Front Cover
 Title Page
 Table of Contents
 Spraying and dusting
 Some decline diseases of citrus...
 Some fruit blemishing diseases...

Group Title: Bulletin - University of Florida. Agricultural Experiment Station - no. 587
Title: Handbook of citrus diseases in Florida
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00027498/00001
 Material Information
Title: Handbook of citrus diseases in Florida
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 157 p. : ill. ; 23 cm.
Language: English
Creator: Knorr, Louis Carl, 1914-
Suit, R. F ( Ross Frisbie ), 1904-
DuCharme, E. P ( Ernest Peter )
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1957
Subject: Citrus -- Diseases and pests -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Statement of Responsibility: by L.C. Knorr, R.F. Suit, and E.P. DuCharme.
General Note: Cover title.
General Note: Includes index.
Funding: Bulletin (University of Florida. Agricultural Experiment Station) ;
 Record Information
Bibliographic ID: UF00027498
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida
Resource Identifier: aleph - 000926798
oclc - 01719617
notis - AEN7498

Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
    Title Page
        Page 1
    Table of Contents
        Page 2
        Page 3
        Page 4
    Spraying and dusting
        Page 5
        Page 6
        Page 7
        Algal Disease
            Page 8
            Page 9
            Page 10
            Page 11
            Page 12
            Page 13
            Page 14
            Page 15
            Page 16
        Black rot
            Page 17
            Page 18
            Page 19
            Page 20
            Page 21
        Brown rot
            Page 22
            Page 23
            Page 24
            Page 25
            Page 26
            Page 27
            Page 28
            Page 29
            Page 30
        Cassytha infestation
            Page 31
        Citrus-nematode disease
            Page 32
            Page 33
            Page 34
        Clitocybe mushroom root rot
            Page 35
            Page 36
        Cold damage
            Page 37
            Page 38
            Page 39
            Page 40
            Page 41
            Page 42
            Page 43
            Page 44
            Page 45
        Diplodia dieback
            Page 46
            Page 47
        Diplodia root rot and collar rot
            Page 48
        Dodder infestation
            Page 49
            Page 50
            Page 51
            Page 52
            Page 53
            Page 54
        Felt fungus infestation
            Page 55
            Page 56
        Foot rot
            Page 57
            Page 58
            Page 59
            Page 60
            Page 61
            Page 62
            Page 63
            Page 64
        Greasy spot
            Page 65
            Page 66
        Heart rot
            Page 67
            Page 68
            Page 69
            Page 70
        Juvenile spot
            Page 71
            Page 72
            Page 73
            Page 74
            Page 75
            Page 76
        Lichen infestation
            Page 77
        Lime anthracnose
            Page 77
            Page 78
            Page 79
            Page 80
            Page 81
            Page 82
            Page 83
            Page 84
            Page 85
            Page 86
            Page 87
        Mesophyll collapse
            Page 88
            Page 89
            Page 90
            Page 90
            Page 91
            Page 92
            Page 93
            Page 94
            Page 95
            Page 96
            Page 97
        Rio grande gummosis
            Page 98
            Page 99
            Page 100
            Page 101
            Page 102
            Page 103
            Page 104
            Page 105
            Page 106
            Page 107
            Page 108
        Shell bark
            Page 109
            Page 110
            Page 111
            Page 112
        Sooty blotch
            Page 113
        Sooty mold
            Page 114
        Spanish moss infestation
            Page 115
            Page 116
        Sphaeropsis knot
            Page 117
        Spreading decline
            Page 118
            Page 119
            Page 120
            Page 121
            Page 122
            Page 123
            Page 124
            Page 125
            Page 126
            Page 127
            Page 128
        Star melanose
            Page 129
            Page 130
        Stylar-end rot of lemons and limes
            Page 131
        Thread blight
            Page 132
            Page 133
            Page 134
            Page 135
            Page 136
            Page 137
            Page 138
            Page 139
            Page 140
        2,4-D injury
            Page 141
        Water damage
            Page 142
            Page 143
            Page 144
        Wood pocket
            Page 145
            Page 146
            Page 147
            Page 148
            Page 149
            Page 150
        Yellow spot
            Page 151
            Page 152
            Page 153
        Page 154
        Page 155
        Page 156
        Page 157
    Some decline diseases of citrus trees
        Page 159
    Some fruit blemishing diseases of citrus
        Page 160
Full Text


The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source

site maintained by the Florida
Cooperative Extension Service.

Copyright 2005, Board of Trustees, University
of Florida

Bulletin 587

June 1957

J. R. BECKENBACH, Director
(A Contribution from the Citrus Experiment Station)

Handbook of




Melanose Greasy Spot


I I L 3 1 I__






Felt Fungus


Bulletin 587

J. R. BECKENBACH, Director
(A Contribution from the Citrus Experiment Station)

Handbook of



Fig. 1.-Aftermath of spreading decline, one of the more
destructive diseases of citrus.

June 1957

Introduction ................. .... .. .............. ....... .... ...... 3
Spraying and Dusting .. ................ ........... .......... ..-- ..--------........... 5
Citrus Diseases in Florida (In the text, common names of diseases are
arranged in alphabetical order. In the following list, they are re-
lated to parts of the tree affected. Scanning of this list may help
growers to determine the specific diseases with which they are con-
Parts affected

Algal Disease ........... .......... X
Anthracnose .....................---... X
Black Rot .........................-.. X
Blight .................................... -
Brown Rot .......................... X
Canker ...................................... X
Cassytha Infestation ......-.. -
Citrus-Nematode Disease ...... -
Clitocybe Mushroom
Root Rot ..........-..-...--- --
Cold Damage ................-........ X
Damping-Off ...........--....... -
Decline ...........................-.. -
Diplodia Dieback ....... .......... -
Diplodia Root Rot
and Collar Rot .....--..... -
Dodder Infestation ............. -
Exanthema .............................. X
Exocortis ............................... -
Felt Fungus Infestation ...... X
Flyspeck ................................. X
Foot Rot ............................. -
Greasy Spot ..................... -
Gummosis ..............-......... -
H eart Rot .............................- -
Juvenile Spot ......................... -
Leprosis ................................-- X
Lichen Infestation ............. X
Lime Anthracnose .................. X
M elanose ................................. X
Mesophyll Collapse .............. -
Oleocellosis ......................- ...... X
Psorosis ..................................
Rio Grande Gummosis .......... -
Russetting .............................. X
Scab .- ................................. ..... X
Shell Bark .......................... -
Sooty Blotch ........................... X
Sooty Mold ..-........................ X
Spanish Moss Infestation..... -
Sphaeropsis Knot ...............- -
Spreading Decline ...........--- -
Star Melanose ...................... X
Stylar-End Rot of
Lemons and Limes ..... X
Thread Blight ......... ............ X
Tristeza ......- ... ..........--- ... -
2,4-D Injury ........................... X
Water Damage ................... -
Wood Pocket .......................... X
Xyloporosis ................-- ........ -
Yellow Spot ............... .......... -

& Twigs















& Trunks















- 8
- 11
X 19
X 32

X 35
X 42
X 44

X 48
X 57
X 118

X 133
X 141
X 142
X 147

Handbook of

By L. C. KNORR, R. F. SUIT, and E. P. DUCHARME 2

Unsatisfactory growth of a citrus tree may result from any
one of a number of causes, including: (a) nutritional deficiencies,
(b) climatic excesses, (c) mechanical injuries, (d) genetic ab-
normalities, (e) insect damage, or (f) diseases. This handbook
is concerned chiefly with the troubles of citrus that are caused
by diseases and, in general, leaves to other bulletins the dis-
cussion of difficulties due to malnutrition, hereditary weaknesses,
and insects.3 Nevertheless, some non-infectious troubles have
been included here, first because they resemble, and consequently
need to be differentiated from, infectious diseases; and second,
because they have not as yet been brought together in any handy
work of references.
Diseases may be caused either by infectious or by non-infec-
tious agents. Among the infectious organisms that attack citrus
are fungi, bacteria, viruses, nematodes, and seed plants.
Fungi are thread-like plants, generally microscopic in size,
that injure hosts on which they lead a parasitic existence. Fungi
may inhabit the soil and be transported by splashing water, as
is the case with Phytophthora parasitica, one of the fungi caus-

Based in part on previous bulletins describing the citrus diseases of
Florida, including Agricultural Experiment Station Bulletins 229 by A. S.
Rhoads and E. F. DeBusk and 463 by R. F. Suit.
The writers are indebted to Dr. W. C. Price for editorial assistance. The
present edition has been improved through valuable suggestions contributed
by various specialists. For assistance so generously given, the authors
wish to thank W. L. Thompson, M. H. Muma, R. M. Pratt, F. E. Fisher,
K. W. Loucks, E. F. Hopkins, I. Stewart, C. D. Leonard, H. J. Reitz, and
J. W. Sites.
SThe following publications may be consulted for troubles of citrus
caused by agents other than disease:
Regarding nutritional disturbances:
Fla. Agr. Exp. Sta. Bul. 536: Recommended fertilizers and nutri-
tional sprays for citrus.
Fla. Dept. Agr. Bul. 93: Malnutrition symptoms of citrus with prac-
tical methods of treatment.
Regarding insect damage:
Fla. Agr. Exp. Sta. Circ. S-5: Identification of Florida red and
purple scale on citrus trees.
Fla. Agr. Exp. Sta. Bul.: Citrus insects and their control. (In press.)
Regarding climatic excesses, injuries, and genetic abnormalities:
Fla. Dept. Agriculture: Citrus industry of Florida.

Florida Agricultural Experiment Stations

ing foot rot. Other fungi are present on dead twigs and are
spread by rain, as occurs with the organisms responsible for
melanose. Some fungi are large enough to be seen, as, for in-
stance, the mushrooms of Clitocybe tabescens that grow out of
injured trunks.
Bacteria differ from fungi principally in the matter of size,
being very much smaller: one bacterium of the type that causes
bacterial canker of citrus measures 1/12,500 inch in length.
They may be spread in a manner similar to fungi.
Viruses are disease-producing particles smaller yet than bac-
teria; they multiply only in living cells of animals or plants, to
the detriment of these hosts. Some viruses, like the one that
causes tristeza, are transmitted from tree to tree by insects;
others are spread through the use of infected budwood.
Nematodes (eelworms) are microscopic or barely-visible
worms, the feeding habits of which cause injuries that resemble
those due to fungi, bacteria, or viruses. Nematodes may be
carried on infested roots of nursery stocks or, in certain cases,
in infested soil.
Parasitic seed plants are flowering plants that obtain susten-
ance by sucking food and water from their hosts. Dodder (Cus-
cuta spp.) is one of the parasitic seed plants that injures citrus
trees in Florida.
The non-infectious diseases are not contagious, yet may
equally interfere with normal development of the plants. Ex-
amples are scalding due to frost, shading brought on by epi-
phytes, and leaf spots caused by physiological or genetical dis-
In the field, the disease problem is not so formidable as the
scope of this bulletin might suggest. As in man, a state of
health is the best possible protection from disease. Bag for
bag, more diseases are controlled by fertilizers than by fungi-
cides. Diseases are also controlled when trees are located prop-
erly with respect to drainage and to frost protection.
However, there are two types of disease that are not appre-
ciably influenced by proper food and location, and these are the
bud-transmissible diseases and the diseases to which even healthy
trees are prone (e.g., melanose, scab, spreading decline, etc.).
To control the bud-transmissible troubles, it is necessary to
select budwood from parent trees that are free of disease. Dis-
eases that can thus be prevented in the nursery are psorosis,
xyloporosis, exocortis, and to a large extent tristeza. In at-

Citrus Diseases in Florida

tempts to regulate the quality of budwood, particularly with
respect to virus diseases, the State Plant Board, in cooperation
with the State's citrus nurserymen, have undertaken a budwood
certification program. Only those parent trees found by test
to be free of psorosis, xyloporosis, exocortis, and tristeza are
being registered as sources of acceptable budwood.
Diseases of the second group-those that are caused by fungi
and that affect trees regardless of their thriftiness-are more
difficult to control. In some of these troubles, as for instance
melanose, it is necessary to apply a protective coating to the
surface of leaves and fruits in advance of possible outbreaks.
In years when outbreaks do not develop, the expense of appli-
cations is in a sense wasted; more properly, however, the ex-
pense should be regarded as being in the nature of insurance
premiums, with the policy paying off in years when the disease
is rampant. In the case of other troubles, such as infestations
of dodder, there need be no cause for concern until after an out-
Spraying operations in citrus groves are generally carried
on to control insects. However, it is often necessary also to spray
against certain fungus diseases such as melanose and scab that
attack foliage, twigs, and fruit. Fungicides used for this purpose
are applied so that susceptible tissues are covered during periods
favorable for infection.
Certain principles governing the spraying of citrus trees
should be mentioned:
1. Citrus trees, which are evergreen, and often bear fruit
throughout the year, require care in the application of fungicides.
Since in citrus there is no dormant period in the sense that this
term is used with deciduous fruits like apples, strong sprays can-
not be used.
2. Spray programs should be tailored to individual groves.
Systematic observance of diseases over a period of years may
make it possible to limit spraying to those troubles that affect
particular groves. Sour orange scab, for instance, can be found
year after year in some groves and not at all in adjacent proper-
3. Sprays for disease control may generally be combined
with insecticidal and nutritional sprays. Thus, using melanose
and scale as an example, copper and a scalicide may be mixed in

