIT2


Introduction
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 importance of the particular diseases
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 diseases and insects
often goes hand in hand.
It is the purpose of this bulletin to present the most recent
information on the citrus diseases that might occur in a grove
so that a grower can obtain a knowledge of various diseases and
employ the best known methods for their control.

Importance of Disease Control
Diseases are not apt to become less numerous in a grove or
to disappear unless proper control measures are undertaken. As
a rule, very little thought or attention is given to a disease until
it is well established in a 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.
Growers will do well to inspect their trees periodically for dis-
eases and conditions that need correcting. 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 threatened. With a moderate

IA condensed revision of Florida Agricultural Experiment Station
Bulletin 229, "Diseases of Citrus in Florida," by Arthur S. Rhoads and
E. F. DeBusk.
2 The writer is indebted to Dr. A. F. Camp for helpful criticisms and
suggestions in the preparation of this bulletin.







Florida Agricultural Experiment Stations


amount of study and observation, growers can determine 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 these diseases and to
make applications of fungicides and the follow-up oil spray 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 ac-
complished if control measures are applied carelessly or in-
differently. The control of many diseases, such as melanose and
scab, must be effected by preventive measures before the sus-
ceptible parts become infected. Even in the case of other
diseases 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 prevention of certain fungous 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
intervals 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 and, in many cases, the two are con-
trolled simultaneously by a combination spray.
Citrus trees are more difficult to treat successfully with
fungicides and insecticides than deciduous fruit trees, since they
are evergreen and retain their foliage throughout the year 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.
One of the results arising from the use of fungicides on
citrus trees is the indirect effect on the increase of insect pests.








Parasitic Diseases of Citrus in Florida


It is generally known that there is a tendency for scale popula-
tions to increase following either fungicidal or nutritional sprays
which leave a heavy residue on the foliage. This heavy residue
is as much of a factor in scale increase as is the possibility of
killing the entomogenous fungi with the sprays. It should be
noted also that the foliage of trees bordering dirt roads is
covered with varying amounts of dust and has a higher scale
population than the trees farther from the road. Various grove
operations such as cultivation and spreading of fertilizer are
also apt to leave excess dust on the foliage which may also result
in increased scale populations. The efficiency of the oil spray is
in inverse proportion to the amount of residue that the previous
sprays left on the foliage. For example, oil sprays are somewhat
more effective when they follow 3-3-100 bordeaux mixture than
when 6-6-100 bordeaux mixture is used, and they are still more
effective when they follow proprietary copper sprays. When-
ever nutritional or fungicidal sprays are applied to citrus the
usual oil spray for scale control should not be omitted. Conse-
quently, periodical examination of the grove following the use
of these sprays should be made to determine when the oil spray
should be applied. In spite of these drawbacks a certain amount
of spraying 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 fruit and foliage.
The number and frequency of spray applications will vary
according to the disease, growth conditions of the tree, and
climatic conditions. The efficiency of a single spray application
may not extend beyond from 10 to 14 days, even without rain-
fall. Where rapid growth occurs, considerable new and un-
protected growth may develop in a few days. To achieve best
results, fungicides should be applied often enough to give com-
plete 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 applied as a coarse mist it tends to collect in drops








Florida Agricultural Experiment Stations


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. Dusting has
'been found to be less effective than spraying for the control of
citrus diseases. Sprayers should be properly cleaned after use
and should be overhauled from time to time to maintain them in
proper working order.

Fungicides
A fungicide is generally used as a preventive and, to be
effective, must be applied at the proper time, which is usually
before the disease is visible. A fungicide merely forms a pro-
tective 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 suscep-
tible parts, before they germinate and infect the tissues.
It 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 require care and judg-
ment, as the efficiency of any spray solution will depend upon
proper preparation and the manner and timeliness of its applica-
tion. 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 most effective fungicides for the control of citrus
diseases are those that contain copper. Such fungicides are
bordeaux mixture and the various proprietary or neutral copper
fungicides. The newer neutral copper fungicides give com-
mercial control when properly applied, although bordeaux
mixture is slightly more effective. However, these new materials








Parasitic Diseases of Citrus in Florida


are easier to mix and do not leave as much visible residue on the
leaves as bordeaux mixture.
Within recent years many organic compounds have been de-
veloped that have fungicidal value. A number of these have
been tested and others are being tested for the control of citrus
diseases. To date none of these organic fungicides have proved
equal to the copper fungicides for citrus disease control. Sulphur
is not an effective fungicide for the control of citrus diseases and
is used primarily as an insecticide. In many cases wettable
sulphur has replaced lime-sulphur in the spray schedule.

Spray Program
Each year a spray and dust schedule for disease and insect
control is compiled by the Advisory Committee of the Florida
Citrus Commission. The program is based on information fur-
nished by the Citrus experiment Station, the Florida Agricultural
Experiment Station, Florida Agricultural Extension Service,
Florida State Plant Board, Florida State Horticultural Society, the
Bureaus of Plant Industry and Entomology and Plant Quarantine
of the U. S. Department of Agriculture, professional consultants
and commercial firms. The spray and dust schedule may be
obtained from the county agricultural agent or by writing to
Florida Citrus Commission at Lakeland, the Agricultural Experi-
ment Station or the Agricultural Extension Service at Gaines-
ville, the U. S. Department of Agriculture Laboratories at
Orlando or St. Lucie, or the Citrus Experiment Station at Lake
Alfred.

Diseases Caused By Parasitic Fungi, Bacteria
and Viruses
Melanose
Melanose caused by Diaporthe citri Wolf (Phomopsis citri
Fawc.)/ccurs 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 described 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 im-


2-463








Parasitic Diseases of Citrus in Florida


are easier to mix and do not leave as much visible residue on the
leaves as bordeaux mixture.
Within recent years many organic compounds have been de-
veloped that have fungicidal value. A number of these have
been tested and others are being tested for the control of citrus
diseases. To date none of these organic fungicides have proved
equal to the copper fungicides for citrus disease control. Sulphur
is not an effective fungicide for the control of citrus diseases and
is used primarily as an insecticide. In many cases wettable
sulphur has replaced lime-sulphur in the spray schedule.

Spray Program
Each year a spray and dust schedule for disease and insect
control is compiled by the Advisory Committee of the Florida
Citrus Commission. The program is based on information fur-
nished by the Citrus experiment Station, the Florida Agricultural
Experiment Station, Florida Agricultural Extension Service,
Florida State Plant Board, Florida State Horticultural Society, the
Bureaus of Plant Industry and Entomology and Plant Quarantine
of the U. S. Department of Agriculture, professional consultants
and commercial firms. The spray and dust schedule may be
obtained from the county agricultural agent or by writing to
Florida Citrus Commission at Lakeland, the Agricultural Experi-
ment Station or the Agricultural Extension Service at Gaines-
ville, the U. S. Department of Agriculture Laboratories at
Orlando or St. Lucie, or the Citrus Experiment Station at Lake
Alfred.

Diseases Caused By Parasitic Fungi, Bacteria
and Viruses
Melanose
Melanose caused by Diaporthe citri Wolf (Phomopsis citri
Fawc.)/ccurs 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 described 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 im-


2-463








10 Florida Agricultural Experiment Stations

portant 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.
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 pro-
gressively with the advanc-
ing age of the trees. In gen-
eral, those conditions which
promote the accumulation of
dead wood may be consid-
ered predisposing factors to
melanose development.
SSymptoms of Melanose.-

twigs and fruit of citrus
trees, attacking these parts
only while they are suf-
ficiently young to be suscep-
tible to infection.
On the Leaves. Mela-
nose first appears on the ten-
der young leaves as minute,
dark, circular depressions
with yellowish margins (Fig.
2). As the leaves harden
the spots become flush with
the surface and later become
distinctly raised, with some-
what irregular margins.
They are then rough and
Fig. 2.-Early stage of melanose on mahogany brown in color
grapefruit leaf, showing the yellow-
bordered spots. and the discolorations in ad-
vance of them are no longer
evident (Fig. 3). If the infection is severe, the leaf partially
loses its natural green color and becomes pale green or yellowish-








Parasitic Diseases of Citrus in Florida


green. The lesions occur on both surfaces but are most numer-
ous on the upper. The markings quite often assume circular or
ring-like patterns varying in diameter from one-eighth to one-
fourth inch. In case of severe infection, solid scar tissue of






,''



























Fig. 3.-Portion of grapefruit leaf showing character of mature
melanose spots. (Twice natural size.)

various forms may result. The foliage on the watersprouts and
other succulent shoots is frequently so severely attacked that
it is permanently distorted (Fig. 4). When severe leaf infection
occurs on leaves that are about half-grown, such leaves become
rV















occurs on leaves that are about half-grown, such leaves become








Florida Agricultural Experiment Stations


Fig. 4.-Distortion of foliage on young grapefruit shoot as a result
of severe melanose infection.








Parasitic Diseases of Citrus in Florida


yellowish and a high percentage of them will drop, resulting in
a partial defoliation.
On the Twigs.-The disease on the twigs (Fig. 5) appears at
first very much as it does on the leaves but the minute corky out-
growths ultimately
become more raised
than on the leaves or
fruits. If the infec-
tion is severe, the
twigs ma y become
more or less com-
pletely covered b y
scar tissue and may
die.
On the Fruit. -
Lesions on the fruit
(Fig. 1) are similar
to those on the leaves.
They are at first dark
and sunken, later be-
coming brown, dis-
tinctly raised, and
rough to the touch.
When the infection is
slight, the melanose
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 Fig. 5.-Melanose on orange twigs.
sugar. If the infec-
tion is more severe the melanose specks are much more numer-
ous and sometimes closely resemble the eruptions resulting from
exanthema ammoniationn). As on the other susceptible parts,
however, the spots may form lines, curves, circles, and other








Florida Agricultural Experiment Stations


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 "sharkskin" effect, may involve












,.^ c ^ .;',






Fig. 6.-Melanose on grapefruit, showing tear-streaked effect.

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 results (Fig. 7). Fruits
moderately to heavily infected by melanose have a rough,
sandpaper-like surface, the rough russeting being termed "rust"
by some growers. In severe cases of infection the fruit may be
dwarfed and, where badly scarred, may crack open as it ap-
proaches maturity.
Cause of Melanose.-Melanose was shown by Stevens, in 1912,
to be due to Phomopsis 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








Parasitic Diseases of Citrus in Florida


S $ '
J t" tt. -." .
.5, .r .
"" "- "..^ "i


Fig. 7.-Melanose on grapefruit, showing mud-cake type.