Florida Agricultural Experiment Stations

and applied out of the same spray tank for the control of both
these troubles. Care must be used, however, to mix only those
pesticides that are compatible. For information on safe combi-
nations and suggested programs, see the Spray and Dust Sched-
ule, revised and issued annually by the Florida Citrus Commis-
4. Scale control should follow applications of fungicidal and
nutritional sprays. Heavy particulate residues on foliage lead
to increases in scale populations; in fact, there is a direct corre-
lation between the amount of spray deposit and the build-up of
scales. Fungicidal applications, furthermore, kill beneficial fungi
that normally parasitize scales and hold populations in check.
Because of these two factors favoring the multiplication of
scales, it is recommended that fungicidal and nutritional sprays
be followed by applications of scalicide.
5. The proper number of applications varies with the disease,
with climatic conditions, and with state of growth. The efficiency
of a single application may not extend beyond 10 to 14 days.
Residual effectiveness is governed by chemistry of the material
applied, by weather conditions that may accelerate or retard
fungicidal breakdown or cause fungicides to be washed off, and
by rapidity with which unprotected foliage emerges. Timing is
important in spraying and dusting. For most benefit, fungi-
cides should be applied to foliage when leaves are susceptible to
fungus attack and in advance of fungal penetration. Properly
timed, fungicides form a chemical envelope around the foliage
that prevents invasion of pathogenic organisms. Wastefulness
results when sprays are applied to foliage already attacked or
when new or susceptible foliage is not present. Suggestions as
to timing, fungicides, and dilutions are contained in the Spray
and Dust Schedule which is part of the Better Fruit Program
issued by the Florida Citrus Commission.
6. Fungicides must be applied properly, using suitable equip-
ment in good working order. Sprays should be applied to insure
thorough coverage. The spray liquid should be forced through
the orifice of the nozzle disc with adequate pressure to produce
droplets sufficiently small to minimize run-off, yet coarse enough
to achieve penetration of the canopy. Air blast sprayers pro-
vide sufficient turbulence for coverage of both outside and inside
leaves even though sprays are mist-like; but applications made
with a high-pressure sprayer should consist of droplets some-
what coarser. Pressure of 500 pounds per square inch is needed

Citrus Diseases in Florida

to produce proper droplet size. Worn nozzle discs, however,
cause a drop in pressure and will result in deposition of more
spray material than recommended amounts, thereby causing
wasteful run-off. Decreasing the size of the disc opening below
that advocated will not produce smaller droplet size, but will re-
sult in a smaller cone of spray with consequent reduction in
coverage. During spraying the agitator should be kept running;
failure to operate the agitator continuously may result in spray
burn from chemicals that have settled and become concentrated
at the bottom of the tank.
After use, spray machinery should be cleaned thoroughly to
minimize corrosion, especially after the application of such ma-
terials as bordeaux mixture and zinc-lime. All traces of growth-
regulating substances such as 2,4-D and 2,4,5-TP should be
flushed out with 2 pounds of washing soda in one-third of a
tankful of water.
7. Short cuts in spraying may lead to trouble. It is true
that economies can be effected sometimes by not spraying fruit
intended for the processing plant. However, it should be pointed
out that omission of blemish-preventing sprays may result in
smaller sizes and lower yields of fruit. Russetted fruit, for ex-
ample, is often reduced in size, and late in the season becomes
"mushy" and unacceptable to the processor. Lack of rust-mite
and greasy-spot control may also cause leaf injury and serious
8. Use the latest recommendations for spraying and dusting
as contained in the Spray and Dust Schedule. The Schedule,
which is part of the Better Fruit Program, is revised annually
and contains latest information regarding recommended chemi-
cals, dilutions, and timing, based on information furnished by
the Citrus Experiment Station, the Florida Agricultural Experi-
ment Stations, the Florida Agricultural Extension Service, the
Florida State Plant Board, the Florida State Horticultural So-
ciety, the Bureaus of Plant Industry and Entomology and Plant
Quarantine of the U. S. Department of Agriculture, and profes-
sional consultants and commercial firms. Copies of the Better
Fruit Program may be obtained from county agricultural agents
or by writing to the Florida Citrus Commission at Lakeland, the
Agricultural Experiment Station or the Agricultural Extension
Service at Gainesville, the U. S. Department of Agriculture Lab-
oratories at Orlando, or the Citrus Experiment Station at Lake


8 Florida Agricultural Experiment Stations


Algae are plants similar in structure to filamentous fungi.
They differ from fungi in possessing chlorophyllous pigments
that enable them to manufacture their own food. Being self-
sufficient in this respect, they are not generally found to lead
pathogenic existences. Nevertheless, one species of alga, Cepha-
leuros virescens, is widely known in the tropics and sub-tropics
as a pest of crop plants, particularly of citrus and tea. Damage
is brought about not by parasitism but by growth and expansion
of algal filaments inside the tissues of the host, causing bark on
twigs and branches to rupture. Citrus fruits may also be dis-
colored by the superficial growth of algal colonies on the rind.


Fig. 2.-Algal spots o
citrus leaf. Spots are grey
ish during the vegetativ
stage and reddish brow
while fruiting.

Symptoms.-Though algal disease
is found on many varieties of citrus,
it is destructive principally on limes
and lemons. The first symptom to oc-
cur following penetration of twigs is a
thickening of the bark. With time, in-
vaded areas crack open and scaling of
bark becomes evident (Fig. 3). Growth
at ends of branches is restricted as
food- and water-conducting vessels are
Disorganized. This is followed by
chlorosis of the foliage, leaf drop, and
eventual death of twigs. Under con-
ditions favorable for the development
of algal disease, branches up to 2
inches in diameter may be killed. De-
structive attacks result in poor trees
Sand in reduced fruit yields.
The diagnosis of algal disease can
n easily be confirmed by noting the con-
e spicuous presence of the causal organ-
n ism. Associated with early stages of
bark splitting are reddish-brown felty

patches (Figs. 2, 3) averaging 1/2 inch in diameter when oc-
curring as single colonies, but covering considerable lengths
of twigs when colonies are merged. The red stage is present
during summer months. At other times of the year, twigs and


Citrus Diseases in Florida 9


Fig. 3.-Algal disease on branches of Meyer lemon showing (a) felty
patches of the fruiting alga, (b) cracking of the bark following invasion
of cortical tissues, and (c) ultimate ragged appearance of affected branch.


10 Florida Agricultural Experiment Stations

leaves are coated with a greenish powdery growth, this being
another stage in the life cycle of the same pathogen.
Fruits, particularly those that are overmature, may become
spotted with blackish growths that under the hand lens resemble
miniature colonies of a moss, with rootlike structures radiating
from a common center (Fig. 4). The fruit blemish, known as
algal spot, that this organism is capable of causing, may on oc-
casion be so severe as to impair the market value of affected
crops. The fruit-spot-
al i' ting phase of algal dis-
ease occurs principally in
areas with high humidi-
ties, as along coast
a, areas and in groves
S where canopies of trees
are so heavy as to inter-
fere with proper ventila-

to become affected are
sweet orange, grapefruit,
lemon, and lime.
Cause. The organ-
ism responsible for algal
L disease is Cephaleuros
Sb virescens Kunze (syn. C.
mycoidea Karsten, My-
Fig. 4.-Algal spot on rind of citrus coidea parasitica Cun-
fruit. Magnification s h o ws moss-like ningham). In its ulti-
growth of the algal plant as it would appear mate development, this
on inspection with a hand lens.
pathogen is a 1i c h e n,
which is the union of an alga with a fungus. While in the green-
colored, lichenous form, the organism is harmless to its host.
In the stage, however, in which the organism exists dissociated
as an alga and produces reddish-brown spots on leaves and
branches, it may become destructive to certain varieties of citrus.
Control.-Control of algal disease is rather easily obtained
through maintenance of trees in a state of vigor. Spacing of
trees should be such as to prevent crowding and consequent
build-up of humidity. Trees regularly sprayed with copper, as
for the control of melanose or scab, are seldom attacked.
Serious outbreaks of algal disease can be controlled by a
two-point program consisting of (1) the thorough pruning of

Citrus Diseases in Florida 11

diseased and dead limbs, and (2) the spraying of trees with
bordeaux mixture according to the following schedule:
a) Bordeaux 6-6-100 plus 2 quarts oil emulsion, applied in
December or January before start of the spring flush.
b) Bordeaux 4-4-100 plus 2 quarts oil emulsion, applied just
in advance of the rainy season in June, or as soon as the
reddish stage of the alga is observed.
c) Bordeaux 4-4-100 plus 2 quarts oil emulsion, applied 1
month after the spray in (b) above.
Instead of bordeaux mixture, a neutral copper may be applied
in (b) and (c) above. The usual addition of insecticides should
be made as needed, but if sulfur is used, oil must not be included.
Sprays should cover thoroughly the bark of all twigs and
branches. After use of the above-described clean-up program,
further attacks of algae may be held in check by the annual
application of copper used in melanose control.

See Black Rot.
See Exanthema.

Two diseases are covered, unfortunately, by the single term
anthracnose. One, caused by the fungus Gloeosporium limetti-
colum Clausen, affects the young growing tissues of Key lime;
the other, associated with the fungus Colletotrichu m gloeospori-
oides Penz., occurs on mature or weakened leaves, twigs, and
fruits of grapefruit, orange, lemon and other varieties of citrus.
The differences between the two diseases are sufficiently wide
to justify their consideration separately. In deference to cus-
tom, the name anthracnose is retained for the disease occurring
on grapefruit, orange, and lemon. The disease on lime is given
the name lime anthracnose and is discussed under that heading.
Symptoms.-Anthracnose fungi invade tissues, whether in
fruits, leaves, or branches, that have become weakened or in-
jured. Certain names have been given these invasions accord-
ing to parts affected. Thus, "withertip" refers to che dying
back of twigs and their invasion by Colletotrichum. "Anthrac-
nose" refers more particularly to the same sequence in fruits
and leaves; and anthracnosee russetting" to staining of the rina


Florida Agricultural Experiment Stations

caused by run-off of spore-laden
water over surfaces of the fruit.
The symptoms of anthracnose
are discussed under these three

Fig. 5.-Anthracnose or wither-
tip on twigs of lemon, showing
sharp line that separates dead from
living wood. The black specks on
the dead, whitened wood are fruit-
ing bodies of the Colletotrichum

Weakening of twigs followed
by infection with anthracnose
fungi and by eventual death is
called withertip. Twigs and
branches affected are those that
are either mature or senescent,
though young twigs may occa-
sionally show withertip, espe-
cially after damage from cold,
drought, scales, or purple mites.
The dying back progresses slow-
ly, with leaves on affected stems
turning yellow, withering, and
dropping; but at times die-back
may proceed rapidly, in which
case leaves wither and dry be-
fore abscission has taken place.
Gumming may occur along the
invaded stem or at the juncture
between healthy and affected
tissues. Following death of dis-
eased portions, there is a devel-
opment of minute black pimples
-the fruiting bodies of the fun-
gus (Fig. 5).
Withertip is frequently seen
during winter months after root
systems of trees have been in-
jured by drought. Trees may
lose leaves and fruit from affect-
ed branches and die back. Much
withertip may result from hur-
ricanes and even from prolonged
winds of velocities insufficient to
cause immediately visible dam-
age. In its early stages, before


Citrus Diseases in Florida

anthracnose fungi have invaded twigs defoliated by drying
winds, withertip is difficult to distinguish from purple-mite in-
jury, especially when the causal mites or their casts are no
longer present.

Fig. 6.-Anthracnose on grapefruit.

When anthracnose fungi are found in dying or dead areas on
the surface of citrus fruits, the condition is commonly referred
to as anthracnose fruit spot. Spots vary from small specks to
areas a quarter inch or more in diameter (Fig. 6). They are
reddish-brown at first, dark brown to black when fully developed.
They are circular, or nearly so, and are generally sunken below
the surface of the rind. Lesions become dry and hard, and are
occasionally found dotted with minute, black fruiting bodies
that in humid weather exude pinkish masses of spores, which
can be seen with a hand lens. Ordinarily, only the rind is affect-


14 Florida Agricultural Experiment Stations

ed but with time the disease may extend through the rind and
into the pulp beneath, to which it imparts a disagreeable or bit-
ter taste. Often, other fungi become established as secondary
invaders in decayed areas, accelerating the rate of decay. Symp-
toms then vary with the particular secondary fungus involved.
Certain strains of the anthracnose fungus may produce a
soft, pliable decay that causes affected fruits to drop. Another
type of anthracnose spotting results from fungus invasions of
punctures caused by thorns or twigs. Such injuries, especially
common on grapefruit, are characteristically black, irregular in
shape, and conspicuous.
Anthracnose fruit spot occurs most often on overripe fruits,
especially in the case of early and midseason varieties of grape-
Anthracnose russetting of fruits is produced by spores of
anthracnose fungi that are washed over fruits from dead twigs
on which the organism is fruiting. When, under certain condi-
tions, these spores germinate on the rind, a russetting of the rind
results. In some cases, this russetting develops as a large blotch;
in others, it produces tear staining somewhat fainter than the
tear stain due to rust-mite injury.
Wounded or weakened leaves are generally invaded by an-
thracnose fungi which bring about death and drying of invaded
tissues. With time, minute black spheres, arranged in concentric
zones, appear in the papery dead areas; these are the fruiting
bodies of the fungus. Anthracnose leaf spots develop quite com-
monly on the foliage of rough lemon seedlings in the nursery
and may lead to defoliation (Fig. 7).
Cause.-The anthracnose fungus, Colletotrichum gloeospori-
oides Penz., has world-wide distribution, and is to be found uni-
versally on the surfaces of live and dead tissues of both healthy
and diseased trees. Several strains of the fungus are known to
exist, and this gives rise to the possibility that the amount of
injury to citrus trees and fruits may depend on strain virulence
as well as on tree condition.
Anthracnose may occur on trees of any size, in nursery or
grove. It rarely develops on trees that have been maintained
in a vigorous condition, but is common in trees that are weakened
or injured from inadequate fertilization, overcropping, drought,
cold, spray burn, and attacks by insects and other diseases.


Citrus Diseases in Florida

Fig. 7.-Anthracnose leaf spots on rough lemon sprout, showing papery
brown areas within the leaf blades and partial defoliation of the twig.

Environmental conditions also influence development of an-
thracnose. Drought and severe cold produce susceptible trees,
and warm and high humidities stimulate growth of the fungus.
As already mentioned, the anthracnose fungus is no longer
considered to be the primary cause of disease. Rather the term
anthracnosee" merely describes one of the species of secondary
invaders present in wounded or weakened tissue. Since the an-
thracnose fungus occurs on nearly all surfaces of citrus trees,
it is more realistic to regard the actual cause as being the agent
that injures or weakens a trees. Tissue may be rendered suscep-
tible to the anthracnose fungus by various agents and the result-
ing troubles have in the past been given various names, as for
1. Cold storage spots-a physiological breakdown of the rind
that develops from prolonged storage of fruit under refrigera-


16 Florida Agricultural Experiment Stations

2. Winter die-back-a leaf- and fruit-drop occurring in the
winter months when root systems have been injured by drought
or fluctuating water tables. This trouble may appear suddenly
on a few trees and seem to spread through the grove, affecting
trees both young and old.
3. Wind damage-a die-back of twigs following periods of
high winds that are not sufficiently severe to show immediate
damage. Anthracnose may appear also after hurricanes that
provide injuries for the entry of fungi.
4. Hail injury-a spotting of fruits resulting from injury
due to hailstones. Often the spots are noticed long after hail
has done its damage and the hailstorm has been forgotten. In-
volvement of secondary organisms or growth of fruit following
injury may obscure the original cause.
5. Post-maturity breakdown-a rind breakdown due to over-
ripeness. Certain strains of the anthracnose fungus may pro-
duce a soft, pliable decay and fruit-drop in the field, whereas
other strains are apparently responsible for a pitting of the rind.
6. Mechanical injuries-a miscellaneous collection of wounds
resulting from thorn scratches, limb chafing, chemical burn, in-
sect punctures, and the like. Blemishes caused by the subse-
quent invasion of anthracnose fungi occur most frequently on

Fig. 8.-Black rot as it affects sweet orange fruit. The apparently
sound fruit at left displays no external symptoms to indicate that it is
infected. At right is the same fruit cut to reveal the extent to which black
rot has invaded the central core and segments.


4-: -
:, .



Citrus Diseases in Florida 17

Control.-Since the anthracnose fungus is not the primary:
cause of disease, control practices should be directed toward
agents that open tissues to this universally-present fungus.
Trees slightly affected by die-back may throw off the disease if
in vigorous condition, but those trees that are severely involved
can scarcely recover without help. Dying limbs should be pruned
below the last trace of disease, following which pruning wounds
should be disinfected and water-proofed as described under the
control of foot rot.
Control practices should also be aimed at providing condi-
tions for the vigorous growth of trees. Health and vitality of
trees can be assured by proper fertilization, disease and insect
control, and acceptable cultural practices.
In the nursery, anthracnose assumes a somewhat more im-
portant status; it can cause serious destruction of foliage in seed-
lings of rough lemon and Rangpur lime. For many years an-
thracnose leafspot has been controlled with bordeaux mixture
or its equivalent in fixed copper. These materials give fairly
adequate results, but their use is disadvantageous because, with
repeated applications, accumulations of copper in the soil may
reach toxic levels. Preliminary trials with some of the newer
organic fungicides which do not contain copper indicate that good
control may be obtained with ferbam, zineb, or captain at rates
of 11/2 to 2 pounds per 100 gallons of water. With either copper
or organic compounds, it is necessary to keep new foliage covered
with fungicide; this means that applications should be made at
10-day intervals during periods of active growth.

See Greasy Spot.

(Alternaria Spot, Alternaria Rot)
Some strains of Alternaria fungi cause a spotting and decay
of citrus fruits known as black rot. This disease has long been
one of the minor rots affecting oranges, tangerines, and lemons,
but, with the advent of frozen concentrate, crops containing even
small percentages of black rot are being rejected at the cannery.
In the manufacture of frozen concentrate, a few black-rotted
fruits are sufficient to produce batches of juice having off flavors
and high microbial counts.

BLACK ROT (ctd.)

18 Florida Agricultural Experiment Stations

Black rot was first described in 1901 by Pierce, who noted its
occurrence in California as early as 1892. In Florida it was first
reported in 1911. The same or a similar decay has also been
described from Arizona, Texas, California, the West Indian
Islands, South America, Africa, Europe, Asia, and Australia.
It was thought formerly that attacks of black rot were con-
fined to the Navel orange, but the disease is now known to attack
other varieties, including Hamlin, Jaffa, Parson Brown, Pine-
apple, Ruby Blood, and Valencia oranges and tangerines. The
disease appears to be most prevalent on early varieties.
Symptoms.-The disease is conspicuous just before color nor-
mally develops in the fruit. Infected fruits turn orange prema-
turely, often several weeks before the color break in healthy
fruits. Some infected fruits drop while others remain on the
tree. Fruits that appear sound reveal, when cut, a rather solid
and black decay near the stylar or "blossom" end. The affected
zone may extend through the central portion of the fruit (Fig. 8)
before becoming evident on the surface as a tan discolored spot
that eventually turns black. It is because there is often no ex-
terior evidence of decay that some infected fruits pass inspection.
Such fruits break down in transit or, in the juice plant, become
the source of high spore counts. The pulp and juice surrounding
the blackened central portion of the fruit have an unpleasant,
bitter taste, though other parts of the fruit may be affected
only slightly.
Lesions due to Alternaria fungi are not always easily dis-
tinguished from those due to anthracnose fungi; at times, in-
deed, both fungi may be present in the same spot.
Cause.-Black rot is caused by a fungus, Alternaria citri
Ellis & Pierce. This fungus is a weak pathogen and attacks
mainly fruits that have been adversely affected by harsh weather
such as high temperatures and winds at time of fruit setting.
Spores of this organism are believed to lodge in openings of the
fruit such as the navel or in breaks at the stylar end of oranges
without navels. Apparently, infection does not occur until after
fruits have attained sufficient size to distend the navel or other
openings. In laboratory experiments, it was not found possible
to infect fruits without injuries.
Moisture is an important contributory factor, judging from
the fact that the disease is most prevalent following periods of
high humidity or rainfall.

BLACK ROT (ctd.)

Citrus Diseases in Florida 19

Blossom-end rot may occur also on fruit in groves that have
been over-fertilized. In such cases the fruit apparently grows
with such rapidity that tissues at the blossom end do not de-
velop normally. This provides openings through which infec-
tions take place.
Control.-Only occasionally is black rot of much importance.
The trouble can be avoided to some extent, particularly in varie-
ties known to be highly susceptible, by picking fruit as soon as
mature and before black rot appears. If, on the other hand,
black rot has developed, fruit should be picked late, thus allowing
any that are infected to drop. Late-picked fruit should be satis-
factory for making either frozen concentrate or canned juice.

One of the first citrus diseases described in Florida is blight,
a trouble of distinctive appearance but still of uncertain cause.
In the years that followed Manville's initial report in 1883, blight
became the subject of many papers in which the disease was
often referred to by other names, including go-back, wilt, dry
wilt, leaf wilt, leaf curl, orange blight, limb blight, Plant City
disease, and roadside decline. Blight has often been confused
with other troubles (e.g., foot rot, Diplodia root rot, Clitocybe
root rot) that show somewhat similar symptoms.
According to information gathered by the State Plant Board
during its Citrus Survey of 1954-55, blight affects more than
15,000 trees in the groves of Polk County alone. It is exceeded
in importance only by heart rot, foot rot, and spreading decline.
Blight accounted for 6 percent of the total number of trees af-
fected by all diseases.
Blight is present throughout the citrus-growing area of
Florida, but appears to be most frequently encountered in the
central portion of the East Coast area. It is apparently confined
to the State of Florida, though a trouble of somewhat similar
description has been reported recently from Texas and from
Symptoms.-The predominant symptoms of blight are wilting
of foliage, die-back of tops, and excessive development of sprouts
along the lower branches (Fig. 9). Wilting and dieback may
occur on single limbs but more generally are present through-
out the entire top. Blight attacks trees that are vigorous and
renders them worthless in a few months to a few years. Al-
though trees as young as 6 years have been found affected, it

BLIGHT (ctd.)

Florida Agricultural Experiment Stations

is trees 15 years old or more that are most frequently affected.
Blight often breaks out in trees that have had a record for high
productivity. It has been stated repeatedly that there is no
varietal resistance to blight, that all rootstocks are subject
equally to attacks.

I~ I.

~, i
.& r

Fig. 9.-Citrus tree affected by blight, showing advanced stage in which
leaves have dropped, branches have died back, and sprouts occur at center
of tree.

The sequence of symptoms begins with a wilting of foliage,
noticeable first during hot, drought weather but later persisting
even during rainy spells. Wilting is followed by leaf drop, then
by the dying back of defoliated branches. During rainy seasons,
affected trees develop a copious growth of water sprouts along
the central axis of the trunk and along the lower branches.
This sprouting distinguishes blighted trees from those affected
by spreading decline.
Distribution of affected trees varies from grove to grove.
Usually, it is haphazard, with trees adjacent to affected ones
remaining healthy; but occasionally, blight appears to spread
spontaneously and rapidly in all directions.

BLIGHT (ctd.)

Citrus Diseases in Florida 21

Affected trees never recover. When reset in other locations,
they do not shake off effects of the disease but continue to de-
teriorate until they finally die. Generally, the rate of dying is
slow, with trees lingering for years in a partially defoliated,
completely unproductive condition.
Cause.-Despite the amount of study to which blight has
been subjected during the past 65 years, there is still no agree-
ment as to the cause of this disease, nor, for that matter, is there
any unanimity as to whether blight is contagious. While not
hazarding an explanation as to its cause, various investigators
have reported that blight is not the result of cold, drought, weu
weather, hardpan, or improper fertilizers. It is generally recog-
nized that blight is prevalent in trees growing on the best soils
and in locations ideally suited for the growth of citrus.
Blighted trees frequently show deficiency symptoms, espe-
cially of zinc and manganese, but such symptoms are not cor-
rected by soil applications of appropriate elements.
Some authorities relate blight to certain types of soil, as for
instance. East Coast soils that are underlaid by coquina. Others
associate the trouble with adverse pH values, and attribute the
decline of trees, especially along roadsides, to the high alkalinity
of limestone fills used in highway construction.
It is possible that some cases of blight are due to root rot.
Although a typically blighted tree shows remarkably few roots
that are dead and blackened, it may be that, as in certain fu-
sarium diseases of plants, toxins are translocated from rotting
roots to living parts of the tree, and that small amounts of these
poisons are sufficient to cause wilt and leaf drop.
Some cases diagnosed as blight, particularly in old sweet
orange and grapefruit trees on so-called rough lemon rootstocks,
may actually be xyloporosis. Many trees of this description,
when examined at the bud union, show the pits and pegs charac-
teristic of infection by xyloporosis virus. Pitting occurs in the
wood of the stock under the bark just below the bud union. Cal-
lus rolls frequently accompany this symptom; as in xyloporosis,
overgrowth above the bud union is indicative of a blockage of
food being conducted from leaves to roots. That trees showing
these symptoms are not on true rough lemon stocks can be con-
firmed by the fact that foliage on rootsprouts of such trees is
remarkably free of anthracnose, that leaf margins are promi-
nently notched or crenulated, and that oil glands are unusually
translucent. Observational evidence indicates that these rough

BLIGHT (ctd.)
22 Florida Agricultural Experiment Stations

lemon variants are hybrid seedlings rather than nucellar seed-
lings, and as such they carry a certain susceptibility to xylo-
porosis virus that is introduced with infected budwood.
Control.-In the absence of certainty regarding the cause
of blight, little can be said about control-except to mention a
few lessons that have been learned from practical experience
with this trouble: (1) Blighted trees never recover; therefore,
expensive procedures aimed at curing blight ought to be re-
jected. (2) The correction of deficiency symptoms, as by the
application of indicated elements, will not restore blighted trees
to health. (3) No regrowth of healthy tops can be obtained by
hatracking blighted trees. (4) Blighted trees should be pulled
and replaced when their presence is no longer profitable. (5)
There is no danger of infecting resets by planting them in holes
from which blighted trees have just been pulled.
For the control of other wilting and dieback diseases which
might be confused for blight, see under Diplodia root rot, Clit-
ocybe root rot, foot rot, lightning injury, and water damage.