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








Florida Agricultural Experiment Stations


develop and, about the middle of 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 suscep-
tible. The length of time that twigs have been dead has an
important bearing on their production of melanose spores.
Those that have been dead but a few months are much more
likely to harbor the organism, although these twigs may serve as
a source of inoculum well into the second year after they have
been killed.
Another important factor necessary for the development of
melanose is moisture, in the form of either rain or heavy dew
or fog. The spores of the causal fungus developed in the pustules
are imbedded in a gelatinous matrix, which swells when
moistened. The spores are then forced out of the microscopic
mouths of the pustules in minute, tendril-like masses, after
which rains 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 remain susceptible.
It has been found that, citrus foliage and shoots are very
susceptible 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, ordinarily,
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 suf-
ficient 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 melanose, based on diameter measurements, varies
from an inch for tangerines, 11/2 inches for oranges, to 21/
inches for grapefruit.
Severe outbreaks of melanose can be traced definitely to
rainy periods occurring as a rule in April or May after the mela-








Parasitic Diseases of Citrus in Florida


nose spores have had a chance to develop in abundance and
before the young fruits have had a chance to outgrow the sus-
ceptible stage. The sudden showers, heavy dews and fogs that
come earlier do not appear nearly so favorable for infection
as do cloudy, rainy periods of two to three days' duration. The
rainy periods that commonly 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. A moderate
amount of fruit infection almost invariably takes place when
March rains occur. Ordinarily, however, principal infections
do not occur before the latter part of April or early May, since
April usually is one of the driest months of the year.
Control of Melanose.-As early as 1896 Swingle and Webber
showed that melanose outbreaks could be greatly reduced by
applications of copper sprays. 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 general practice to prune for melanose control.
However, it later became evident that thorough pruning to elimi-
nate the dead and dying twigs and branches did not prove to be
a consistently effective, practical, or economical method of con-
trolling this disease in groves of any considerable size. In view
of this it has become customary to rely upon spraying in con-
junction with pruning.
A single application of a copper fungicide, such as a 3-3-100
bordeaux mixture or neutral or proprietary copper sprays used
at equivalent copper content, has been found to give excellent
control of melanose. This spray should be applied within one to
three weeks after the fruit has set. During years of higher than
normal spring rainfall it is advisable to apply the spray about a
week after the fruit sets in order to obtain more effective disease
control. Under such conditions a delay of the spray application
for two to three weeks or longer would result in the fruit be-
coming infected before spraying and disease control would be
less satisfactory than if the spray had been applied sooner. If
melanose has been severe in the past and brighter and cleaner
fruit is desired, a second copper spray should be applied three


3-463








Florida Agricultural Experiment Stations


to four weeks after the usual melanose spray. It has been found
that 1-40 lime-sulphur solution has but little effect in controlling
melanose and that dusting with either copper or sulphur mix-
tures has failed to give satisfactory control.
There are two important secondary benefits from spraying
with copper fungicides for melanose control. One is the reduc-
tion in 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 satis-
factory control of melanose. The other is the reduction in the
tendency of the fruit to develop ammoniation. Spraying with
copper fungicides will help to eliminate this trouble in many
instances. A marked beneficial effect commonly occurs also in
the trees themselves that may be suffering from dieback.
Star Melanose
( Peculiar markings, similar to melanose lesions in their color
and corky raised character but characterized by the fact that
many of them are more or less stellate or star-like in outline,
occur occasionally on citrus foliage (Fig. 8) and fruit. The
markings occur on either surface of the leaves, although more
commonly on the upper surface. They may be numerous or few
and widely scattered. The individual lesions vary in size up to
1/16 inch in diameter. The one characteristic is the longi-
tudinal splits in the corky tissue, which is not present in normal
melanose lesions, and also the star-like shape of the markings.
The star melanose is a result of copper injury. Usually this
type of lesion does not develop unless the copper sprays (copper-
wettable sulphur or copper-oil) are applied rather late in the
spring and during the summer. It appears that the application
of a copper spray on young melanose lesions induces excessive
cork formation which results in the split eruptions and star-like
appearance. When copper sprays are applied at the correct
time there are usually no new melanose lesions and the spray
prevents the infection. Therefore, star melanose will probably
not occur if the copper sprays are applied at the proper time.
Scab
Citrus scab (caused by Elsinoe fawcetti Bitanc. and Jenkins
(Sphaceloma fawcettii Jenkins)), lemon scab, sour scab, verru-







Parasitic Diseases of Citrus in Florida


Fig. 8.-Star melanose on orange leaf following spraying with
a copper fungicide.

cosis, as the disease has been variously termed, is widely dis-
tributed throughout Florida and the Gulf states, where it causes
serious injury to a number of species and varieties of citrus
fruits. This disease 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


~C~si!







Florida Agricultural Experiment Stations


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 con-
ditions are favorable to its development.
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 sour orange, lemon, and some
of the tangelos are extremely susceptible. Grapefruit and shad-
dock are quite susceptible to infection, as are also the King,
Temple, mandarin, and Satsuma oranges, which belong to the
mandarin group of oranges. Sweet or round oranges are rarely
attacked and may be considered quite resistant. The Mexican
or Key lime, kumquat, citron and Cleopatra mandarin are ap-
parently immune.
The greatest commercial injury caused by scab is to the
grapefruit crop. It is owing to the large increase in grapefruit
plantings 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
Foster 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.
Losses occasioned by this disease result primarily from heavy
premature dropping of young fruit and a marked reduction in








Parasitic Diseases of Citrus in Florida


the grade of the fruit that reaches maturity, much of which is
rated as culls. In severe attacks, from 50 to 90 percent of the
fruit of susceptible varieties in an affected grove may be injured.
An average estimate of the scab losses to Florida growers is
about 1,000,000 boxes of fruit annually. This means a con-
siderable increase in the cost of producing grapefruit, tanger-
ines, 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 species and varieties of Citrus. These parts
are attacked only while 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 tissue,
at first pale yellowish orange in color, later becoming pink, then
drab and, finally, dark olive drab as the leaf matures and
hardens. The infected spots often run together and cover large
areas with a corky, scabby growth. Badly infected leaves be-
come very much crinkled, distorted, and stunted, having very
little resemblance to normal foliage (Fig. 9).
On the Twigs.-The characteristic effects of the scab disease
are the development of small masses of similar corky out-
growths on their surfaces. The centers of infection may remain
entirely separate or may run together to form larger, somewhat








Florida Agricultural Experiment Stations


Fig. 9.-Scab on leaves and twig of sour orange.

elevated 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 pro-
jections or conical growths extending from the surface (Fig.
10). On the grapefruit, infected areas tend to flatten out and
the fruit which reaches maturity may show merely large scabby








Parasitic Diseases of Citrus in Florida


Fig. 10.-Scab on Temple orange, showing warty projections.

patches of considerable extent, the entire surface being covered
in severe cases (Fig. 11). 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.
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 in 1906 by Fawcett, who incor-
rectly assumed it to be Cladosporium citri Massee. It was placed







Florida Agricultural Experiment Stations


Fig. 11.-Grapefruit severely affected with scab.

in the form genus Sphaceloma by Jenkins, who made a critical
study of it and named it Sphaceloma fawcettii in 1925. Recently
Bitancourt and Jenkins found the perfect stage of the citrus scab
fungus and described it as a new species, Elsinoe fawcettii.
The casual fungus lives over the winter period on the at-
tacked leaves and twigs, which serve as sources of infection the
following spring. 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 only while the growing parts are very young and tender.
The leaves are most susceptible in the early stages of growth
and become entirely resistant by the time they reach a width of








Parasitic Diseases of Citrus in Florida


one-half inch. Young fruits are extremely susceptible to infection
immediately after the falling of the petals and they become
progressively resistant until they reach immunity, which, in the
case of grapefruit, occurs by the time they have attained a di-
ameter of about three-quarters of an inch.
Severity of scab may vary greatly from year to year, de-
pending upon climatic conditions prevailing during the stage of
growth when the trees are susceptible to attack and the abund-
ance 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. Under
average Florida conditions, moisture rather than temperature
is the important limiting factor in natural infection. It is not
necessary, however, that the moisture be in the form of rain,
since the disease is commonly quite severe, even during springs
of minimum rainfall, in localities where heavy dews or fogs
occur during the critical period of infection or in regions of low
hammock or flatwoods soils, which are usually damp localities.
In the high, dry, rolling hill sections, scab is a negligible factor
during ordinary 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.
It has been shown that the temperature range for severe
infection is between 590 and 730F. Under Florida conditions the
mean temperatures seldom exceed these limits from the time new
growth appears in the spring until the fruits are no longer
susceptible to infection. During the warmer summer months
temperatures may drop low enough for periods sufficiently long
to permit infection to occur. Thus severe infection may occur on
nursery stock during the summer when abundant rains coincide
with the presence of immature foliage, although the mean
monthly temperature may be above 730F.
Control of Scab.-Scab can be controlled satisfactorily and
economically in most cases by the use of bordeaux mixture, pro-
vided the spraying be done with thoroughness at the proper time.
Since scab infection takes place very quickly under favorable


4-463








26 Florida Agricultural Experiment Stations

circumstances, it is difficult to control this disease in years
when frequent rains occur while the fruit is setting. Since scab
attacks the tissues only while very young and tender, spraying
must be done early in spring if the fruit is to be protected.
In trees where the overwintering foliage is severely infected
it is advisable to make two applications of copper sprays. The
first spray may be either 6-6-100 bordeaux mixture or a neutral
copper spray containing equivalent copper content. This spray
should be applied just prior to the spring flush of growth and
serves to cover the old scab lesions and thereby materially reduce
scab infections. The second spray should be applied as soon as
the petals have fallen and may be 3-3-100 bordeaux mixture or
its equivalent in neutral copper materials. If the overwintering
foliage is but slightly infected a single thorough pregrowth ap-
plication of 6-6-100 bordeau mixture usually will give adequate
protection from scab infections. On the other hand, a 3-3-100
bordeaux mixture pregrowth application followed by another
3-3-100 bordeaux mixture or its equivalent in neutral copper as
a post-bloom spray also will give excellent control.
Scab may be controlled to a considerable extent by removing
the sources of infection not essential to commercial fruit produc-
tion. 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 mixture 5-5-100 plus 2 quarts
of oil emulsion. 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.
Canker

Canker (caused by Xanthomonas eitri (Hasse) Dowson) is
the worst disease of citrus trees that has ever been introduced
into the Gulf states. Had it become well established in Florida
it would undoubtedly have caused disastrous results to the







Parasitic Diseases of Citrus in Florida


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 eradication of
this disease, which occurred in 515 properties scattered through
26 counties. Through the effective control campaign and sys-
tematic inspection of grove properties carried out by the State
Plant Board, it appears that canker has been eradicated 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 outstanding
accomplishments in the history of plant disease control. Con-
stant vigilance 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 been present
also 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, Mississippi,
Alabama, and Florida. In Florida it was probably first intro-
duced from shipments of triofoliate 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 that this
disease was a serious menace to the citrus industry and its true
cause was not demonstrated until 1915.
Canker attacks all kinds of citrus trees of commercial im-
portance in Florida, provided the conditions are favorable for








28 Florida Agricultural Experiment Stations

the development of the disease. Grapefruit is attacked most
severely and for this reason citrus canker threatened to cause
far more serious damage here than in other Gulf states, where
the Satsuma orange, a much more resistant species, 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 commercially re-
sistant. The kumquat is highly resistant. Inoculation experi-
ments have shown that a large number of species and varieties,
hybrids and wild relatives of Citrus are more or less susceptible
to canker. In contrast to the canker as it occurred in Florida,


Fig. 12.-Canker on grapefruit leaves and twig.


~%~i~LBPI
---
1

*~


-


II #


a








Parasitic Diseases of Citrus in Florida


which can be termed Cancrosis A, another form of canker known
as Cancrosis B occurs in Argentina, Uruguay, and Paraguay.
This form of the disease is most serious on lemons and occurs
occasionally on oranges. It appears that the B form is caused
by a strain of the citrus canker organism.
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 observed in the field is the characteristic
spotting produced on the foliage, twigs, and fruits.
On the Leaves. Canker first appears as small watery,
translucent spots usually of a darker green color than the sur-
rounding 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. 12). For example, 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









Fig. 13.-Canker on young twig of trifoliate orange.

leaves of limes and lemons, they are much smaller, frequently
being not 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.