See Psorosis.
See Wood Pocket.

Some of the fungi causing foot rot on trunks of citrus trees
(see Foot Rot) also cause a rotting of fruits on the tree. Brown
rot has long been known in California and at one time, when it
was believed that this disease did not occur in Florida, quaran-
tine barriers against California fruit were erected to prevent
introduction of brown rot. First encountered in Florida in 1951,
the disease has annually become more serious. No exhaustive
survey has been made to determine its distribution in the State,
but numerous groves from Wabasso to Ft. Pierce along the East
Coast, one grove on Merritt Island, and two groves in Hendry
County were found to contain trees with infected fruit in 1953.
The disease was discovered in the interior, near Wauchula, in
1955. Cases are known in which 90 percent of the crop has been
lost from individual trees, and an aggregate loss of 30 percent has
been estimated from some groves.
In Florida, the disease has been seen on early and midseason
oranges, including Pineapple, Hamlin, Navel and Temple. Grape-


Citrus Diseases in Florida 23

fruit also has been found affected. To date, brown rot has not
been observed in this state on late oranges, even when growing
adjacent to heavily-infected early orange varieties.
Brown rot can be a serious problem also during transit, often
causing complete destruction of shipments by the time they
reach their destination.
Symptoms.-Fruit decay produced by brown rot is remark-
ably firm, despite its soft-rot-like appearance. Affected areas
of the rind are dark
greenish-brown and
remain flush with
the level of the sur-
rounding unaffected
rind (Fig. 10). In
later stages, how-
ever, when second-
ary organisms have
gained entry, affect-
ed fruits decompose
rapidly and resem-
ble the type of rot-
ting generally found
in fruits on the
Infection takes
lace 3 to 10 days Fig. 10.-Brown rot affecting sweet orange.
before rotting be- The darkened area is characteristically olive-
comes obvious. The brown and as hard as the surrounding healthy
first symptom on area.
the surface of fruit is a slight discoloration that changes to
various shades of olive drab or brown. Fruits affected by brown
rot emit a strong fermentative odor; detection of this odor in a
grove provides strong presumptive evidence that brown rot is
present. Generally under grove conditions, the causal fungus
cannot be seen growing on affected areas, but when invaded
fruits are stored at high humidity, a white, delicate fuzz, the
strands of the causal fungus, appears. This growth can infect
other fruits by contact, and accounts for spread of brown rot
in bulk storage or packaged fruit.
Cause.-The fungus causing brown rot of fruits in Florida is
Phytophthora parasitica Dastur. It has been known here for
years as one of the causes of foot rot in trees, and the cause of


24 Florida Agricultural Experiment Stations

rot in fruits on the ground. Several theories have been ad-
vanced to account for the fact that, despite long presence of the
fungus in the State, outbreaks of brown rot on fruit in the tree
have not occurred here until recently. One is that a fruit-rotting
strain has only lately been introduced; another is that weather
conditions that favor infection of fruits have not occurred until
It is certain that abundant free moisture is a prerequisite for
infection. Moisture in this sense, however, does not necessarily
mean rainfall. In fact, there is little or no correlation between
inches of rainfall on the one hand and epidemics of brown rot
on the other. More important than inches of rainfall is the du-
ration of wetness. In order for the fungus to complete its life
cycle, ranging from the liberation of spores to their germination
and penetration, it is necessary that fruits remain wet for a rel-
atively long and uninterrupted period. Evaporation of the mois-
ture film during part of the cycle will kill the germinating fungus
and put an end to the infection process. More effective than rain
in maintaining films of water for extended periods is the free
moisture that forms nightly after the dew point has been
reached. Rains can and do play a part, of course, in prolonging
the period that fruits are wet, but rains, on the other hand, are
not conductive to development of brown rot when they occur as
showers that are followed by bright, drying, sunny weather.
Outbreaks of brown rot in Florida take place in late summer
or early fall, generally following prolonged periods of high hu-
midities and cloudy, rainy weather.
Apparently, stage of fruit maturity also is an important
factor governing susceptibility, judging from the observation
that early and midseason varieties are much more frequently
attacked than are late varieties.
Control.-Control of brown rot may be carried out in the
grove or in the packinghouse. Ideally, the disease ought to be
prevented in the grove, and this has been done by applying a
protective bordeaux spray (ll/2-l/-1100) or the equivalent of
neutral copper about the middle of September. In areas where
brown rot is only an occasional problem, spraying may be de-
ferred until immediately after the first appearance of affected
fruits. Sprays should be directed principally at spores on the
ground and toward fruit on low-hanging branches to 6 feet that
might be splashed by soil water. Reduction in the amount of
brown rot can be obtained also by hedging, by clean cultivation,


Citrus Diseases in Florida 25

and by lifting the skirts of low-growing trees, thereby improving
ventilation and putting a space barrier between soil-borne spores
and fruit. Once brown rot appears in the unharvested crop,
spraying will be of help only in those rare instances when weath-
er conditions might again become suitable for infection.
Because fruit containing brown rot may break down in tran-
sit, it is necessary to eliminate sources of inoculum in the pack-
inghouse. In California, this is done by immersing fruits from
2 to 4 minutes either in water at 115-120 F. or in a 13/ percent
solution of soda ash, either hot or cold, or in a bath of copper
sulfate, 1 to 2 pounds in 1,000 gallons. To prevent rind burn,
picked fruits are cured for 24 hours before being subjected to
treatment. Cold solutions prevent infection of healthy fruits
in the washer, but heat is required to kill the fungus in fruit
already invaded.
While the use of copper as suggested above will undoubtedly
control brown rot wherever found, it should be mentioned that
in Florida such treatments tend to increase the amount of stem-
end rot developing in storage. Because brown rot has only re-
cently been discovered in this state, recommendations for the
control of this disease in Florida packinghouses have yet to be
See Foot Rot.
See Xyloporosis.

See Psorosis.
Bacterial canker is a disease no longer known in Florida,
though once it threatened to be the worst catastrophe in the
history of the state's agriculture. Fortunately, the serious na-
ture of canker was recognized early and sufficient sentiment was
created to lead to its complete eradication.
Between 1914 and 1931 more than $2,500,000 of state, fed-
eral, and private funds were spent in combatting citrus canker
in Florida, and the total cost of this disease to the state has
been at least $6,500,000. During this period 257,745 grove trees
and 3,093,110 nursery trees were destroyed incidental to eradi-

CANKER (ctd.)

26 Florida Agricultural Experiment Stations

cation, which occurred on 414 properties in 26 counties. Be-
cause of the systematic inspection of grove properties carried
out by the State Plant Board, there has been no commercial dam-
age to Florida citrus groves by canker since 1922, and no infec-
tions have been found since 1926. Constant vigilance is neces-
sary, however, to be sure that the disease does not obtain an-
other foothold. The danger of reinfection from abroad still re-
mains as a constant threat; in fact, fruits with canker are con-
tinually being intercepted at ports of entry, particularly in
Canker is widely distributed throughout the citrus-growing
countries of the world but is especially prevalent in the Orient.
It is believed to be native to China but it has also been present
in Japan for a considerable time, having been observed there as
early as 1865. Canker was first brought into the United States
around 1910 on trifoliate orange trees shipped from Japan to
Texas, Mississippi, Alabama, and Florida. In Florida it was
probably introduced in shipments of trifoliate orange trees from
both Japan and Texas and also from Satsuma orange trees im-
ported directly from Japan and planted at Monticello in the
spring of 1910. When the disease was first observed in Florida
at Monticello on September 30, 1912, it was thought to be scab,
but when found again at Silver Palm in Dade County in July
1913, it was realized that a new disease was involved. It was
not until 1915 that the actual cause of canker was shown to be a
Canker can attack all kinds of citrus trees of commercial im-
portance in Florida, provided conditions are favorable for its
development. Grapefruit is very susceptible 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 limes (other than Tahiti), sweet oranges, American-grown
lemon varieties, and Satsuma oranges. Mandarin oranges, cal-
amondins, and citrons are commercially resistant. The kum-
quat is highly resistant. Inoculation experiments have shown
that many other species, hybrids, and wild relatives of Citrus
are more or less susceptible to canker.
In contrast to the canker that occurred in Florida, which can
be termed Cancrosis A, is another form known as Cancrosis B
that is present in Argentina, Uruguay, and Paraguay. This

CANKER (ctd.)

Citrus Diseases in Florida

form is most serious on lemons and occurs occasionally on sweet
and sour orange, lime, and citron. It now appears that Cancrosis
B is caused by a strain of the citrus canker organism.
Symptoms.-Canker may attack any part of the tree above
ground, although leaves, twigs, young branches, and fruits are
most commonly infected and these chiefly while still fairly young
and tender. The distinguishing feature of canker as observed
in the field is the characteristic spotting produced on the foliage,
twigs, and fruits.


Fig. 11.-Canker on grapefruit leaves and twig.

On Leaves.-Canker first appears as small, watery, translu-
cent spots usually of a darker green color than the surrounding
tissue and with raised, convex surfaces. The spots, which may
appear on either surface of the leaf, usually become evident on

".' :;-** .- .*' *-' 03- .a--.; -w / .

CANKER (ctd.)

28 Florida Agricultural Experiment Stations

the lower one first. As the disease advances, surfaces of the
spots become white or grayish and finally rupture, exposing a
tan, spongy central mass developed in a crater-like formation of
the ruptured epidermis (Fig. 11). Such spots usually become
surrounded by a yellowish halo that persists even in very old
lesions. Size, color, and abundance of lesions vary somewhat on
different kinds of citrus trees. For example, lesions are largest
on grapefruit leaves, where they may be nearly 1/2 inch in diam-
eter, and are usually not quite so large on leaves of sweet orange.
On leaves of limes and lemons, they are much smaller, frequently
being not more than 1/8 inch in diameter. Old lesions are brown
and corky and may even appear pinkish or dark-colored due to
overgrowth by saprophytic fungi.

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

On Twigs, Branches, and Roots.-Lesions on twigs are com-
mon on the more susceptible kinds of citrus, such as grapefruit,
trifoliate orange, lime, and sweet orange. The spots on young
twigs (Fig. 12) are like those on leaves and fruit. On older
twigs they are more prominent and somewhat irregular in shape,
especially in the case of old spots. Spots show the same spongy
tissue as do lesions found on leaves but acquire a cankerous ap-
pearance, and the surface membranes disappear completely. In
especially susceptible varieties, branches from 2 to 3 inches in
diameter are commonly found affected. While underground
roots of citrus trees have been repeatedly inoculated, with posi-
tive results, canker has never been observed occurring naturally
on roots, despite numerous examinations of badly diseased trees
in heavily infected groves. However, the disease has been found
on roots of grapefruit trees exposed above the surface of the
On Fruit.-Individual canker lesions on fruit have much the
same appearance as on leaves, except that the yellowish halos
are usually absent and the crater-like appearance is more notice-

CANKER (ctd.)

Citrus Diseases in Florida

able (Fig. 13). The spongy, rough eruptions may be scattered
over the surface or several may occur together, forming an ir-
regular, scurfy or scabby mass. Gumming is sometimes associ-
ated with spots formed on fruits. Lesions do not penetrate deep-
ly into the rind, nor cause decay directly, but they do open the
way for fungi that cause a secondary rotting of the fruit.

.. .. .... .-... "_......

Fig. 13.-Canker on grapefruit.
Distinguishing Canker from Other Diseases.-Citrus diseases
that may be mistaken for canker are'scab, leprosis, juvenile spot,
and anthracnose on Key limes. Canker may also be confused,
especially by one comparing it with written descriptions, with
spine punctures, melanose, and certain kinds of spray injury.
The following symptoms are characteristic of canker:
1. Canker lesions may be elevated on both surfaces of the
leaf, especially if lesions are old. Such two-sided infections do
not generally occur in the case of leaf spots due to leprosis, scab,
juvenile spot, anthracnose, and melanose.
2. Canker lesions are concentrically ringed-a characteristic
not common to other spots affecting citrus.
3. Canker lesions possess crater-like depressions at their cen-
ters-a symptom generally requiring a hand lens for detection.
Young canker lesions may not show these craters; on the other

CANKER (ctd.)

30 Florida Agricultural Experiment Stations

hand, lesions due to scab may at times show similar depressions
in the middles of corky areas.
Canker differs from leprosis nailheadd rust) in the size of
spots, which are much smaller than those of leprosis. The con-
sistency of canker lesions is spongy; that of leprosis hard and
glazed. Furthermore, on grapefruit, canker is a common occur-
rence, whereas on the same host, leprosis is practically unknown.
Canker differs from scab in that it penetrates the leaf to
produce more or less equal spotting on both sides. Canker does
not produce leaf distortions nor wart-like outgrowths.
Canker differs from anthracnose on Key limes in that lesions
are not confined to fruit but occur also on leaves, young shoots,
and twigs. Canker on fruit resembles anthracnose on limes
more closely than any other disease, but canker spots do not
occur on sunken places, in distortions, and in splits of the rind,
as is often the case with anthracnose on limes. Moreover, can-
ker lesions do not develop the thick, corky, mound-like out-
growths characteristic of lime anthracnose.
Cause.-Citrus canker is a bacterial disease caused by Xan-
thomonas citri (Hasse) Dowson, a short, rod-shaped, motile or-
ganism that can be seen only with high magnifications of a com-
pound microscope. These bacteria are formed in countless num-
bers in canker spots and are exuded in masses whenever spots
are moistened by rain or dew. Drops of water carrying bacteria
spread the disease and when the organism comes in contact with
susceptible citrus tissue, especially tender foliage, twigs, and
young fruit, new canker spots are rapidly produced. On older
bark of branches and trunks, the bacteria probably gain entrance
through small wounds. Canker organisms can be spread some
distance by wind and driving rains, and any animal or insect
coming in contact with moistened foliage of an infected tree
may 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 citrus plants and spread throughout Flor-
ida on budwood and nursery trees.
The disease often remains inactive for considerable periods
after infections have occurred. Canker may also be carried over
long periods in lesions on older bark, but in soil the canker or-
ganism perishes rapidly.
Control.-The prompt and complete eradication of all canker-
infected trees was found to be the only safe and practical method

CANKER (ctd.)

Citrus Diseases in Florida 31

for checking the disease in Florida. The procedure consists of
burning trees in place with flame throwers, and preventing grove-
to-grove spread by disinfecting workmen's clothes and tools
with corrosive sublimate.
Now that canker has been eradicated in Florida, it is neces-
sary that citrus fruits and budwood from canker-infected areas
not be permitted to enter the United States. It is the business of
plant quarantine inspectors to intercept such unlawful importa-
tions and their work is often made difficult by the overzealous
grower, who, thinking he has discovered a new variety, attempts
to smuggle it across quarantine lines.
Any suspected case of citrus canker should be reported im-
mediately to the State Plant Board, which agency will attend
to its eradication.

Vines of many species use citrus trees to gain support for a
place in the sun. Most of these vines take advantage of trees
simply for support, and if roots or stems of such vines are cut,
the upper portions die. Among the common epiphytic plants
found on Florida citrus trees are (1) crab's eye (Abrus abrus
(L.) W. F. Wight), (2) wild balsam-apple vine (Momordica
charantia L.), (3) passionflower (Passiflora sp.), (4) florist's
fern (Asparagus sp.), and many others.
There are several vines, however, that are distinctly parasitic
and derive all their food and water at the expense of a host.
The effect of year-after-year feeding by these parasites on citrus
is cumulative, and eventually vigor of infested trees is reduced
to a point where adverse growing conditions like low tempera-
tures may kill weakened trees.
One of the parasitic vines occurring on Florida citrus trees is
Cassytha (Cassytha filiformis L.) (Fig. 14), a plant closely re-
lated to the laurels. Cassytha occurs in Central and South Flor-
ida but is seen on citrus mostly on the East Coast from Merritt
Island south. Infested trees look as if covered with a network
or tangle of orange-brown wire. The "wire" consists of stems
approximately the thickness of lead in pencils. Leaves are ves-
tigial to the point of appearing absent. Suckers penetrate the
bark of twigs, obtaining food and water for continued growth.
Cassytha is often mistaken for dodder (Cuscuta sp.), both
being parasitic on their hosts and resembling each other in gen-


Florida Agricultural Experiment Stations

eral appearance. However, Cassytha may be differentiated from
dodder on the basis of 2 characteristics: (1) Cassytha is a pe-
rennial, dodder an annual; (2) strands of Cassytha are longi-
tudinally striated, as can be seen by viewing under a hand lens,
whereas strands of dodder are smooth. For a discussion of
Cuscutas on citrus, see under Dodder Infestation.

Fig. 14.-Cassytha vines mantling an orange tree. Growth resembles
that of dodder, another parasitic vine on citrus, but differs in being more
loose, wiry, and orange-brown.

Control of Cassytha may be obtained by hand-pulling of the
vines, if the attack is light, or if heavy, by hatracking of the
The citrus nematode, Tylenchulus semi-penetrans Cobb, is
responsible for one of the declines affecting citrus trees in Flor-
ida. This microscopic eelworm was discovered in California in
1912 and in Florida in 1913; since then, it has been encountered
in 29 counties of the State and also in most areas of the world


Citrus Diseases in Florida 33

where citrus is grown. In California, the disease produced by
this parasite is called "slow decline". It should not be confused
with "spreading decline", a trouble in Florida that is caused by
the burrowing nematode, Radopholus similis.
The citrus nematode reduces the vigor of parasitized trees
and decreases their productivity. Trees heavily infected may,
however, show no ill effects until they encounter adverse growing
conditions, when decline sets in.
Symptoms.-Trees suffering from attacks by the citrus nema-
tode have sparse, small, and dull grey-green foliage. There are,
however, no specific above-ground symptoms that are useful in
characterizing this disease.
Examination of the roots provides a basis for diagnosing
slow decline. The nematode feeds on rootlets and, in the course
of egg-laying, secretes onto the rootlet surface a sticky substance
to which grains of sand adhere. This sheath of soil particles
around the feeder roots tends to make affected rootlets look some-
what swollen. The cortex of heavily infected rootlets is rather
easily separated from the woody cylinder. Parasitized rootlets
deteriorate more rapidly than unaffected ones; when rot becomes
extensive in rootlets, above-ground portions of the tree begin
to show symptoms of decline.
Cause.-The cause of slow decline is Tylenchulus semi-pene-
trans Cobb, a nematode approximately 1/60 inch in length.
Though too small to be seen with the naked eye, it can be ob-
served readily under the low power of a compound microscope.
A preparation consisting of swollen rootlets placed in water and
allowed to stand for 30 minutes will provide male and female
specimens of both larval and adult stages.
The female adult is sedentary in habit, having its head and
elongated neck imbedded in the cortical layer of the rootlet
while the rest of its body, globular in form, remains exposed on
the surface. The posterior portion of the adult female increases
in diameter with feeding, soon becoming much distended with
food and with one or two eggs. The latter, after being deposited
on the surface of the rootlets, hatch into free-living needle-
shaped larvae. These larvae, male and female, radiate in all
directions in search of food. In this way the nematode is dis-
tributed and so, in turn, is the disease. Further spread of the
nematode is assured by man when he moves infested nursery
stocks from place to place.


34 Florida Agricultural Experiment Stations

The citrus nematode may be found from the surface of the
ground down to levels 12 feet deep. It seems capable of thriving
in soils of all types, from light sand to heavy clay.
The host range of the citrus nematode is confined mainly to
citrus and its relatives, and to olive and persimmon. It includes
sweet orange, sour orange, grapefruit, tangerine, lemon, lime,
shaddock, citron, Mauritius papeda (Citrus hystrix DC.), Ichang
papeda (C. ichangensis Swing.), Citrus celebica var. Southwickii
(Wester) Swing., C. taiwanica Tanaka & Sham., Cochin China
atalantia (Atalantia citroides Pierre ex Guill.), kumquat (Fortu-
nella sp. Swing.), finger lime (Microcitrus australasica (F.
Muell.) Swing.), and trifoliate orange4 (Poncirus trifoliata
A number of other nematodes, including the burrowing nem-
atode which is described under Spreading Decline, have been
found to be parasitic on citrus. One of these is the sting nema-
tode (Belonolaimus gracilis Steiner). It is known to parasitize
citrus only in Florida. The extent of its damage has not yet
been completely evaluated.
The meadow or root-lesion nematode (Pratylenchus pratensis
(de Man) Filipjev) also parasitizes roots of citrus trees in Flor-
ida, as well as in California and Brazil. Meadow nematodes,
like burrowing nematodes, are endoparasitic-that is, they com-
plete their life cycle within the tissues of their host. As yet
no particular injury to citrus has been attributed to the meadow
nematode, though this species is known to be destructive to many
other agricultural crops.
A number of other plant-parasitic nematodes are associated
with roots of citrus, but their importance has not yet been
assessed. Of questionable importance are Hoplolaimus coro-
natus (Cobb), Aphelenchus avenue Bastian, Criconemoides citri
Steiner, Xiphenema americanum Cobb, Hemicycliophora parvana
Tarjan, Trichodorus sp. Cobb, Hexatylus sp. Goodey, and Aphel-
enchoides sp. Fischer.
Control.-The best way to control citrus nematodes is to
plant nursery stock free of this pest. Heat treatment of in-
fected nursery plants has not been entirely satisfactory. Con-
siderable care is necessary in heat-treating citrus so that plants
will not be damaged.
To date there is no satisfactory treatment to free mature
trees of citrus nematodes. No chemicals are available for treat-
*Although reported as a host by various workers, some varieties of this
species have been found to be highly resistant to the citrus nematode.


Citrus Diseases in Florida 35

ing trees in place without injury to such trees. In California
some reduction in nematode populations has been obtained by
treating strips of soil between infected rows with D-D at the
rate of 63 to 180 gallons per acre.
The final resort is to pull infected trees and to treat soil with
D-D, as is done for the control of spreading decline.

Clitocybe mushroom root rot attacks citrus as well as many
other plants, including grapes, forest trees, and ornamentals.
The causal fungus has been known in Florida since 1902, but not
until 1929 was it discovered to be a pathogen of citrus. The dis-
ease on citrus is restricted to Florida, but occurs on other hosts
in the Southeastern states. A similar disease, Armillaria root
rot, is a serious problem in certain citrus-growing areas of Cali-
fornia. The two diseases can be differentiated only by identifi-
cation of the mushrooms; in the case of Clitocybe, the annulus or
collar that occurs around the stem in Armillaria is absent.
Symptoms.-Above-ground symptoms of Clitocybe root rot
are so nearly like those of foot rot as to make differentiation diffi-
cult. As in foot rot, tops of trees decline, with foliage first be-

Fig. 15.-Grapefruit trees attacked by Clitocybe mushroom root rot,
showing dark lesion at base on left, and cluster of fruiting bodies attached
to citrus root on right.


36 Florida Agricultural Experiment Stations

coming pallid, then showing signs of starvation, and finally lead-
ing to dieback. Fruits produced on devitalized branches are
small and frequently drop before reaching maturity. When
decline is well advanced, areas of dead bark, similar to those in
foot rot, develop at the base of trunks (Fig. 15) ; the bark above
dead areas splits and gumming develops. Dead crown roots can
often be seen without scraping away the top soil.
Symptoms of decline caused by Clitocybe mushroom root rot
are the result of damage to the root system and do not appear
until many roots are involved. Development of the disease at
or below the ground line is generally difficult to see without ex-
posing roots. Consequently, it is not easy to locate trees in early
stages of involvement unless fruiting bodies of the fungus hap-
pen to emerge from infected tissues.
In most plants attacked by Clitocybe, there is formed under
the bark a thin, cream-colored fungus mat with radiating mar-
gins. In citrus, however, this sign is rarely seen in above-ground
parts, but in freshly dug roots these threads are found wherever
fungus growth is still active.
Cause.-Mushroom root rot of citrus in Florida is caused by
the fungus Clitocybe tabescens (Scop. ex Fr.) Bres. (Armilla-
riella tabescens (Scop. ex Fr.) Sing.). The fruiting structures
of this organism appear above ground as clumps of toadstools,
usually at the base of trunks (Fig. 15), but these toadstools
should not be confused with other mushrooms that grow singly
or are not attached to the tree.
Infection takes place by means of spores and by contact of
healthy roots with diseased roots or with decaying roots of alter-
nate hosts left when land is cleared. Since the fungus apparently
penetrates noninjured bark, it must be considered an active para-
site. It flourishes also as a saprophyte in stumps and roots of
various trees, especially oaks and Australian pines.
Clusters of mushrooms are produced after the fungus has
attained a certain stage in its development, provided moisture
and temperature conditions are favorable. The season of their
appearance in Florida is generally from September to early De-
cember. When fully developed, the caps are convex, tan-colored,
ranging from 2 to 31/2 inches in diameter and showing white
gills underneath. Clusters emerge and expand to full propor-
tions within a few days, and in warm wet weather decay very
quickly. However, if arid weather follows emergence, they may
dry up and turn blackish but remain recognizable for from 2