Florida Agricultural Experiment Stations


On the Twigs, Branches, and Roots.-Lesions on the twigs
are common on the more susceptible kinds of Citrus, such as
grapefruit, trifoliate orange, lime, and sweet orange varieties.
The spots on young twigs are like those on the leaves and fruit
(Fig. 13). On the older twigs they are more prominent 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 surface
membrane completely disappears (Fig. 14). In especially sus-
ceptible varieties, canker has been found on the bark of mature
branches as large as six inches in diameter and branches from
two to three inches in diameter are quite commonly infected.
While underground roots of citrus trees have been repeatedly
inoculated, with positive results, canker has never been ob-
served occurring naturally on such roots despite numerous ex-
aminations of roots of badly diseased trees that were dug in
heavily infected groves. However, the disease has been found
on roots of grapefruit trees exposed above the surface of the
ground.
On the Fruit.-Individual canker lesions on the fruit have
much the same appearance as on the leaves except that the
yellowish halo is usually absent and the crater-like appearance is
usually more noticeable (Fig. 15). The spongy, rough eruptions
may be scattered over the surface or several may occur together,
forming an irregular, scurfy or scabby mass. Gumming is some-
times associated 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 by noting the
following points:-
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,








Parasitic Diseases of Citrus in Florida 31










































Fig. 14-Canker on stems of grapefruit nursery stock.





Fig. 14.--Canker on stems of grapefruit nursery stock.







32 Florida Agricultural Experiment Stations



'J '

.
















Fig. 15.-Canker on grapefruit.

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 resembles anthrac-
nose on limes more closely than any other disease 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 anthracnose on limes.
Cause of Canker.-Citrus canker is a bacterial disease caused
by Xanthomonas citri (Hasse) Dowson, a short, rod-shaped,








Parasitic Diseases of Citrus in Florida


motile organism 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 moistened by rain or dew. Drops of water contaminated
with bacteria spread the disease and when the organism comes
into contact 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 insect coming into contact with the moistened foliage
of an infested tree is apt to spread the disease to other trees.
The chief agent in the dissemination of canker has been man,
the disease having been brought to America on nursery trees
and spread through Florida on budwood and nursery trees.
The disease often may remain inactive for a considerable
period after infections have taken place before developing the
usual symptoms and evidently can be carried over long periods
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
longevity of the citrus canker organism in soils, it appears that
most agricultural soils cannot long retain the possibility of
disseminating 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, which will attend to the
eradication of the disease.

Foot Rot
Foot rot or collar rot (caused by Phytophthora parasitica
Dastur or P. citrophthora (Sm. & Sm.) Leonian) is probably
more widespread than any other citrus malady, being known to


5--463







Florida Agricultural Experiment Stations


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 widely
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.
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 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 mandarin,
rough lemon, trifoliate orange, grapefruit, lemon, and sweet
orange. The first four of these stocks may be classed as re-
sistant 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 disease which affects
the bark of the trunk usually at or just above the bud union and
occasionally the bark of the crown roots. Foot rot may occur
at one or more places on the trunk at the same time. 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 awater-soaked appearance
and a watery gum is sometimes found beneath it. Both the
diseased bark and the underlying wood frequently have an ill-
smelling, fermented odor. As the disease progresses gum exudes
from other portions of the bark, which in turn die. The exuda-
tion of gum accompanying the early stage of this disease is
rarely abundant. It develops most abundantly in late spring








Parasitic Diseases of Citrus in Florida


Fig. 16.-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.







Florida Agricultural Experiment Stations


and early summer and the exuded gum may be washed away by
rains. 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. 16). The bark remaining alive in advance of
the lesions often develops a callus formation which tends to
delimit the disease and check its further spread, especially in an
upward direction. Following this, the old patches of diseased
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 re-
sumed later. Sometimes the disease continues to spread on the
main crown roots and laterally around the base of the trunk
until the tree becomes more or less completely girdled. The de-
struction of the bark on the trunk rarely extends more than one
or two feet above the ground. The height of the bark lesions
above the ground, however, is by no means always an index to
the progress of disease.

After foot rot has progressed to the point where the root
system or the trunk becomes partially girdled the effect of the
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. De-
clining trees have the appearance of suffering from lack of
nourishment, which is indeed the case on account of the partial
girdling. Such trees may continue 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, become small in the
late stages of decline. In well-advanced cases of the disease
the leaves often become yellow, especially along the midribs, an-
exhibit the characteristic symptoms of acute starvation. The
twigs and smaller branches die first and the larger branches
succumb gradually as the disease progresses, thus giving the
trees a ragged appearance (Fig. 17). When the top becomes
half dead, numerous watersprouts frequently 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 injured by foot
rot frequently exhibit a pronounced wilting and curling.







Parasitic Diseases of Citrus in Florida


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, how-
ever, it may be several years before the attacked trees finally


Yr d~


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







Florida Agricultural Experiment Stations


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 is caused primarily by two
fungi, Phytophthora parasitica Dastur and P. citrophthora (Sm.
& Sm.) Leonian, although other species of Phytophthora have
been found associated with foot rot. These fungi are present in
the soil and infect trees whenever conditions are favorable for
attacking the trees. Infection may occur below, at, or above the
bud union. The fungi gain entrance to the tree by directly
penetrating the bark and through wounds in the bark. Soil con-
taining the fungus and splashed on the trunk of the tree during
rain is a common means of trunk infection. Wounds made in
the bark and left unprotected offer an easy avenue of entrance
Fig. 18.-Advanced case of foot rot, in which the tree broke off long
before it died. The taproot and most of the lateral roots have been dead
a long time.







Parasitic Diseases of Citrus in Florida


for the fungus. In addition to the mechanical injuries of the bark
there is frequently a splitting and gumming of the bark induced
by a heavy application of fertilizers. This bark splitting is more
common where an attempt has been made to bring quickly back
into high production run-down and poorly cared for groves.
Any factor which causes the base of the trunk to remain wet
or damp for long periods also predisposes to infection with Phy-
tophthora fungi.
Control of Foot Rot.-There are two general methods for the
control of foot rot, namely preventive safeguards in connection
with the maintenance 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 planting, and the avoidance of too close planting, too low
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.

*% S MUBMBBNfC MSM







Florida Agricultural Experiment Stations


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-
penning, 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 re-
quires 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 it is first
necessary to clear away the soil for about three feet around the
tree in order to determine the extent of the trouble on the root
crown if the lesion extends below the surface of the soil (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 replacing the
regular disk by one with a large hole. If the disease 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 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 disinfectant, such as bordeaux
paste. Bordeaux paste is made by completely dissolving 1 pound
of copper sulphate in 1 gallon of water and then adding 1 pound
of hydrated lime to the solution. The mixture should be con-
stantly stirred while the lime is slowly added. Since bordeaux
paste loses its physical properties if allowed to stand overnight








Parasitic Diseases of Citrus in Florida


a fresh mixture should be made for each day's work. Avenarius
carbolineum is also a good disinfectant to use when treating foot
rot lesions. All exposed wood surfaces should be painted with a
good waterproof wound dressing to prevent the entrance of
wood-boring insects and decay-producing organisms. It is ad-
visable 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 retarding its subsequent
development. Frequently, the mere removal 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 applicable to
trees on heavy clay soils, since the water accumulating in the
excavations during rainy weather would be fatal to the tree.
When lesions are at or above the bud union it is sufficient to
clean out thoroughly the diseased bark and wood. These exposed
surfaces should be treated with bordeaux paste or avenarius
carbolineum, followed by the application of a good wound
dressing as previously mentioned. This method of treatment is
satisfactory if the lesions are small and do not involve more
than 30 percent of the circumference of the trunk. If the lesions
are more extensive, some benefit might be obtained by treating
them, provided the trunk is not more than three-fourths girdled
and extensive deterioration has not developed in the top of the
tree. Trees that are extensively deteriorated in the top should
be removed.
Trees which have not become weakened too seriously from
foot rot often can be saved merely by planting three or four sour
oranges or other resistant trees about the base of each attacked
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 induces
the development of a new root system, within from three to five


6-463







Florida Agricultural Experiment Stations


years, from the callus formed at the lower limit of the living
bark just above the nearly girdled base (Fig. 20). A con-
siderable 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 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 usually will reveal the presence of foot rot long before it
becomes evident by the way the tops have died back.
Clitocybe Mushroom Root Rot
Clitocybe mushroom root rot (caused by Clitocybe tabescens
(Scop. ex Fr.) Bres.), which is known to attack fruit trees,
grapevines, forest, shade, and ornamental trees, and shrubs,
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.







Parasitic Diseases of Citrus in Florida


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 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.
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 disease
will eventually be found to be of fairly widespread and common
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 attacked
trees.

Fig. 21.-Grapefruit trees attacked by Clitocybe mushroom root rot,
showing bark lesion at base on left and a partially dried cluster of fruit-
ing bodies developed at base on right.







Florida Agricultural Experiment Stations


Symptoms of Mushroom Root Rot.-Symptoms exhibited by
the attacked trees vary greatly with the age and progress of the
disease. In 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
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 attacked trees in this locality were located solely by finding
either fresh or old clusters of the mushrooms. Upon excavating
the soil under the bases of these trees to examine the root system
and prepare them for treatment, 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 disinte-
grated. In all four of these trees the taproots also were 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).
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.
The inference is that the oak and other hardwood roots which
were left in the land when it was cleared may have been 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 ob-
served also 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







Parasitic Diseases of Citrus in Florida


seems to develop very far above the ground line. The mycelium,
or vegetative 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 be-
tween 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 portions 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 ex-
tended 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, tree 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 invariably 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 has declined considerably, basal lesions of dead bark, similar
to those developed in foot rot, usually develop 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.
Cause of Mushroom Root Rot.-The organism causing this
root rot is one of the gill fungi, Clitocybe tabescens .(Scop. ex








Florida Agricultural Experiment Stations


Fr.) Bres., popularly termed mushrooms or toadstools, which
occur in clusters comprising several individuals 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


Fig. 22.-Cluster of the mushrooms or toadstools of the Clitocybe
root rot fungus.

(Armillaria mellea Vahl ex Fr.), especially in regard to habit of
growth, the presence of white fan-shaped mycelium beneath the
bark, the 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-








Parasitic Diseases of Citrus in Florida


morphs which are developed under certain conditions. The
rhizomorphs are very inconspicuous and have been observed only
occasionally in nature.