Citrus Diseases in Florida 37

to 3 months, marking the tree as one having been attacked by
root rot (Fig. 15, right). The presence of either fresh or old
clusters of mushrooms frequently enables one to locate attacked
trees long before presence of the disease becomes apparent in
the tops.
Clitocybe root rot coincides to a large extent with the plant-
ing of land improperly cleared of hardwoods, particularly oaks
and Australian pines. The disease occurs most frequently in
light, well-drained sandy soils, predominantly acid in reaction
and drought in character.
Rootstock varieties appear to be a factor in the development
of this disease. The highest incidence of mushroom root rot
occurs in rough lemon and the disease has never been found in
sour orange.
Control.-To reduce the likelihood of infection from Clitocybe
tabescens, citrus trees should be planted on land only after it
has been thoroughly cleared of stumps and roots, especially if
the land has just grown oaks or other hardwoods.
Once infected, however, trees can be treated in much the
same manner as described under Diplodia root rot. Because of
the underground work necessary in coping with root rot, the
excision of lesions is tedious and time consuming and may often
prove not to be as economical as would the replacement of in-
fected trees.
To prevent the disease from spreading from infected to
healthy trees, a narrow trench may be dug so that all communi-
cating roots are cut and removed, thus removing the bridges
along which the disease may pass. Care should be taken to throw
excavated soil on the side of the trench where the disease occurs.
Much experimental work has been done, both in this country
and abroad, in attempts to control root-rot fungi by chemical
treatments of the soil. Under California conditions, control of
the very-similar Armillaria root rot is obtained by soil injections
of carbon disulfide gas. In Florida, Clitocybe root rot has not
developed sufficient importance to warrant the testing of this
Low temperatures induce various symptoms on leaves, fruits,
and branches that resemble those due to infectious diseases.
Cold damage also may render plants susceptible to weakly path-
ogenic or saprophytic fungi and bacteria so that, in later stages


38 Florida Agricultural Experiment Stations

of cold injury, affected areas would come to be diagnosed as
withertip, Diplodia dieback, or gummosis.
Cold damage varies in expression from chlorotic and necrotic
spots on foliage (Fig. 16) to death of limbs and trunks. Dam-
aged tissues become invaded subsequently by leaf-spotting, fruit-
decaying, and wood-rotting fungi. Generally, first evidence of
damage to leaves, twigs, or green fruit consists of a dark greasy
discoloration. If not greatly chilled, leaves may regain their
normal appearance and remain on the tree apparently uninjured.
With lower temperatures, portions of the leaf blades may be
killed and subsequently become invaded by the withertip fungus.
Such leaves develop gummy spots or turn dark brown and papery
(Fig. 16). Leaf symptoms following still lower temperatures
consist of leaf rolling as if from moisture deficiency. Defoliation
may occur without serious damage to twigs and smaller branches.
However, when frost injury is so severe as to kill leaves and
twigs outright, they wither, blacken, and die.
Cold injury to twigs and smaller branches contributes to the
development of such diseases as melanose and stem-end rot,
withertip, and Diplodia dieback, since the organisms responsible
for these diseases form spores in dead tissues.
On trunks and large branches, cold damage may appear as
splitting or loosening of the bark. Certain areas, especially those
at or near crotches, are particularly sensitive. Localized dam-
age to patches of bark on the trunk and larger limbs are often
mistaken for gummosis, since these may exude gum at a later
Fruits are affected in a number of ways. One of the common-
est symptoms is the appearance of brown spots on the peel caused
by liberation of peel oil and settling of tissues between oil glands.
More severe freezing may produce watery patches on the rind,
and these areas then become invaded by blue and green molds.
The first effects of frost, other than the formation of ice crystals
within fruit, are water-soaked areas or white specks on walls
of segments of the pulp. These specks, consisting of hesperidin
crystals, usually can be seen from 5 to 10 days following injury
to tke fruit. Severely affected fruit falls within a few days of
damage, but fruit less severely frosted may escape detection
until breakdown occurs in transit.
Protection of citrus from the effects of cold are limited to 2
methods: banking of young trees and heating of older ones.


Citrus Diseases in Florida 39

Once freeze injury occurs, further damage by secondary or-
ganisms can be reduced by cutting out dead wood. However,
pruning should be delayed until the following fall or winter so
as to allow the full extent of damage to become visible. Prun-
ing wounds should be treated as outlined in the chapter on foot
See Foot Rot, Diplodia Root Rot and Collar Rot.

See Psorosis.
See Heart Rot.
Crinkle-scurf is a disorder frequently encountered in Florida
groves; it produces a crinkling of leaves and a surfing of bark.
The bark symptoms bear a pronounced resemblance to banded
corky bark, a disease said to be related to psorosis, but transmis-
sion experiments with crinkle-scurf in Florida have failed to
implicate a virus. Rather, the trouble appears to be due to
a genetic abnormality.

Fig. 16.-Cold injury to leaves of sweet orange, showing
graduation in severity of symptoms.



40 Florida Agricultural Experiment Stations

Twisting of foliage caused by crinkle-scurf may be confined
to a single limb or it may occur in foliage throughout the top.
At times, the abnormality is sufficiently striking and bizarre to
create some concern; instances are known where growers have
become alarmed to
the extent of remov-
ing systematically all
affected trees.
Foliage symptoms
consist of a marked
crinkling or twisting
of leaves (Fig. 17)
due to abnormal
curvature of the mid-
ribs and edges. In-
stead of being nor-
mally stra igh t or
slightly curved, the
midribs and e d g e s
'L- are arched or S-
S shaped. Leaves of
-j I- the terminal flush are
most conspicuously
distorted. However,
older leaves do not
entirely lose t h e i r
convoluted appear-
ance. In addition to
being distorted,
leaves stand erect
and appear to wrap
themselves a r o u n d
Fig. 17.-Terminal shoot from a Valencia t wigs supporting
sweet orange tree showing leaves that are
twisted and abnormally erect and clasping, them. This upright
habit makes foliage
look thin and gives the tree the appearance of suffering from
drought. Twisted leaves often show a large centrally-placed
chlorotic area best seen when viewed by transmitted light. In
distinction to chlorotic patterns associated with psorosis, the one
produced by crinkle-scurf is persistent, and is often found in
leaves as far back as the fourth flush.
Bark symptoms are more obscure and are easily overlooked


Citrus Diseases in Florida 41

but occur on nearly every tree showing the crinkle aspect. The
bark abnormality consists of patches of fine corky blisters ar-
ranged in straight horizontal lines (Fig. 18). At times, such
patches encompass the trunk or branch but generally they are
small, approximating the size of a penknife. When affected
areas are shaved tangentially, the underlying cortical tissues are
found to be impregnated with a dark-brown gummy substance.
The discoloration is limited laterally to the width of the ridge
and does not penetrate into the wood.

Fig. 18.-Scurnng of the bark on a Valencia sweet orange tree associated
with crinkle-scurf, showing parallel alignment of rows of corky pimples.

Crinkle-scurf occurs mainly on Valencia and Lue Gim Gong
trees; in fact, it is so nearly an exclusive trouble of these two
varieties that symptoms of crinkle-scurf can be depended upon
to indicate that affected trees are one of the late varieties of
sweet orange.
While with the unaided eye it is difficult to detect certain al-
terations in the growth of affected trees, significant decreases
from the normal do occur, particularly in matters of trunk diam-
eter, size and weight of fruit, and yield. Losses in production
are difficult to assess, but undoubtedly are dependent on the
number of branches per tree that show symptoms of crinkle-


42 Florida Agricultural Experiment Stations

Results of tests to determine the infectiousness of crinkle-
scurf demonstrate that the trouble is not transmissible from af-
fected budwood to healthy stocks. However, buds from af-
fected trees will produce scion shoots that again exhibit crinkle-
scurf. As a consequence, trees showing crinkle-scurf should not
-in the interests of maintaining varieties true to type-be used
as a source of budwood.


Damping-off, whether of citrus or of some other plant, is
essentially a disease of the seedbed. It is a universal trouble
of citrus seedlings and is caused by one or another of several
species of fungi.
Symptoms.-The symptoms of damping-off are much the
same, irrespective of the fungus involved. First indication that
a seedling has been attacked is a wilting of leaves and a water-
soaked appearance of a portion of the stem near the ground
line. Affected stem tissues eventually turn dark, shrink to form
constrictions, and die (Fig. 19). Affected areas develop a soft
rot so that cortical tissues slough and the exposed woody tissues
dry and whiten. Seedlings pulled up at this stage will be found
to have roots consisting of only thin woody fibers. Citrus seed-
lings killed by damping-off do not fall over as do most other
kinds of plants affected by this disease, but remain erect despite
complete girdling.
Damping-off occurs characteristically in spots or patches in
the seedbed. These patches enlarge rapidly when weather is
cold, cloudy, and wet.
Cause.-Damping-off is caused by any one of several species
of fungi, including Pellicularia filamentosa (Pat.) Rogers, Scler-
otium rol/sii Sacc., Pythium spp. and Phytophthora spp. Seed-
lings are susceptible from time of germination until stem tissues
become woody. The disease is favored by cold, wet weather;
generally, damping-off is not a problem during the heat of sum-
mer or the dryness of winter.
Control.-Damping-off organisms are soil inhabitants and
can survive year after year on decaying vegetable matter. Seed-
beds preferably should not be planted on land where damping-off
has been a problem. If use of such land is unavoidable, soil ster-
ilization should be practiced. This may be done by treating plant-
ing sites with steam under pressure as is done in greenhouses,


Citrus Diseases in Florida 43

or by applying a mixture of 1 part formalin (37-40% formalde-
hyde) in 50 parts water at the rate of 1/2 gallon to each square
foot of soil surface. When formalin is used, treated soil should

Fig. 19.-Damping-off of citrus seedlings, showing constrictions along
stems caused by rotting of cortical tissues, also the sloughing of bark and
feeder roots on underground parts.


44 Florida Agricultural Experiment Stations

be covered for a day or two with tarpaulins or sacks to trap
fumes, but before planting, the residual fumes must be dissipated
by spading until all ordor of gas has disappeared. It is generally
safe to plant seeds in formalin-treated soil 2 weeks after treat-
Seeds may be protected from pre-emergence damping-off by
coating them with a fungicidal powder like spergon, cuprocide,
or one of the neutral coppers. One level teaspoonful of fungi-
cide is sufficient to treat a quart of seeds. Mixing of seeds and
protectant can be done in a mason jar or small drum, and the
excess dust screened off after treatment.
If damping-off develops in the seedbed, some protection to
the remaining healthy seedlings can be afforded by spraying
them with 3-3-100 bordeaux mixture or neutral copper at equiva-
lent rates of copper. Enough spray should be applied to wet
seedlings and soil around stems, and applications should be con-
tinued at weekly intervals until spread has stopped.
In addition to selecting clean soil, using seed protectants,
and applying sprays, the proper management of seedbeds is es-
sential for control of damping-off. Proper management includes
the regulation of factors that favor the disease. Irrigation water
should be applied without puddling the soil. Drainage should
be provided that will remove excess soil moisture. Shading
should not be too heavy, and overcrowding should be prevented.
In order to minimize the chances of infection, fruits for seed
should be picked from trees rather than off the ground.

Citrus suffers from a number of troubles that result in grad-
ual weakening so that trees decline in vigor, may no longer pro-
duce fruit, and may eventually die. For many years the word
"decline" has been applied to such conditions, especially when
the cause was unknown. Thus, a grower may speak of water
damage as decline because he does not know the cause. Roadside
decline is used in reference to an unthrifty condition that occurs,
or is most evident, alongside highways. Spreading decline refers
to a specific disease, now known to be caused by the burrowing
nematode. Thus, many common diseases and troubles have
been, and some still are, lumped together under the name decline.
It should be apparent, therefore, that decline is not a specific
disease but merely a term that denotes wasting away of growth

DECLINE tctd.)

Citrus Diseases in Florida 45

or fruit production and that may involve ultimate death of the
It is commonly thought that there are more trees declining
and dying now than there were a number of years ago, but this
viewpoint is open to question. When most groves were deficient
in various mineral elements such as zinc and magnesium, lack
of vigor was a condition frequently encountered. As a result of
this widespread debility, trees which actually died called forth
little attention. Today, with groves well fertilized and in vig-
orous condition, occasional weak or dying trees present a marked
contrast. It is, therefore, questionable whether the percentage
of trees dying from disease is any higher than formerly. Ad-
vancing age of trees in most groves has caused some increase in
the incidence of diseases, as has also the spread of the burrowing
nematode, but these factors hardly equal the losses due to min-
eral deficiencies that occurred in the Twenties and Thirties.
There are many reasons why a citrus tree may be in a state
of decline. Some of the more common causes of decline are foot
rot, root rot, spreading decline, water damage, lightning injury,
nutritional deficiency, heart rot, algal disease, psorosis, tristeza,
xyloporosis, and exocortis. The number of groves where foot
rot and root rot are the cause of decline is probably equal to the
number of groves in which all other causes are involved. De-
tailed discussions of most of these declines are to be found else-
where in this bulletin.
The cause determines the rate at which trees decline and die.
Unfortunately, however, decline is not always noticed in its early
stages; only when foliage becomes sparse or when branches die
does the trouble become apparent. The most rapid decline re-
sults from acute water damage or root rot; at the other extreme,
many years may elapse before a tree shows decline from heart
It is important to differentiate between effects of the primary
cause and of the secondary factors, as for instance, the sunscald-
ing of bark on branches as a result of defoliation. These sun-
scalded areas usually become infected with wood decay fungi and
may cause the loss of branches, though they are purely of sec-
ondary origin brought on incidental to loss of foliage.
The pattern made in a grove by affected trees provides addi-
tional information regarding the cause of decline. Some diseases
affect only scattered trees while others affect trees down the
row or throughout a block. For instance, if decline is due to

DECLINE (ctd.)

46 Florida Agricultural Experiment Stations

diseases like psorosis, xyloporosis, and exocortis, the pattern will
consist of an affected tree here and there, this pattern reflecting
the fact that causal viruses are transmitted through the bud-
wood. If, on the other hand, the pattern involves all trees in a
block, there is more likelihood that the trouble is due to a con-
dition present throughout the grove, as, for instance, standing
water, nematodes, or lack of fertilizer.
In attempting to classify such declines, certain observations
should be made in order to determine causal agents. The follow-
ing procedure may be helpful:
1. Examine the pattern made by affected trees in the grove.
It may be desirable to make a map of the affected locality, indi-
cating trees that are stricken. If trees are confined to a solid
area, and this area has been increasing in size, and the affected
trees are weak in foliage, the trouble may be spreading decline.
If, similarly, affected trees are in a definite area, but the trouble
has come on rather suddenly, it may be caused by water dam-
age, and the soil should be examined for evidence of water-log-
ging and the roots for rotting.
2. When affected trees are scattered irregularly through the
grove, the limbs and trunk should be inspected for the scaly le-
sions of psorosis, the base of the trunk for lesions of foot rot,
and the roots for evidence of root rot. Other troubles such as
lightning injury also can result in a scattered pattern of affected
If the above analysis is made, the relatively meaningless term
"decline" can be replaced by the much more informative name
of the disease actually involved. The various types of decline,
including those caused by foot rot, root rot, blight, psorosis,
xyloporosis, exocortis, and tristeza, are described in this bulletin
under these respective names.

See Shellbark.
Under grove conditions, it is usually difficult to distinguish
between dieback due to Diplodia and that due to Colletotrichum
anthracnosee). Laboratory tests are needed to identify the two
fungi involved. At times, both fungi are found in the same
lesion, under which circumstances they probably do more injury
in combination than when each is present alone. Diplodia die-
back is known in many tropical and subtropical lands where


Citrus Diseases in Florida 47

citrus is grown, and occurs generally distributed in all citrus
groves, being found on the surfaces of both healthy and diseased
trees. The fungus is considered a feeble pathogen in that it is
dependent for entry on weak or wounded tissues.
Symptoms.-Diplodia dieback starts from the tips and ex-
tends for some distance down limbs, ending in a sharp line;
gumming may be present or absent. The whole twig, branch, or
trunk may become involved. Affected bark develops a reddish-
brown color and becomes dotted with small black pustules-the
fruiting bodies of the fungus. The wood underneath turns black.
Occasionally, attacks may take place not at the tip but at
some point well back from it, causing girdling of the twig. As
a result, leaves above the affected ring quickly wither and the
twig dies. An internal gumming and death of bark occurs at
points of attack, and a few drops of gum may flow to the surface.
Another manifestation of Diplodia dieback, especially in
grapefruit and lemon trees, is the involvement of fruits on at-
tacked twigs. Such fruits decay and eventually become mum-
Cause.-The fungus Diplodia natalensis Pole-Evans is found
in tissues affected by Diplodia dieback. It is the same fungus
that also occurs in lesions of Diplodia collar rot, crotch rot of
mandarins, shellbark or decorticosis of lemons, and stem-end
rot of fruit.
Environmental conditions that predispose a tree to infection
are (1) cold injury, (2) droughts that cause a sudden checking
of growth, (3) wounds, and (4) heavy bearing of fruit, espe-
cially in trees that grow on light, sandy, impoverished soils.
Among grapefruit varieties, Triumph is notably susceptible to
this disease.
In years of drought, much dieback is encountered in young
trees just set out in grove formation. The killing of twigs starts
from the top and proceeds down the trunk. Isolations from
affected tissues yield principally Diplodia fungi, but the primary
cause of the trouble is actually the lack of water. Freezes may
injure young trees in a similar manner, following which Diplodia
becomes established.
Control.-To prevent attacks of this disease, reliance must
be placed largely on proper fertilizer and cultural practices as
outlined under the control of anthracnose. Trees maintained in
a good state of vigor will be less adversely affected by unfavor-
able conditions that are beyond the grower's control.


48 Florida Agricultural Experiment Stations

Once injury has taken place, further damage can be pre-
vented by promptly removing infected branches, going well back
into healthy wood to make the cuts. Pruning wounds should
be disinfected and covered with waterproof dressing as men-
tioned under foot rot.

In addition to Phytophthora foot rot and Clitocybe root rot,
another rot of the foot or collar and of the roots exists in Florida
citrus groves, namely Diplodia collar rot and root rot. Though
similar in outward appearance to Clitocybe root rot, Diplodia
root rot is generally the cause of decline in trees on wet heavy
soils, Clitocybe root rot being unknown in such locations. Di-
plodia root rot is thought also to be involved in the so-called road-
side decline.
Symptoms.-A sudden collapse of the foliage of citrus trees
is usually the first symptom of this disease. When this happens,
inspection of affected trees may reveal dead bark around the
base of trunks. This collar-rot type of girdling is usually ac-
companied by gumming at the margins of lesions. On the other
hand, more frequently rotting of bark is confined to the roots.
Often, the entire root system is involved, with bark so much
disintegrated that pathogenic organisms cannot be isolated.
Effects of Diplodia rot on the tops of trees are similar to those
caused by Phytophthora foot rot or Clitocybe root rot. Diplodia-
infected roots, however, are characterized by a black discolora-
tion of wood beneath the dead bark. Diplodia rot frequently
follows damage due to excessive rainfall or flooding by irriga-
tion water, and symptoms of decline generally do not appear
until a period of drought has set in. Flooding asphyxiates the
lower roots first, leaving only the surface ones to sustain the
tree. When, with subsequent drought, the surface roots also
are killed, the tree goes into a sudden wilt and declines. Trees
damaged by a fluctuating water table frequently show a broom-
like mass of dead roots along the taproot.
The pattern of spread in the field differentiates this disease
from spreading decline, with which it is sometimes confused.
In cases of Diplodia rot, it is isolated trees, either singly or in
groups of two or three, that are affected, whereas in spreading
decline, trees are affected along an unbroken front.
Cause.-Isolations from early stages of this trouble generally
yield the fungus Diplodia natalensis Pole-Evans, which is the


Citrus Diseases in Florida 49

imperfect stage of Physalospora rhodina (B. & C.) Cooke. This
is the same fungus that has been described as causing Diplodia
rot in citrus fruits, and it has also been found present in such
diseases of non-citrus plants as Casuarina trunk rot, Cotoneaster
collar rot, branch rot of Erythrina and Ficus, and twig rot of
persimmon. There is no certainty, however, whether the fungi
isolated from affected tissues are the direct cause of death or
whether they enter a tree after it has been weakened by other
factors. The latter possibility appears to be the more probable
explanation, since the incidence of Diplodia root rot is higher
in groves where water damage has occurred or where soil pH
values are above the optimum, as along highways where crushed
lime rock is used as a base. Trees struck by lightning but show-
ing no visible effects also develop Diplodia root rot.
Control.-Unfortunately, Diplodia root rot is rarely observed
until trees have become completely girdled. Infected trees sel-
dom recover and should be pulled and replaced as soon as they
become unprofitable. Soil fumigation of the replanting site
with D-D may promote sanitary growth of the reset by killing
pathogenic fungi that increase in decaying roots.
In the case of trees suffering only from the collar rot phase,
diseased bark may be cut out and treated as described under
foot rot, provided not more than one-third the circumference of
the trunk is affected.
If further studies substantiate the probability that Diplodia
plays only a secondary role, it may become possible to correct
the cultural malpractices that are primarily responsible for this
type of decline.

Citrus trees are occasionally hosts to various species of cus-
cutas. These are leafless plants lacking chlorophyl and appear-
ing to cover trees in a tangle of orange-colored string. Cuscutas
have many local names, being known variously as dodder, love
vine, devil's guts, pull down, gold thread, hellbind, hailweed,
and vermicella. Of all agricultural weeds, except possibly Can-
ada thistle, dodder has been most legislated against. Fortu-
nately dodder has never become a widespread pest of citrus in
Florida, though in the Lesser Antilles its presence has become so
troublesome that landowners are enjoined by law to destroy all
dodder occurring on their properties.
Dodder is a vine belonging to the morning-glory family. It
contains no green pigment and is therefore parasitic, withdraw-


50 Florida Agricultural Experiment Stations

ing nutrients from the tree instead of using it merely for support
as do such chlorophyl-containing vines as crab's eye, wild balsam-
apple vine, or cypress vine. An infestation of dodder usually
originates from seed. On emergence from the soil, a thread-like
seedling twines around weeds, using the latter first as hosts and
later as ladders on which to get into the tops of citrus trees.

Fig. 20.-Dodder on grapefruit twigs. The clusters on this leafless vine
consist of flowers and seed cases. At left are shown the tightly encircling
vines; these coils make hand pulling ineffective since any that are over-
looked can regenerate the dodder.


Citrus Diseases in Florida 51

Once the seedling has found a host, it losses connection with the
soil, and thenceforth carries on a parasitic existence. On citrus,
the dodder most commonly encountered (Cuscuta americana L.)
takes a number of turns around a stem, sending in approximately
8 suckers per turn, and then projects its growing tip across
space in search, as it were, of new twigs on which to extend its
range (Fig. 20).
Control of dodder, once the parasite has become established,
is difficult if attempted during the growing season. It is neces-
sary to pull out not only all loose strands but also all tightly clasp-
ing sucker coils. To do this properly, each branch must be taken
separately, examined down its entire length, and rubbed to re-
move the dodder bracelets. To remove dodder from one bearing
tree may take as much as 48 man-hours.
An easier method of controlling dodder is to let winter tem-
peratures kill the vines and to keep the ground under parasitized
trees cultivated so that germinating dodder seedlings will not
have a chance to reinfest trees. This procedure works satisfac-
torily in the case of sweet orange, grapefruit, and tangerine
trees. However, when tops are Tahiti lime, it is necessary to
hatrack affected trees to eradicate dodder suckers because in this
variety dodder can overwinter under the bark and re-emerge the
following spring.
On the lower East Coast of Florida, another parasitic vine
may be found on citrus. It is often confused with dodder;
actually, however, it is a close relative of the laurels. For a dis-
cussion of this pest, control of which is somewhat different due
to its perennial growth, see under Cassytha.

EXANTHEMA (Ammoniation)
Exanthema causes a dieback of branches in citrus trees sim-
ilar to that produced by Diplodia; hence, these two types of die-
back are often confused. Exanthema also affects fruits, under
which circumstance the name ammoniation is often used. The
symptoms of exanthema are: (1) formation of bark-covered
gum pockets at or near the nodes on succulent shoots, (2) bark
excrescences made by gummose tissue forced through longi-
tudinal or rounded breaks, (3) stained terminal twigs, (4) glossy
brown gum-soaked spots on fruit, often accompanied by a more
or less horizontal splitting of the fruit, and (5) gum in angles of
segments surrounding seeds.
Exanthema is caused by a deficiency of copper. While it has
long been held that this disease may also be due to over fertili-


52 Florida Agricultural Experiment Stations

zation with nitrogen or practices that lead to a high accumula-
tion of nitrogen in plant tissues, it is now known that these
factors are not directly responsible for exanthema, and are im-
portant only when copper is severely lacking.
Exanthema can be controlled by the application of bordeaux
mixture, 1-1-100, preferably just prior to or during the flush of
growth in which exanthema is expected. Also effective, but less
rapidly so, is the spreading of copper sulfate crystals (not the
finely ground "snow") on the soil under the canopy of trees, 2
pounds per adult tree. The regular use of fertilizers containing
copper will prevent the occurrence of exanthema.
At one time exanthema appeared to be universally present in
Florida citrus groves; nowadays, however, with routine fertiliza-
tion with minor elements, including copper, the trouble is seldom
encountered. Symptoms taken for exanthema are today more
often due to troubles unrelated to copper deficiency. For ex-
ample, vertical splitting of the fruit as opposed to the horizontal
or irregular splitting mentioned above, is caused not by copper
deficiency but by physiological factors as yet incompletely un-
derstood. Likewise, lesions on fruit resulting from melanose
are occasionally mistaken for those caused by exanthema.