Since the fungus can infect 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 suf-
ficiently 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 moisture and temperature conditions are favor-
able. The season of their appearance in Florida is chiefly from
October to early December. They are sufficiently distinctive
that they should not be confused with other mushrooms or
toadstool-like fungi which may occur around trees occasionally
but be without any pathological significance. When fully de-
veloped, the caps are convex in shape, tan-colored, and range
from 2 to 31/2 inches in diameter (Fig. 22). The clusters attain
their full development within a few days under favorable con-
ditions 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 fruiting bodies of the fungus the disease
appears to be distinguishable from foot rot only by finding the
characteristic mycelial growth of the fungus between the bark
and the wood of the roots, which is best done in those below the
surface of the ground, where better moisture conditions prevail.
Control of Mushroom Root Rot.-It is far easier to take steps
to prevent the occurrence of mushroom 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 de-
sirable to remove all roots as thoroughly as is practical and to







Florida Agricultural Experiment Stations


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.
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 aeration of the
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 crown roots exposed for aeration and
drying has proved to be very beneficial in checking the develop-
ment of these root-rot fungi. Considerable experimental work
has been done, both in this country and abroad, in attempting
to control Armillaria and other root-rot fungi by various chemi-
cal treatments of the soil, but without sufficiently consistent
success to justify any recommendations.
When grove trees are attacked by Clitocybe root rot, control
measures will be determined to a large extent by local circum-
stances. Two main problems are presented: (1) the prevention
of the spread of the disease to adjoining unattacked 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.
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 crown since, after the
disease has progressed to this extent, death of the tree is in-
evitable. 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-
server that something is wrong with the tree. However, at-
tacked trees often can be located some time 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








Parasitic Diseases of Citrus in Florida


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 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 be
worked away from the 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 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 crown (Fig. 23). Special effort should be made to remove
all oak or other foreign tree roots that may be encountered
under the citrus trees being treated. All dead roots of either

Fig. 23.-Grapefruit tree treated for Clitocybe mushroom root rot,
showing the dead roots and diseased areas of bark cut out.
-Ve







Florida Agricultural Experiment Stations


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, crown, lateral roots, or tap-
root 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 examine
for lesions under the lateral roots, especially where they leave
the 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 con-
tainer 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. If
allowed to remain, these may act as sources of infection. All
trees not worth treating should be dug up, care being taken to
remove and burn all roots. When 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 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
crown may be left exposed indefinitely if preferred. If the tops
of the trees have died back they should be dehorned proportion-
ately and pruned of all dead wood. The location of the treated
trees should be recorded and they should be inspected peri-
odically to detect any subsequent development of the root-rot
fungus.
Diplodia Collar Rot and Root Rot
(Caused by Diplodia natalensis Pole-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 suddenl\become
girdled. A slight amount of gumming usually accompanied this
collar-rot type of girdling at the margins of the lesions aad







Parasitic Diseases of Citrus in Florida


Diplodia has been found associated with the early stages of this
trouble. 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.
Unfortunately, this trouble is rarely observed by growers until
the trees have become more or less completely girdled. When
examined at this stage, entire root systems of young trees are
often found to be more or less dead and with the bark so far
disintegrated 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.
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
bordeaux paste or avenarius carbolineum. 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
them properly when they occur, since these bark-attacking fungi
often gain entrance through injured places.
Within recent years numerous cases of root rot have been
observed in which Diplodia appeared to be the causal organism,
although the trouble may have been initiated by some other
agent. Diplodia-infected roots are characterized by the black
discoloration of the wood beneath the dead bark. This black
discoloration is not characteristic of mushroom root rot. Any
factor which weakens roots predisposes them to root rot. For
example, the incidence of root rot is higher in groves where
some water damage is apt to occur and in areas where the soil
j pH tends to be above the optimum, which is often the case along
highways where crushed lime rock was used as a base. In ad-
dition, trees struck by lightning without showing visible effects
S usually have some roots injured which later succumb to root rot.
At present there is no known method of control. Trees infected
with root rot will probably never recover and should be removed
as soon as it becomes unprofitable to keep them in production.







Florida Agricultural Experiment Stations


Gummosis

The exudation of gum by citrus trees is an indication of an
abnormal condition affecting the tree (Fig. 24). This gumming
can be caused by a number of factors, such as foot rot, Diplodia
collar rot, mushroom
.. root ro t, psorosis,
incompatibility of
,rootstock, chemical,
mechanical, and in-
sect injuries, and ex-
v cessive applications
of fertilizer. Gum-
ming lesions may be
found at the base of
the trunk, at various
places on the trunk,
and on the larger
limbs and branches.
If the lesions are
not caused by a dis-
ease and do not be-
come infected by sec-
ondary organisms
they will gradually
heal over (Fig. 25).
However, it is pref-
erable that the gum-
ming lesions be
treated to prevent
secondary infection.
Such areas of bark
should be cleaned off
with a wire brush,
dead and discolored
bark cut out, and the
Fig. 24.-Close-up view of portion of grape- ex posed surfaces
fruit trunk affected with gummosis just above
the bud union. Note copious exudation of gum painted with bor-
down center of trunk; also note cracks in deaux paste as for
bark at left that probably will exude gum the control of foot
later.








Parasitic Diseases of Citrus in Florida


rot. If the gumming has been caused by the use of excessive
fertilizer it is beneficial to broadcast two pounds of copper sul-
phate crystals on the soil around each tree.


Fig. 25.-Portion of grapefruit trunk with old, inactive case of gum-
mosis. 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. (Photograph by A. F. Camp.)







Florida Agricultural Experiment Stations


Psorosis
Psorosis3 (caused by virus, Citrivir psorosis Fawc.) was first
described in 1896 and occurs throughout the citrus-producing
section of Florida, affecting tangerine, orange, and grapefruit
trees. Tangerines are extremely susceptible to psorosis but seem
to withstand the ravages of the disease longer than either
oranges or grapefruit. Of the oranges, Valencia appears to be
most severely affected. The sour orange is highly resistant,
although not immune, to the disease.
Symptoms of Psorosis.-Psorosis is a disease affecting the
bark of trunks, main limbs, branches and occasionally, even large
twigs. Although psorosis is generally classed as a gum disease,
gum exudes only during the active 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 development of
the disease, its inconspicuous appearance, and lack of effect upon
the foliage in the earlier stages, frequently prevent it from being
noticed by growers until it has become well advanced.
Psorosis usually begins as an inconspicuous spot in which the
bark dies and becomes infiltrated with gum. This usually occurs
about the time the tree is from 10 to 15 years old. 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 from discolora-
tion, except for a slight yellowish tint. As the disease pro-
gresses, 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, psoro-
sis may completely encircle the trunk or limb without causing
serious injury to the underlying wood.
Up to a certain point in the development of psorosis, new bark
forms under the outer bark that loosens and scales off, but this
bark becomes diseased and the scaling process continues. The
bark within the affected area thus becomes considerably
thickened, rough, and brownish, giving this area a more or less

3 This name is from the Greek word meaning ulcer. It is pronounced
so-ro'-sis.







Parasitic Diseases of Citrus in Florida


.


tx


M^ U.-


f ^''' ^
^ ^^


^^ ^^ w .-

'*t ^' -<<'*


Fig. 26.-Psorosis on orange tree, showing well advanced stage in which
the disease has spread entirely around the divisions of the trunk.







Florida Agricultural Experiment Stations


swollen and distinctly ulcerated appearance. In this stage the
lesions are very conspicuous because of the pronounced breaking
up of the outer bark into large flakes, scales, or shaggy strips
which curl away from the trunks or branches as they slough off
(Fig. 26). After the lesions encircle the trunk or branch they
continue to increase in length. Scaly bands are thus formed
which vary in length from a few inches on the smaller 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
disease frequently involves the entire main trunk above the bud
union and also many of the main limbs. Independent lesions may
occur farther up on the limbs and even on the smaller branches
of the tree.
In well-advanced stages of the disease, internal gum forma-
tion begins in the cambium and embryonic wood cells and a
yellowish discoloration of the inner bark may appear, due to the


-w~


Fig. 27.-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.)







Parasitic Diseases of Citrus in Florida


gum infiltrating the tissues. A cross-section of an attacked
trunk or branch in the late intermediate stage of the disease may
not show any discoloration except dark brown lines of closely
crowded gum canals in the outer layers of the wood (Fig. 27).
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
cambium or growing layer may, in certain cases, become dis-
colored in this way but remains firm and hard unless wood-
decaying 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 that
had been attacked. The leaves of this part of the top become
reduced in size and number. Twigs die back, followed by the
death of the smaller branches, and this part of the tree 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. 28).
Cause of Psorosis.-It has been definitely established that
psorosis is caused bya virus. The disease has been transmitted
from diseased to healthy trees by budding and by leaf, bark, and
root grafts. A virus is not a specific organism like bacteria or
fungi, but is a protein material which is able to cause a diseased
condition within the host. This protein material is spread by sap
from diseased plants or by buds, scions or stocks taken from
diseased plants. In the case of psorosis, the virus is spread only
by^Judding-or-grafting.
Once a tree becomes infected with the virus it never recovers
and the virus is systemic within.lbthe-Jee; that is, all parts of
the tree have the virus. Usually a diseased tree will not show
the bark symptoms until it is at least 10 or 12 years old, although
occasionally psorosis has been found on younger trees. Trees
which have psorosis may also show mosaic-like symptoms on the
young leaves, even though bark symptoms are not present.
These symptoms may be a flecking or oak-leaf pattern of lighter








Florida Agricultural Experiment Stations


colored areas in the half-grown leaves. When the leaves become
mature the symptoms tend to disappear. It is rather difficult
to diagnose the leaf symptoms, and injuries due to thrips, purple


f


th


Fig. 28.-Orange tree dying on one side from psorosis. Two of the
three main limbs have been girdled by an old lesion at the junction of
the limbs.







Parasitic Diseases of Citrus in Florida


mites, and other insects, as well as hail, windblown sand, and
spray injury might be mistaken for psorosis.

Control of Psorosis.-Most of the psorosis originates in the
bud wood used in propagation. Psorosis could be eliminated if
bud wood was taken from trees that were free of the disease.

The manner of treatment for psorosis will vary in accordance
with the stage to which the disease has progressed. 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 affected, there is
little hope for successful treatment. When the bark on the trunk
is badly killed or the top of the tree as a whole appears stunted
and shows dead branches, the tree should be removed if it is
unproductive. An those cases where the bark lesions are not
usually more than three or four inches in diameter or do not
extend more than one-fourth to one-third of the way around the
trunk or large limb, some success can be obtained by treatment.
The principle governing the treatment of psorosis by bark
scraping is the elimination of the diseased outer bark tissues
without interfering with the production of new bark by the
cambium. The affected bark should be scraped deep enough to
eliminate the discolored layers and the surrounding bark, not
visibly affected, should be scraped for a distance of at least six
to eight inches above, at least six to eight inches below, and at
least four to six inches on each lateral side beyond the margin of
the affected area. The depth of scraping on the bark not visibly
affected beyond the lesion should not be deeper than to cut
through the green outer layer and to expose the white layer.
Care should be taken not to scrape so deeply as to injure the
inner bark and the cambium layer. The scraped area should
then be painted with bordeaux paste. (See discussion on control
of foot rot for formula for bordeaux paste.) Within from three
to six months from the date of treatment the outer corky portion
of the scraped bark will crack loose and slough off, exposing the
new bark. Bark treated during the spring months appears to
recover most rapidly. It is probable that the treatment will not
eliminate the psorosis but will retard the development of the
bark symptoms for a few years and thus prolong the usefulness
of the tree.







Florida Agricultural Experiment Stations


Tools.-A number of different types of scraping implements
may be used for scraping bark attacked by psorosis or foot rot.
The most important feature of such tools, in addition to con-
venience and efficiency, 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 of
gum pockets in the bark. The box scraper comprises a re-
versible, 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 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. The type of scraper in common use in California is
made of spring steel and consists of a curved blade sharpened
on both edges and at its end and set in a wooden handle. The
handle is about 71/ inches long and 1/2 inches in diameter,
while the steel part is about 51/ inches long with the curved
blade being about 21/ inches of the length of the steel part.
Scaly Bark or Nail-Head Rust
Scaly bark or nail-head rust (causal organism not definitely
known), 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. Originally, scaly
bark was reported to occur only in Florida but has now been
found in China, the Philippines, Mexico, India, Java, Japan,
Egypt, South Africa, Argentina, and Brazil. The disease was
first observed in Florida in Pinellas County about 1860. By
1925 it was known to be present in Pinellas, Hillsborough, Semi-
nole, Volusia and Brevard counties, with a few scattered cases
in the counties in the central part of the state. Today, scaly
bark is found in only a few groves which do not receive proper
care. This disease is known as "lepra-explosiva" in Argentina
and in the past 15 years has caused the death of hundreds of
acres of grove in that country. Information from Argentina
indicates that the disease may be caused by a virus which is
possibly spread by a certain species of mite.







Parasitic Diseases of Citrus in Florida


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 infection may be observed occasionally on grapefruit,
tangerine, and mandarin orange trees. The latter, however,
are very resistant 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 poorly under-
stood. It has been suggested that moisture at the right season
for infection is an important factor in the severity of this
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 in former years, it has decreased
greatly within the past few years without any special steps
being taken to combat it. This marked decrease in the preva-
lence and severity of scaly bark within late years is believed to
be due to the increase in the use of fungicidal sprays and to
the improvement in grove management. At present the dis-
ease seems to be most virulent in badly neglected or abandoned
groves.
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 younger branches up to about one-half
inch in diameter, and to the fruit. 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 incon-
spicuous greenish-yellow spots which have a somewhat trans-
lucent appearance (Fig. 29). According to Fawcett, twig in-
fections may occur at any time of the year but the largest
number appears 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 ob-
served 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 10
months, become from one-sixth to one-half inch in diameter,







Florida Agricultural Experiment Stations


raised above the surface of the surrounding bark, rusty in color,
and with well defined margins. As the spots grow older the
bark becomes glazed, then brittle, develops longitudinal cracks,
and finally breaks into
small flakes and scales
(Fig. 29). 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
reddish-brown, scaly bark
(Fig. 30) ; 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 de-
cidedly rough and scaly
and the branch is finally
killed as a result of girdl-
ing. The water sprouts,
which develop abundantly
when badly attacked trees
are cut back, are es-
pecially susceptible to in-
fection.
Fig. 29.-Scaly bark spots on seedling On the Larger Branches
orange twigs, showing early stages of the
disease. The youngest stages show as and Trunk. Where the
slight yellowish spots. (Twice natural size.) disease is of long stand-
ing, the original spots are no longer distinct and the surface of
the affected bark becomes broken into thin flakes or scales.
Ultimately the trunks and larger branches acquire an extremely








Parasitic Diseases of Citrus in Florida 63



r ~~s~


Fig. 30.-Scaly bark on watersprouts of sweet seedling orange tree,
showing indeterminate stages of the disease. (Natural size.)







Florida Agricultural Experiment Stations


fl~A~i


S 'I ll /4...

SA -



Fig. 31-Advanced stage of scaly bark on sweet orange tree, showing
roughened bark on the branches and nailhead rust spots on the fruit.







Parasitic Diseases of Citrus in Florida


rough and shaggy appearance (Fig. 31). Pieces of bark one-
half inch or more in size scale loose, sometimes over areas of
considerable size. New bark often forms under the old as the
latter cracks off and only rarely do dead areas on the trunk or
large limbs result from this disease. Where the disease has
been of several years' standing, the branches often exhibit
band-like lesions encircling them, which closely resemble pso-
rosis.
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. 27, 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 off.
Twigs and branches attacked by scaly bark frequently exhibit
more or less formation and exudation of gum.
On the Fruit. Where characteristic dark-colored, sunken
spots develop (Fig. 32) the disease is quite distinctive and is
commonly termed nailheadd 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 becomes hard, glazed, and brittle and the spots eventually
become surrounded by a yellowish border.







Florida Agricultural Experiment Stations


Fig. 32.-Nailhead rust spots on orange.


Cause of Scaly Bark.-Fawcett's investigations indicate that
a fungus similar to Cladosporium herbarum Lk., which has been
known as C. herbarum Lk. var. citricolum Farl., may be re-
sponsible for the initial spots and that Colletotrichum gloeo-
sporioides Penz., which causes withertip, 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, also are frequently found in the
dead twigs and branches that accompany this disease under
Florida conditions. Considerable work has been done in Florida
recently by West in isolating and studying the organisms as-
sociated with scaly bark lesions. A number of fungi and bac-
teria have been obtained but none of them have reproduced the


1~r.

r~a. ~ci


cc; ~"
c.~."1 io








Parasitic Diseases of Citrus in Florida


disease when they were inoculated into citrus twigs of various
ages.
Scaly bark is quite slow in its growth, developing so
gradually into its destructive form that growers often are un-
aware of its importance until it has been present for several
years. It seems clearly evident that the disease can be spread by
picking crews and in pruning operations.
Control of Scaly Bark.-Scaly bark can be controlled and fruit
losses 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 before
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
bordeaux-oil emulsion early in February before they bloom. All
twigs and branches, as well as the main limbs, should be sprayed
thoroughly, since the diseased spots are almost entirely located
on these parts. A second application of bordeauxoil 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 appli-
cation 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 advisable
to make one or two applications of bordeaux-oil emulsion during
the year following the treatment.
The above outline for the control of scaly bark is applicable
to a grove in which the disease is severe. Actually, today, there
are very few groves in which the disease is present. Scaly bark
has been largely eliminated by the improved spray, pruning and
fertilizer programs which are in use today. There is no need
for specific scaly bark control measures if a grove receives proper
care.







Florida Agricultural Experiment Stations


Anthracnose
Anthracnose (caused by Colletotrichum gloeosporiodes Penz.
and Gloeosporium limetticolum Clausen) has been regarded as a
frequent source of trouble in Florida citrus groves. Although
most attention has been called to this disease in Florida,
especially in the publication by Rolfs in 1904, the same trouble
has been reported in nearly every citrus-growing country.
Two forms of these diseases occur. One attacks the young
growing tissues of the Key lime. The more common and wide-
spread form usually attacks only mature, weakened leaves,
twigs, and fruit of oranges, lemons, grapefruit, and other citrus
trees. The latter form of anthracnose may occur on trees of
any size from nursery trees to old bearing ones. It rarely
develops on trees that have been maintained in a vigorous con-
dition but is confined largely to trees that have become weakened
or injured as a result of inadequate fertilization, overcropping,
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 be-
comes accentuated. In reality, anthracnose on citrus trees other
than limes is largely a secondary trouble. The importance of
anthracnose has been so overemphasized in past years that it
still constitutes 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 this
disease cannot be regarded of as much importance in Florida
as it was formerly.
In southern Florida and the West Indies anthracnose fre-
quently causes serious damage to 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
anthracnose is the Dominican Thornless lime, which supposedly
originated as a sport from the West Indies. The organism which
causes anthracnose of the lime is somewhat different from that
causing this disease of other citrus fruits and is much more
severe in its effect.
Symptoms.-Since there are a number of different phases of
this disease, depending upon the part of the tree attacked, the








Parasitic Diseases of Citrus in Florida


Fig. 33. Withertip on small
lemon twigs, showing the sharp
line of demarcation between the
dead and living wood and the num-
erous black fruiting bodies de-
veloping on the dead wood.


symptoms will be discussed un-
der the subheadings withertip,
anthracnose, and russeting.
Withertip.-The term "with-
ertip" is applied to the wither-
ing 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 mature
twigs and branches that are at-
tacked by the fungus. The dying
back often progresses slowly,
with the leaves turning yellow,
withering and dropping gradu-
ally. Ordinarily, after the fun-
gus travels down the twigs and
smaller branches a greater or
lesser distance, a slight exuda-
tion of gum occurs and the dis-
ease becomes self-limited, end-
ing at a sharply defined line
which separates the diseased
from the healthy tissue. After
the diseased wood dies the char-
acteristic, minute black pustules
or fruiting bodies of the fungus
develop abundantly (Fig. 33).
Outbreaks of a trouble re-
sembling withertip often develop
during the winter months after
the root systems of trees have
been injured by drought or fluc-
tuations 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







Florida Agricultural Experiment Stations


green and may develop new leaves 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 old
bearing ones. In such cases it is a question whether the trouble
should be regarded as withertip or as merely a natural balancing
of the top of the tree inevitably following the reduction of the
root system.
Outbreaks of withertip often follow prolonged high winds,
even though they are not sufficiently severe to cause any ap-
parent damage to the trees. Withertip also develops abundantly
in trees that have been severely whipped by hurricanes,
especially when the tissues of the leaves, twigs, and branches
have been injured by extreme desiccation or by the action of
salt spray. Such injuries, together with the numerous abrasions
and wounds resulting from winds of hurricane intensity, afford
excellent places for the spores of the universally present an-
thracnose fungus to gain entrance. Owing to the rapid propa-
gation of the fungus in weakened or injured tissues of citrus
trees, it is difficult to distinguish between injury initiated by the
fungus and that initiated by injuries alone, after which the
fungus has invaded the tissues as a saprophytic or semi-parasitic
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 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 be girdled
by an attack farther back, in which case they fall over and
hang lifeless as if attacked by some insect. Young leaves in-
fected 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.
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.







Parasitic Diseases of Citrus in Florida


The anthracnose blemishes vary from small specks to spots
ranging from a quarter of an inch or more in diameter. At first
these spots are reddish-brown in color but when fully developed
they become dark brown to black, circular or nearly so, sunken
below the general level of the surface, dry and hard (Fig. 34),


Fig. 34.-Anthracnose on grapefruit.


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 of
entrance for other fungi which break down the fruit. Conse-
quently, fruit affected by anthracnose or similar spotting is
worthless for shipping. On very mature, overripe fruit certain








Florida Agricultural Experiment Stations


strains of the fungus may produce a soft, pliable decay 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 blem-


W .
L ~ jL T.


Fig. 35.-Anthracnose blemish developed in thorn puncture on grapefruit.
(1 5/16 natural size.)

ishes are especially common on grapefruit (Fig. 35) and cause a
large amount of fruit to be culled at the packinghouse. Other
fungi than the anthracnose fungus, however, also commonly
contribute to the decay of fruit following such injuries.
Spots of the same general nature as anthracnose blemishes
may be caused also by organisms other than the anthracnose
fungus. Such spots also develop frequently following a physio-
logical 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.







Parasitic Diseases of Citrus in Florida


Cold storage spots, now considered to be a physiological
breakdown of the rind, have been confused with anthracnose in
the past. Thus, a number of spots other than those due to this
fungus unquestionably have been included under anthracnose
in the earlier literature on this disease. The presence of the
fungus in these spots does not constitute proof that it was re-
sponsible 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 stimulated
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.
Anthracnose of the lime, which is quite distinct from that on
the other citrus fruits, does not depend upon lowered vitality of

Fig. 36.-Anthracnose of young Key limes resulting from infection of
fruits developing during the rainy summer period. Note the numerous
prominent corky spots, deformation, and splitting of the fruit.







Florida Agricultural Experiment Stations


the tree, and is much more severe. Infection of 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. 36).
Sometimes the corky out-
Sgrowths will be sloughed
F" off, leaving shallow or
deep depressions, depend-
ing upon the severity of
the attack.
Russeting.-Staining of
the rind of citrus fruits,
commonly known as rus-
seting and tear-staining,
is sometimes produced by
the spores of the anthrac-
nose 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 anthrac-
nose, merely result in a
certain amount of russet-
ing of the rind. In some
cases this russeting de-
velops as a large blotch
Fig. 37. Anthracnose russeting on (Fig. 37), while in others
grapefruit, caused by spores of the fungus it develops as a tear-stain
on the dead twig above washing down over effect reported by Rolfs in
the fruit.
1904. However, most of
the russeting on citrus fruit is caused by rust mite. The anthrac-
nose russeting is similar to, but fainter than rust mite russeting.
Cause of Anthracnose.-Anthracnose and the accompanying
injuries on citrus trees other than limes are caused by fungus








Parasitic Diseases of Citrus in Florida


Colletotrichum gloeosporioides 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 distribution.
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
somewhat upon the virulence of the strain of the fungus as well
as upon the condition of the tree.
Fruiting bodies of the fungus develop as minute, dark,
slightly raised, densely massed pustules on the twigs, leaves,
fruits, and dead bark of citrus trees (Figs. 33 and 38.). 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 diseased trees.
This fungus, however, with the exception of the species on
lime, rarely attacks growing tissues unless trees are in a
weakened or run-down condition. Trees weakened from over-
bearing, 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 anthracnose is
most abundant and causes the greatest injury. For this reason,
there has been considerable difference of opinion among investi-
gators and growers as to the part played by the fungus in any
given outbreak of anthracnose.
In regard to the anthracnose previously described on the
lime, which is caused by Gloeosporium limetticolm Clausen, there
never has been any doubt as to the parasitic nature of the organ-
ism involved, since it attacks young growing tissue and not the
mature tissue.
Control of Anthracnose.-Since the most important con-
tributing conditions for the development of anthracnose and
related troubles of the common varieties of Citrus, other than
limes, are those which have to do with the weakening of the
tree and the tissues, it is of primary importance to maintain the
health and vitality of the trees by proper care in fertilization,
cultivation, and other grove practices.
In the case of anthracnose of limes, where infection of young
shoots, blossoms and young fruits takes place very rapidly during








Florida Agricultural Experiment Stations


Fig. 38.-Grapefruit leaves attacked by the anthracnose fungus. These
leaves were turning yellow and falling from the tree. Note the characteris-
tic fruiting bodies of the fungus, consisting of concentric zones of minute
black pustules formed on the dead areas of the leaves.

rainy weather, control of the disease can be accomplished only
by timely and repeated applications of a fungicide, such as
bordeaux-oil emulsion or bordeaux mixture 5-5-100, to protect
the developing parts while they are susceptible to the disease.
One application should be made at the time of blooming and be
followed by one or two others at intervals of from 10 days to
two weeks while the young fruits are forming. When the







Parasitic Diseases of Citrus in Florida


bordeaux-oil emulsion is used on the tender young fruit it is
advisable to reduce the oil content to 1/ of 1 percent. Close
watch should be kept for evidence of increase of scale-insects,
which may have to be combatted by the application of oil
emulsion.

Diplodia Twig and Branch Dieback

(Caused by Diplodia natalensis Pole-Evans)
Species of Diplodia have been reported on citrus trees, es-
pecially twigs and dead parts, in nearly every important citrus-
producing country. In Florida and the West Indies a species,
apparently identical with Diplodia natalensis, is responsible for
a number of citrus troubles, including a sudden dying back of
the twigs and branches, gumming, a destructive stem-end rot of
the mature fruit, and apparently a collar-rot type of girdling of
the trunks of young trees. That this fungus was active in the
death of twigs of both citrus and peach trees in Florida was
first shown by the inoculation experiments of Fawcett and Burger
in 1911. In 1915 Earle and Rogers reported that Diplodia was
a serious factor in the dying of twigs and branches of citrus
trees in the Isle of Pines. This form of citrus dieback was re-
ported from Cuba by Bruner in 1917. Nowell (1923) considers
that Diplodia is important as a secondary agent in the dying
back of lime twigs in the Lesser Antilles. But relatively little is
known concerning the different disease manifestations caused by
this widespread fungus. The effect of Diplodia in causing an
important fruit decay, however, is well known.

Symptoms of Diplodia Dieback.-Attacks of Diplodia on
citrus trees may manifest themselves in a number of different
ways. Probably the most common form of attack is a rapi'I
dying back of the twigs and smaller branches which closely re-
sembles withertip. In fact, this trouble is often called withertip
and can be distinguished only by laboratory examination. The
twigs may die back from the tips for some distance, ending at a
sharply defined line which demarks the diseased from the
healthy tissue. In such cases the point at which the progress of
the disease becomes arrested usually is characterized by a slight
exudation of gum. In other cases no limiting line is developed








Florida Agricultural Experiment Stations


and the entire twig and possibly part of a larger branch may be
involved and killed (Fig. 39). Under favorable moisture con-
ditions the bark that has been killed eventually becomes dotted
by numerous minute, black pustules which are the fruiting
bodies of the fungus. Instances have
been noted from time to time, especially
on lemon and grapefruit trees, where
the fungus follows the attacked twig
and infects a fruit borne on it, the de-
caying fruit often mummifying on the
dead twig. In still other cases the at-
tack may not begin at the end of a twig
or branch but at some point well back
on it, causing its girdling, as a result of
which the leaves above the girdled place
quickly wither and the attacked twig or
branch dies. An internal gumming and
death of the bark occurs at the attacked
point and a few drops of gum may exude
on the surface.


Fig. 39.-Diplodia die-
back on lemon branch,
showing exudation of gum
at base of lesion.


Cause of Diplodia Dieback.-It has
been found that the types of injury de-
scribed above are caused by a species of
Diplodia apparently identical with Dip-
lodia natalensis Pole-Evans. A number
of conditions may be mentioned as con-
tributing to attacks of Diplodia on the
trees. The most important of these
seem to be the presence of wounds,
maturity of woody tissues while still
weak, and deficient or unfavorable nu-
tritional conditions. The sudden check-
ing of growth with the setting in of
drought and heavy fruiting on poorly
nourished trees on light sandy soils are
mentioned by Earle and Rogers as sup-
plying favorable conditions for attack.
The anthracnose fungus is frequently
associated with Diplodia and Fawcett








Parasitic Diseases of Citrus in Florida


believes that these fungi probably do more injury in combina-
tion than when each is working alone.
Control of Diplodia Dieback.-Remedial measures for the
control of Diplodia twig and branch dieback comprise the prompt
cutting off of attacked branches well back into the healthy wood
and the painting of all wounds with a good antiseptic and water-
proof wound dressing to prevent reinfection. For the prevention
of attacks of this disease, however, reliance must be placed
largely on such cultural and fertilizing methods as will keep the
trees in a thrifty condition. Trees maintained in a good state of
vigor will be less adversely affected by unfavorable seasonal con-
ditions which may be beyond the grower's control and less sus-
ceptible to attacks by Diplodia and a number of other fungi.

Sphaeropsis Knot
The disease known as Sphaeropsis knot (caused by Sphae-
ropsis tumefaciens Hedges) is found occasionally in Florida,
where it has been known to occur since 1911. It has been re-
ported from all sections of the citrus-growing district but is a
relatively unimportant trouble. This disease is widely dis-
tributed in Jamaica, where it has caused considerable damage to
lime and orange trees. It occurs as a minor disease in Cuba. It
is known to occur naturally on lime, grapefruit, sweet orange,
and sour orange trees, but small knots have been produced arti-
ficially by inoculation on lemon, tangerine, and trifoliate orange.
Symptoms of Sphaeropsis Knot.-Knot may occur on trees of
any age and old growth appears to be attacked as readily as
young. The knots vary in size from three-eights of an inch to
two and three inches in diameter and may occur singly or with
several close together on the branches (Fig. 40). They are
usually round but variations sometimes occur as elongated
swellings more or less closely crowded along the stems.
The first indication of infection is a slight swelling of the
branch, which ultimately develops a well-formed knot at the
point of infection. In the early stages of growth the surface of
the knot is light-colored and rather smooth. With age the sur-
face becomes darker and more or less fissured and an old knot
from which the bark has disappeared may be almost black,
rough, and deeply furrowed. Frequently, in rapid growth, knots




Florida Agricultural Experiment Stations


.4


Fig. 40.-Citrus knot on grapefruit branch.







Parasitic Diseases of Citrus in Florida


of large size develop with the bark intact and the surface may
remain smooth and light in color. Although knot is quite viru-
lent and destructive in Jamaica, the disease is found only rarely
and in but a very mild form in Florida.
Cause of Sphaeropsis Knot.-The disease is caused by the
fungus Sphaeropsis tumefaciens Hedges, which penetrates and
grows within the tissues, stimulating the formation of the
typical galls or knots. Living parts of the fungus may be de-
tected in the tissue of apparently healthy twigs several inches
beyond the knots.
According to Hedges and Tenny, who made a detailed study
of the disease and the fungus causing it, fruiting bodies have not
been found on naturally occurring knots, although a few strains
of the fungus have been found to produce fruiting bodies on
galls induced by inoculations in the greenhouse. The exact mode
by which the disease is disseminated is not known but the fungus
is capable of living for several years in the tissues of an at-
tacked tree.
Control of Sphaeropsis Knot.-In Jamaica, where the disease
has proved quite troublesome, it has been kept under control by
cutting out all affected parts of the tree. This method is fairly
successful where attacks are slight and confined to the smaller
branches and twigs. The affected parts should be cut off several
inches below the lowest gall so as to be sure to remove all infected
tissue. Removing the affected branches probably will suffice
for the control of this disease in Florida.

Heart or Wood Rot
Dead or decayed wood (caused by various fungi) is frequently
found in citrus trees. Such areas may be in the interior of the
trunk or large branches or on the surface. This condition is
found especially in trees that have been frozen back severely, top
worked or headed back, sunburned, attacked by foot rot or other-
wise diseased and the wounds not properly treated. In the first
stages of decay the wood is firm but is discolored. Later the
affected wood becomes soft and punky or spongy and gradually
disintegrates. Boring insects and ants enter such wood and
help to break it down.
Any injury to the bark which exposes the wood serves as an.







Florida Agricultural Experiment Stations


infection point for the development of heart rot or wood rot.
The two most common types of bark injury in citrus trees are
pruning wounds and sunburned bark. Once wood rot is initiated
it gradually spreads throughout the heart wood of the tree. This
spread is slow and in a large tree may be many years before any
serious effects occur. Branches in which wood rot has originated
at a sunburned area are weakened and may break under the
strain of a large crop or during hurricanes. Trees which have
extensive heart rot throughout the trunk and larger branches
are extremely subject to breaking during a storm. Coupled with
the structural weakening from heart rot is the gradual declining
of the tree as the condition progresses.
There are many fungi which can cause heart or wood rot.
Most of these belong to the group called Basidiomycetes and are
commonly called wood-rotting fungi. Ganoderma sp. is one com-
monly found in Florida. There are several other fungi which
are found also in the heart or wood rot complex. Some of them
have bracket and semi-woody types of fruiting bodies that ap-
pear on trunks or branches of infested trees. A few others
produce a toadstool or mushroom type of bodies. One of these
is Clitocybe tabescens, which is discussed elsewhere in this
bulletin.
The best control for heart or wood rot is prevention. Once
the rot becomes established it is very difficult to alleviate the
condition. Since the spores of these wood-rotting fungi cause
infection only through exposed wood, all pruning wounds over
an inch in diameter should be covered with a good pruning paint
on the same day that the pruning is done.

Any bark that becomes sunburned should be cut out and the
wound covered with pruning paint. A little attention to the care
of these two types of wounds will eliminate heart rot and make
better trees. If wood rot becomes established, but has not ad-
vanced too far, decayed and diseased wood can be cut out. The
wound should then be treated with avenarius carbolineum and
later painted with a pruning paint. If the cavity will hold water
it should be trimmed or cut in such a way as to provide drainage
after rains. The filling of cavities with a material does not
usually pay for the expense incurred and the open cavities with








Parasitic Diseases of Citrus in Florida


the wood properly dug out and treated can be inspected more
easily for further decay. The use of surgery on a tree is an
expensive item and the proper care of wounds as they are made
is the best procedure.
Damping-Off
Damping-off (caused by Rhizoctonia solani Kiihn and Sclero-
tium rolfsii Sacc.), a universal trouble of seedling plants, com-
monly attacks citrus seedlings. This is primarily a disease of
young seedlings, attacking them at any time after the seeds
germinate until the stem tissues become woody. The first indi-
cation of damping-off usually is a watersoaked or brown area of
decaying tissue on the stem, at or near the surface of the ground
(Fig. 41). This lesion extends up the stem and girdles it to a
point about a half-inch above the ground. The soft decayed
cortical tissue then sloughs off and the exposed woody tissue
dries up and becomes whitened. The seedlings usually are com-
pletely girdled before they wither and die and do not fall over as
in the usual form of damping-off (Fig. 42).


Fig 41.-Damping-off of citrus seedlings. Note the lesions
at the ground line.







Florida Agricultural Experiment Stations


Damping-off occurs characteristically in spots or patches in
the seedbed. These spots may enlarge considerably if soil con-
ditions are favorable for the spread of the causal organism. A
number of different soil-inhabiting fungi have been found as-
sociated with the damping-off of citrus seedlings, but Rhizoctonia
solani Kiihn has been found to be the most common cause in
Florida. Sclerotium rolfsii Sacc. also has been found responsible
for the damping-off of citrus seedlings in some cases.
Control.-A number of conditions have been found to favor
the development of damping-off. Among these are poor drainage,
improper watering, excessive shading, a high degree of humidity,
overcrowding, and planting on infected soils. Control measures
consist largely in the careful management of the seedbed to avoid
these conditions. Only well-drained soil should be selected for a
citrus seedbed. Since infection is most common at the ground
level, this part should be kept as dry as practicable. If the loca-
tion selected has been used previously for a seedbed and damping-
off occurred at that time the soil should be disinfected before the
Fig. 42.-Damping-off of citrus seedlings.
%..
,"j "..,'} 1../ -..!J', "*,








Parasitic Diseases of Citrus in Florida 85

new seedbed is established. This may be done by treating with
steam under pressure, as is done in the case of soil sterilization
in greenhouses, or by use of 1 part of commercial formaldehyde
to 50 parts of water, applied at the rate of 1/. gallon to each
square foot of soil surface. After the soil has been treated it
should be covered for a day or two with gunny sacks or canvas to
permit the fumes to thoroughly penetrate the soil and kill the
soil fungi. Soil which has been treated should be worked up two
or three times to allow the fumes to escape. It is safe to plant
seed about two weeks after the soil has been treated.
To protect seed from pre-emergence damping-off and to in-
crease stands, they may be treated with a seed protectant such
as spergon, cuprocide or one of the neutral coppers. About 1 level
teaspoon of material per quart of seed should be sufficient. The
seed and protectant can be mixed in a small drum or other dust-
tight container. The excess dust is then screened off and the
seed planted.
If the disease shows up in young seedlings, spray them with
5-5-100 bordeaux mixture or a neutral copper at an equivalent
copper content. Sufficient spray should be applied to the seed-
lings so that the liquid runs down the stem and wets the soil
around it. The spray should be applied at weekly intervals until
the damping-off has been controlled.

Sooty Mold
Sooty mold (caused by Capnodium citri Berk & Desm.) is
abundant throughout the citrus-growing sections of the world
wherever whiteflies, aphids, scales, and other honeydew-excret-
ing insects feed on the trees. It is a black fungous growth which
forms a filmy membrane over the surfaces of the leaves, twigs.
and fruit of citrus and many other trees.
The sooty mold fungus does not attack citrus tissue but is
simply a saprophytic organism developing on the surface of the
plant parts in the honeydew excreted by the larvae of such
insects as whiteflies, aphids, and scale-insects. These insects are
gregarious in their habits and, consequently, the sweetish liquid,
termed honeydew, that is excreted by them collects in con-
siderable quantities on the various parts of the tree. This
sweetish excretion also proves very attractive to ants. If the
insects are few or the amount of honeydew is slight this black







Florida Agricultural Experiment Stations


mold may occur in lim-
ited spots but, when the
accumulation b e comes
abundant, the entire sur-
face of the leaves, fruits,
and twigs may be cov-
ered by a dense, continu-
ous black membrane
composed of the closely
interwoven vegetative
filaments of the fungus.
Spores are borne on the
filaments and, through
the agency of wind, are
carried from tree to tree.
Sooty mold is entirely
superficial and may be
rubbed off or often
pulled loose from the
leaf (Fig. 43). With age,
under very dry condi-
tions or where the trees
have been sprayed with
oil emulsion or soap solu-
tion, it may become
broken up and detached
and blown off in frag-
ments by the wind.
When sooty mold fol-
lows the attacks of
whiteflies or other honey-
dew-excreting insects oc-
Fig. 43.-Sooty mold on grapefruit leaf. curring in enormous
The light areas show where the film of numbers on citrus trees,
closely interwoven fungus filaments has
been partly detached. it prevents the sunlight
from reaching the leaves
and interferes greatly with the normal functioning of the leaves.
It may seriously retard the growth and cause light blooming and
fruiting. Fruits that are covered with the fungus are retarded
in ripening, usually color unevenly, and are often small in size.








Parasitic Diseases of Citrus in Florida 87

Since sooty mold occurs merely as
a saprophyte in the honeydew ex-
creted by certain insects, especially
whiteflies, aphids, and scale insects,
it can be prevented by controlling
these insects. This may be done
either by the use of natural enemies,
such as entomogenous fungi and in-
sect parasites, or spraying with oil
emulsion. The latter will prove very
effective in cleaning the trees of sooty
mold.

Felt Fungus

In Florida and the tropics the
twigs and branches of citrus trees are
frequently encircled by growths of
the felt fungus (Septobasidium pseu-
dopedicellatum Burt). This fungus
also occurs on a number of other
trees. It causes little, if any, direct
damage to citrus trees. The fungus
appears most frequently on the
twigs, sometimes extending along the
leaf-stalks and bases of the leaves. It
appears as a soft, felt-like covering,
light brown to gray in color, which
encircles the twig on which it may be
growing and envelops it for a dis- Fig. 44.-Felty fungus en-
tance of from one to three or more circling live grapefruit twig.
inches (Fig. 44). The surface of the
growth is smooth and compact or membrane-like, while beneath
this the mass is soft and spongy. The fungus grows over the
surface and does not appear to penetrate the bark tissue.
Patches of it occasionally occur on the leaves.
The felt fungus may be found in many groves but is more
prevalent in moist situations such as groves in low hammocks.
Since there is no serious injury caused by the felt, no specific
control methods are warranted. It may be eliminated by
pruning out the twigs and branches bearing the fungus. Also








Florida Agricultural Experiment Stations


the fungus is rarely found in those groves that regularly re-
ceive a melanose spray.

Flyspeck and Sooty Blotch
The flyspeck fungus occurs occasionally on citrus and other
fleshy fruit in Florida. This fungus (Leptothyrium pomi (Mont.
& Fr.) Sacc.) derives its name from the minute black specks con-
sisting of closely interwoven fungous filaments, about the size
and color of flyspecks, that develop on the surface of the fruit
(Fig. 45). This fungus may or may not be accompanied by a
sooty growth of dark-colored fungus filaments which spread over
the surface of the rind in blotch-like spots. The latter fungus
(Gloeodes pomigena (Schw.) Colby) is termed sooty blotch. It is
different from the previously described sooty mold, which de-
velops after infestations of the whitefly, and may occur when the
insect is absent.
No damage to the rind results from the presence of either
the flyspeck or sooty blotch fungus but the appearance of the
fruit is marred. The sooty blotch fungus usually will be re-
moved in the washing and polishing process but the flyspeck
fungus is more persistent and may result in lowering the grade
of the fruit.

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Br^ .c "r\ FI 1 s:- *. *"'.A


Fig. 45.-Flyspeck fungus on grapefruit. (Slightly enlarged.)








Parasitic Diseases of Citrus in Florida


The development of these superficial fungi are favored by
shade and humid atmospheric conditions. They are most likely
to develop late in the winter in groves with moist, humid con-
ditions and on fruits that have remained on the trees long after
they have become ripe. Their development is favored by a rank
growth of weeds about the lower branches of the trees. The
occurrence of these fungi is not of sufficient frequency to war-
rant control measures.
Thread Blight
Thread blight (caused by Corticum stevensii Burt) is one of
the minor citrus diseases in Florida, which rarely causes the
grower any trouble. The causal fungus occurs on several dif-
ferent woody plants scattered over the state. It forms long
brown threads or strands on the twigs and fruit (Fig. 46).
These often spread out on the under side of the leaves, forming
a thin, whitish weft of fungous filaments. Here and there along
the twigs small, round, hard bodies, called sclerotia, are formed.
By means of these bodies the fungus is enabled to live through
unfavorable periods and perpetuate itself in more favorable sea-
sons. Spores are developed from the fungous wefts on the
attacked leaves. In some instances the fungous filaments pene-
trate the leaves and cause their death. The fruit may have
several threads or strands running over the surface or, at times,
it may be covered with a cobwebby weft of brown fungous
filaments.
The thread blight fungus is restricted to rather limited lo-
calities in the southeastern United States, apparently because of
the high humidity and high temperature required for its develop-
ment. In Florida these favorable conditions occur chiefly in groves
surrounded by heavily wooded swamps or hammock forests.
In case this disease becomes of sufficient importance to
justify control measures, its development can be checked easily
by a single application of bordeaux-oil emulsion at the beginning
of the rainy season.
Black Rot
Black rot of oranges (caused by Alternari citri Ellis &
Pierce) is widely distributed throughout the citrus region of
Florida but it is only in occasional years that the disease becomes
sufficiently prevalent to cause considerable damage.
This disease was first described in 1901 by Pierce, who noted








Florida Agricultural Experiment Stations


Fig. 46.-Thread blight on grapefruit.


it occurring in California as early as 1892. The same or similar
decays of oranges have been reported to occur also in Arizona,
Cuba, South Africa, and in Victoria, Australia.
It was formerly thought that attacks of black rot were con-
fined to the navel orange but the disease has long been known to
attack other varieties. In Florida the occurrence of this trouble
has been observed on Hamlin, Jaffa, Parson Brown, Pineapple,
Ruby Blood, and Valencia oranges and on tangerines. The
disease appears to be most prevalent on the early varieties. In








Parasitic Diseases of Citrus in Florida


one case 25 percent of the fruit had developed and undoubtedly
a larger percentage eventually developed the disease.
Symptoms of Black Rot.-The most characteristic external
symptom of black rot on oranges is the premature coloring of
full-sized or nearly full-sized fruit. The disease is most con-
spicuous just before the normal crop colors. At this time the
infected fruits color sooner than the remaining sound fruits and
turn a deep-orange color. Many of the diseased oranges fall
from the trees early in the season, while others which were
perhaps infected later or in which there has been a very limited
development of the causal fungus remain on the trees. Such
fruits often look sound on the exterior but, when cut, have a dry,
black decayed place within, at, or near the stylar end.
Black rot begins at the stylar, or so-called "blossom," end of
the fruit. The causal fungus seems to make but a weak attack
upon the tissues of the fruit and has very limited power to
spread, working mainly along the core. The rind is not affected
at first and becomes involved only after the fungus has made
considerable progress within the fruit, in which case it becomes
discolored over the affected interior. The exterior light-brown to
blackish discoloration usually is confined to a limited portion
near, and usually at one side of, the navel or stylar end. The
fungus evidently exerts a stimulating effect upon the physio-
logical process of the fruit and, even though the diseased por-
tion of the orange is small, the whole orange develops a high
color prematurely.
When an infected orange is cut in half lengthwise the full
extent of the rot becomes apparent. Underneath the brownish
or dark-colored spot the tissue is brown or, in more advanced
stages of decay, greenish-black. The blackening may extend
through the central portion of the fruit (Fig. 47). This decay
does not soften the fruit as rapidly as other rots and is confined
more to the interior of the fruit. In some cases there is no
visible sign on the exterior to indicate that black rot is present.
In such a case a grader naturally is going to let the fruit go to
market, and the disease will become apparent later. When the
black rot has attained an advanced stage of development a
prominent greenish-brown to blackish spot makes its appearance
at the stylar end. At this time the core is black and the adjoin-








Florida Agricultural Experiment Stations


Fig. 47.-Black rot of orange. Sectional view showing extent of the decay.
ing fruit cells are beginning to break down. The pulp and juice
in close proximity to the blackened central portion of the fruit
have an unpleasant bitter taste, while the balance of the fruit is
affected but slightly.
Cause of Black Rot.-Black rot is caused by a fungus known
as Alternaria citri Ellis & Pierce, the spores of which are believed
to lodge in the natural openings of the fruit, which may be
either the navel or a small imperfection at the stylar end in the
case of other oranges. Infection apparently does not take place
until after the oranges have grown to sufficient size to distend
the navel or other natural opening. There seems to be a direct
relation between the occurrence of black rot and moisture, as
the disease is most prevalent after periods of humid or rainy
weather.
Control of Black Rot.-As a rule, the infection at the stylar









Parasitic Diseases of Citrus in Florida


or blossom-end of oranges is not of sufficient importance to
justify special control measures. If the variety showing in-
fection is mature the crop should be picked as soon as possible,
for the longer the fruit remains on the trees the greater may
be the amount of rot. A pre-harvest spray has not been reported
as effective, but in two instances some benefit was obtained with
a neutral copper spray thoroughly applied as soon as the presence
of the rot was observed.
In general, blossom-end rot is most apt to occur on fruit in
groves that have been over-fertilized. In such cases the fruit
grows so fast that the tissues at the blossom-end do not grow
properly. This leaves an opening through which the infection
can take place.

Stylar-End Rot of Tahiti Limes
Tahiti limes, and sometimes also lemons, frequently develop
a firm, light-colored collapse of the rind at the stylar, or so-
called "blossom," end of the fruit. This type of rot (cause un-
known) is of frequent occurrence in Florida and has been re-
ported also from Cuba and California. It starts as a light to
drab-colored sunken area at the base of the nipple or tip (Fig.

Fig. 48.-Stylar-end rot of Tahiti limes.


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: 1
r








Florida Agricultural Experiment Stations


48) and progresses until from about one-third to one-half the
fruit is involved. In occasional instances the trouble may start
at the stem end or on one side of the fruit. The affected portion
of the rind has a water-soaked appearance and is sunken some-
what below the level of the healthy surface, but remains firm.
The tendency of the fruit to develop this decay on trees about
the time it ripens constitutes an undesirable feature of this
otherwise highly desirable lime variety.
The cause of this trouble is not known but it has been
thought to be a physiological breaking down of the rind, since
cultures made in the early stages of the trouble consistently fail
to yield an organism. In the later stages of the trouble, however,
Colletotrichum, Diplodia, Oospora, and other fungi have been
found associated with the disease in Florida. These fungi, which
appear to be purely secondary, rapidly bring about the decay of
affected fruits after the trouble has started.
Since stylar-end rot occurs in greatest abundance as the fruit
approaches maturity, part of the loss can be avoided by picking
the fruit before it becomes too mature.

Diseases Caused By Algae and Lichens and Parasitic
and Epiphytic Flowering Plants
Citrus trees commonly harbor a great variety of plants, in-
cluding algae, lichens, mosses, ferns, dodder, Cassytha, and air
plants. With the exception of the parasitic plants, these miscel-
laneous plants are of little or no particular consequence in the
life of the trees. Most of them are merely attached to the
surface of the bark, although some may penetrate the outer
portion of the bark. The more important of these plants are
discussed in the following.

Algae
Many algae are found on the bark of the trunk and branches
of citrus trees. They are responsible for the green to yellowish-
green coating on the bark and may occur on the leaves under
conditions of high humidity. The majority of the algae are not
harmful and do no damage. In those groves regularly sprayed
with copper for melanose control algae are not usually found.
One alga (Cephaleuros virescens Kunze) is parasitic on
citrus. The disease is known to occur principally in the southern








Parasitic Diseases of Citrus in Florida


half of the state. Although it occurs occasionally in the central
part of the state, the disease is most prevalent along the east and
west coasts and extreme southern part where conditions are
more generally favorable for the growth of the parasite. This
alga also occurs in the West Indies, South America, East Indies,
India and Africa.
In Florida, Cephaleuros virescens has been found on ap-
proximately 50 species of plants. It has been found on the fol-
lowing kinds and varieties of citrus: Tahiti and Key limes;
Meyer and Ponderosa lemons; Thompson Pink, Royal, Foster
Pink, and Marsh grapefruit; Pineapple, Navel, Hamlin, Blood,
Temple, Jaffa, Orlando, and Valencia oranges; Dancy tangerine;
Mineola tangelo; kumquat and calamondin.
Usually the disease is not noticed until the red fruiting stage
of the alga is present on the bark of the twigs and branches.
The infected areas may be small spots up to an inch in diameter
or the spots may have coalesced to cover the surface of a branch
for a few inches up to several feet. The extent of the diseased
area is an indication of the length of time twigs or branches have
been infected. During most of the year the lesions are covered
with a thin greenish-grey velvety layer of algal growth. During
May, June, July and August the velvety growth assumes a brick
red color due to the presence of the fruiting structures of the
parasite.
On the leaves round greenish-grey spots occur (Fig. 49).
These are more noticeable when the alga is fruiting, at which
time red-brown color predominates. A fruit blemish also may
occur. The lesions on the fruit are dark brown to almost black
in color and slightly raised with irregular margins.
The infection of the bark by the alga causes considerable
injury to the trees in some cases. The bark on the twigs of the
current flush of growth or on the larger limbs is not subject to
infection. As a result of the alga penetrating the tissues im-
mediately below the epidermis of the bark the bark becomes
thickened and the twigs or branches become enlarged at the
places attacked. The attacked bark eventually becomes cracked
and scaly and the infected branches are stunted and the foliage
is sparse. If new bark cannot form rapidly enough to make up
for the loss caused by the parasite, death of the diseased twigs
and branches will result. If the new bark is formed in sufficient








Florida Agricultural Experiment Stations


amounts the outer diseased bark will crack, become scaly and
slough off. (Fig. 50). When the twigs or branches are in this con-
dition their appearance may be confused with that of scaly bark
which is an entirely different disease.
| Under conditions favorable for the de-
velopment of the disease, particularly
on limes and lemons, the death of
numerous twigs and branches up to
two inches in diameter has occurred.
In some cases the branches may be
only weakened. Attack by the alga
results in a poor tree and a reduced
yield of fruit.
SThe disease is not difficult to con-
S trol. The trees should be kept in a
vigorous state of growth by the proper
Suse of fertilizers. Dead and severely
Sl diseased branches should be pruned
out and burned. If the trees are
planted too close and considerable
difficulty is experienced with the dis-
Fig. 49.-Alga spot on ease it would be advisable to thin out
citrus leaf. the grove.
In those groves that are regularly sprayed with a copper
fungicide the algal disease is not usually found. If the disease
becomes serious a thorough pruning accompanied by the follow-
ing spray schedule as a clean-up program is suggested.
1. Bordeaux mixture 6-6-100 plus two quarts of oil emulsion
applied in December or January before a flush of growth starts.
2. Bordeaux mixture 4-4-100 plus 2 quarts of oil emulsion
just before the rainy season starts or as soon as the red stage of
the alga is first observed.
3. Bordeaux mixture 4-4-100 plus 2 quarts of oil emulsion one
month after the second application.
In the second and third applications a neutral copper at
equivalent copper concentration may be substituted for the
bordeaux mixture. The usual additions of insecticides can be
made as needed. If sulfur is used omit the oil. In spraying for
the control of this disease the spray should be directed so as to
cover thoroughly the bark of all twigs and branches.








Parasitic Diseases of Citrus in Florida


Fig. 50.-Scaly condition of bark on citrus twigs induced by Cephaleuros
virescens Kunze. (Photograph by E. P. DuCharme.)








Florida Agricultural Experiment Stations


After the disease has been brought under control by the
clean-up program, the use each year of a copper spray for
melanose control should give satisfactory control also of the
alga if the twigs and branches are thoroughly sprayed.
Lichens
Lichens frequently occur on the trunks, branches, twigs, and
leaves of citrus trees and are sometimes mistaken for injurious
pests by those unfamiliar with them. Lichens belong to one of
the lower forms of plant life, in which a fungus and an alga live
together with mutual benefit to each other. As a rule, they are
not at all parasitic but live entirely on the surface and obtain
their food material chiefly from the air, or dead, functionless
tissues of the bark on which they grow. In addition to growing
on the trunks and branches of citrus trees, they may be found
growing on fence posts, rocks, and other inanimate objects. In
groves, lichens are more apt to be abundant in damp, shady
situations and are frequently conspicuous on old, neglected, and
run-down trees. They thrive in the moist, humid climate here.
There are many varieties of lichens. A very common form
is a grayish-green, paper-like growth that occurs on the bark.
This growth may vary from small spots or patches to areas so
continuous and extensive as to cover trunks and limbs com-
pletely (Fig. 51). The edges are usually lobed and curl upward.
Another lichen appears in the form of a round, pinkish-white
area, adhering closely to the bark. Still another form appears
as small, black dots on a white, indefinitely outlined background.
The leaf-inhabiting lichens are less common and appear as more
or less numerous, small, rounded, grayish spots closely resembl-
ing algal spots (Fig. 52).
Lichens are considered harmless so far as any direct injury
produced by their own growth is concerned, but detract greatly
from the well-kept appearance of the trees. They are apt to
grow more abundant on twigs and branches of weak, slow grow-
ing trees than on vigorous, rapidly growing ones. Since the
lichens do not penetrate the bark deeply and establish no true
parasitic relation with the tree, it is supposed that any damage
they may cause is due to their mechanical interference with the
respiration and transpiration processes which go on through the
lenticels, or breathing pores, of the bark.




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