EXOCORTIS (Scaly Butt)
Exocortis is a serious disease of trifoliate orange and of cer-
tain citranges that are used as rootstocks. The prevalence of
the exocortis virus is obscured in Florida because rootstocks sus-
ceptible to it are not generally grown in the state. In the event,
however, that trifoliate orange or some of the citranges become
popular here, as they have in other citrus-growing areas, the
real destructiveness of this disease will be felt.
The causal virus probably occurs throughout groves in Flor-
ida, existing in a masked form in standard stock-scion combina-
tions. Budwood from infected trees is necessary to introduce
the virus into trifoliate orange seedlings; nonbudded seedlings
are not known to be infected. Certain scion varieties are rather
.generally infected, others only occasionally, and a few appear
to be exocortis-free. The disease has not been observed in tri-
foliate orange when the top varieties have been, among sweet
oranges, the Argentine variety Calderon, and among mandarins,
Satsuma. Only occasional cases of scaling have been seen when
the scion variety is the Argentine common sweet orange, a
midseason variety similar to the Pineapple. On the other hand,


Citrus Diseases in Florida


Fig. 21.-Scaling produced by the exocort-s virus in the rootstock
portion of a tree on trifoliate orange.


54 Florida Agricultural E l, i, ii,, ,, Stations

exocortis appears frequently when scion varieties are, among
sweet oranges: Washington and Thompson Navel, Valencia,
Lue Gim Gong, Jaffa, Ruby Blood, and certain South American
varieties; among mandarins: Clementine, the Australian variety
Thorny, and the Argentine Common, Campeona, and Cutro;
among grapefruit: Thompson, Duncan, Triumph, Marsh Seed-
less, Pernambuco, and Florida; among lemons: Eureka, Lisbon,
English, and Lumma; among kumquats: Nagami; and among
limes: Key.
So far as is known, spread of exocortis is brought about solely
by the use of infected budwood. There is no evidence that in-
sects are responsible for dissemination from tree to tree, nor
that the virus is seed-transmitted.
Symptoms.-Susceptible trees may be affected in one or both
of two ways: by scaling of bark and by stunting of growth.
Both symptoms usually appear together, though stunting may
occur in the absence of scaling. Stunting is usually severe, so
that, for example, a 30-year-old tree may not exceed 5 or 6 feet
in height.
Scaling of susceptible rootstocks, a more reliable indication
of exocortis than stunting, may be limited to scattered cracks
in the outer bark. Generally, however, scaling involves the
whole rootstock portion of the tree, giving it the appearance
of a shellbark hickory (Fig. 21). Scales consist of dry, thin
layers of bark varying in size from bits 1/4 inch square to vertical
strips 5 or 6 inches long. Although bark shelling also takes
place on the upper main roots, the action of soil organisms work-
ing on dead tissue causes the scales to disappear, sometimes as
rapidly as they are formed.
In healthy trees, the trifoliate orange portion of the trunk
outgrows the scion portion, so that immediately below the bud
union a shelf or shoulder is formed. In trees affected by exocor-
tis, this shoulder is much less apparent or may be missing al-
The diagnosis of exocortis is at times complicated by the fact
that in certain areas of the state a rootstock has been used, ap-
parently the variety Everbearing Lemon, that causes a shoulder-
like growth confusingly similar to that of trifoliate orange. In
addition, such rootstocks may also develop an exocortis-like
scaling. It is not yet certain whether this scaling results from
presence of the exocortis virus in this variety or whether scaling
is due to shellbark, a condition common to lemons in Florida.


Citrus Diseases in Florida

Exocortis scaling first becomes evident as a cracking and
scaling of rootstock bark 3 to 8 years after budding. Stunting
appears somewhat later than shelling, and as a rule results in
trees that are not worth their care.
Control-There is no known method for curing trees infected
with exocortis. Therefore, in order to control the disease, it is
necessary to exclude the virus during
the growing of plants in the nursery. I
This can be done by using healthy
scion wood in the budding operation.
It is possible to obtain scion wood free
of exocortis virus by selecting it from
normal-appearing parent trees that are
8 years or more old that are on trifo-
liate orange rootstocks.
Since exocortis affects only trifo-
liate orange and its hybrids, another
way of avoiding this disease is through
use of tolerant rootstocks. Such root-
stocks include the commercial varie-
ties rough lemon, sour orange, sweet
orange, Cleopatra mandarin, and
grapefruit. .

In Florida and in the tropics, twigs
and branches of citrus trees are fre-
quently encircled by growths of the
felt fungus (Septobasidium pseudo-
pedicellatum Burt). This fungus is
present also on plants of a number of L
other genera. Septobasidia cause lit-
tle, if any, direct damage to citrus.
The fungus appears most frequently
on twigs, sometimes extending along
leaf stalks and bases of leaves and onto Fig. 22. Felt fungus
fruits. It appears as a soft, felt-like twi ng live grpefuit
covering, light-brown to gray in color,
that encircles twigs and envelops them for a distance of from 1 to
3 or more inches (Fig. 22). The surface of the growth may be
smooth and compact or membrane-like, but the structure under-
neath is soft and spongy. Growth is superficial and does not ap-

56 Florida Agricultural Experiment Stations

pear to penetrate the bark. Felt fungi are most prevalent in
humid locations such as low hammocks.
Since this fungus causes no serious injury, control methods
are not warranted. If for any reason, however, it is desired to
eliminate these felty patches, limbs and twigs bearing them
may be pruned out. Further growth can be stopped by the same
copper sprays that are applied for the control of melanose.

See Rio Grande Gummosis.

See Leprosis.
Flyspeck is a fruit-spotting disease of minor importance in
Florida. It is caused by Leptothyrium pomi (Mont. & Fr.) Sacc.,
and derives its common name from the minute black specks
formed on the rind (Fig. 23). At times speaking is accompanied
by another fungus, Gloeodes pomigena (Schw.) Colby, that pro-
duces a dark film on the rind, called sooty blotch (which see).

*i i

y ^ ',, ". ......

Fig. 23.-Flyspeck on grapefruit, showing the small fruiting bodies
that give this disease its common name. (Slightly enlarged).


Citrus Diseases in Florida 57

Flyspeck and sooty blotch are found chiefly on fruits that are
overripe. Neither fungus causes injury. However, when preva-
lent, these fungi may mar fruit sufficiently to lower its grade in
the packinghouse. While washing and polishing operations re-
move sooty blotch, they do not remove flyspeck.
Development of both these diseases is favored by high hu-
midities. Rank growth of weeds around lower branches of a
tree is one factor that may produce an undesirable degree of
humidity. Low vitality of the tree is thought to predispose
fruits to attack.
Flyspeck and sooty blotch are not of sufficient importance in
Florida to warrant control measures.

FOOT ROT (Phytophthora Foot Rot, Brown Rot
Gummosis, Collar Rot)
Foot rot is the most frequently encountered disease on the
trunks of citrus trees in Florida. First discovered in the Azores
in 1834, the disease was later found in Mediterranean countries,
and by 1863 had destroyed thousands of trees in Italy and Sicily.
In Florida, foot rot was first noted about 1876, though its im-
portance was not felt until after 1880. Between 1834 and 1914
foot rot spread to practically all citrus-growing areas of the
world, where it became a problem, especially in sweet seedling
groves. The disease led ultimately to an almost universal change
from seedling culture to the growing of trees on resistant root-
The purpose of a foot-rot resistant rootstock is to elevate the
susceptible scion portion of the trunk above the ground in which
are harbored the foot-rot fungi. When bud unions are too low,
it is possible for rain to spatter these fungi onto susceptible scion
portions of trunks and thus to initiate foot-rot lesions above the
bud unions. In such ways foot rot becomes a problem not only
in seedling trees but also in trees that are budded too low. At
times the disease may affect nursery stock, attacking either sus-
ceptible varieties grown for rootstocks or susceptible varieties
occurring above the bud union.
Citrus varieties differ in susceptibility to foot rot. The order
of resistance among rootstocks commonly used in Florida ap-
pears to be as follows, starting with the most resistant: trifoli-
ate orange, sour and bittersweet orange, Cleopatra mandarin,
rough lemon, grapefruit, sweet orange, acid lime, lemon, and

FOOT ROT (ctd.)

58 Florida Agricultural Experiment Stations

sweet lime. The first 4 show sufficient resistance to be recom-
mended for plantings on heavy or wet soils. Troyer citrange
shows considerable resistance in trials under California condi-
tions, but use of this rootstock in Florida should await results
of experiments to determine resistance to other diseases.
Some variations within varieties also occur. Thus, among
grapefruits, which are generally susceptible, the variety Imperial
is as resistant as sour orange, whereas Pernambuco is as sus-
ceptible as some of the limes. Among sweet oranges, Navel is
much more susceptible than Valencia.
Symptoms.-Foot-rot lesions generally occur at the union in
budded trees or at the crown in seedling trees. The first symp-
tom is a water-soaking of bark that appears as a dark spot on
the trunk. When scraped, this area is seen to consist of dis-
colored tissues that at first occur only in the thin surface layers
of bark, but that eventually may extend through the cambium
and into the wood. In early stages, the decayed bark is firm and

Fig. 24.-Foot-rot lesion in the sweet-orange scion portion of a trunk.
Tipped growth of the tree has brought susceptible tissues close to the
ground line where conditions for the disease are favorable.

FOOT ROT (ctd.)

Citrus Diseases in Florida 59

intact, but with age it becomes shrunken and cracked, shredding
in lengthwise strips as it dries. Much gumming may accompany
advanced stages of foot rot, particularly in late spring and early
summer, though because of the solubility of gum in rain water,
this symptom is not always present. Affected bark is at first ir-
regular in outline; with age, it becomes sunken below the level
of healthy bark and is sharply demarcated (Fig. 24). The bark
remaining alive in advance of lesions often develops callus rolls
that check further spread, especially in an upward direction.
In some cases the disease appears to be arrested, only at a later
date to resume extension. Ultimately, the lesion may encircle
the trunk, causing the tree to die, but vertical spread is gen-
erally limited to 1 to 2 feet from the ground. Decay may also
proceed into the roots (Fig. 25), under which circumstance foot
rot may easily be confused with root rot. Lesions below ground
are invaded by secondary organisms that involve the bark in a
wet, sour-smelling, fermentative rot.

Fig. 25.-Soil removed from around a sweet-orange tree on sour-orange
rootstock, revealing the extent to which foot rot has invaded the root



FOOT ROT (ctd.)

60 Florida Agricultural Experiment Stations

After foot rot has progressed to a point where the root sys-
tem or the trunk is partially girdled, the top develops a charac-
teristic decline (Fig. 26). The foliage at first is hard and dull,
assuming a light yellow cast, and eventually becoming distinctly
yellow, especially along the midribs. These are the characteristic

Fig. 26.-Orange tree nearly dead from girdling by foot rot.

~k-7~3il~Lr ~~~,r,

FOOT ROT (ctd.)

Citrus Diseases in Florida 61

symptoms of acute starvation brought on by girdling. The leaves
of successive flushes are smaller, and twigs, branches, and finally
limbs succumb, giving the tree a distinctly ragged appearance.
Such trees may continue for a time to bear fair amounts of fruit,
and occasionally, just before dying, may set a large crop. Fruit
sizes become smaller in late stages of decline. When half the
top is killed, numerous water sprouts may develop from the
branches. A characteristic development of foot rot, differenti-
ating this disease from root rot, is that the tree often dies ir-
regularly, one side failing while other sides are still sound.
During dry spells, trees girdled by foot rot develop an early and
pronounced wilting and rolling of the leaves.
The rate of dying due to foot rot varies considerably. In
severe cases, trees may be dead within a year from first appear-
ance of symptoms. Usually, however, affected trees do not suc-
cumb but decline to the extent that the circumference of the
trunk is damaged. Small lesions may be overgrown by new bark,
in which case the tree recovers fully.
Cause.-Foot rot in Florida is caused primarily by two fungi,
,Phytophthora parasitica Dast. and P. citrophthora (Sm. and
Sm.) Leonian, although other species of Phytophthora are known
to be involved elsewhere. These fungi are found in the soil and
will attack trees whenever conditions are favorable. Important
factors contributing to the initiation of foot rot are the cool air
and soil temperatures of early spring and abundant soil mois-
ture. Injuries at such times afford the fungus an entrance into
tissues where it becomes established and grows without further
need of the climatic conditions that favored its entry. The
causal fungus may also enter trunks through cracks made by
the growth of adventitious shoots near the bud union. This
invasion is often the result of planting trees so deep that the
susceptible variety is at or below the ground level. Invasion of
the fungus may also take place in the absence of wounds, espe-
cially if wet soil is in contact with susceptible bark for sufficient
time. Any factor that keeps the trunk wet for prolonged periods,
especially when temperatures are low, predisposes a tree to in-
fection. Entry may be effected also through bark that has been
split by heavy applications of fertilizers, as when attempts are
made to force neglected groves into quick production.
Acidity of the soil is known to influence the survival of Phy-
tophthora spores. In experiments with soils having differing
pH values, it was found that the largest number of root infec-

FOOT ROT (ctd.)

62 Florida Agricultural Experiment Stations

tions occurred at pH 5.4 to 7.5; that moderate infection took
place at 4.8 to 5.0, and that only slight infection appeared be-
tween 4.3 and 4.5.
The rate at which lesions develop, once infection occurs, is
related apparently to physiological conditions within the plant.
Increased activity, as exists during periods of shoot growth, is
thought to render the host more susceptible and to lead to exten-
sion of the decay.
Invasion of affected trees by rapid-acting wood-rot fungi may
accelerate decline and be responsible for rotting and breaking
of trees at or below the ground line, sometimes before all the
trunk has been girdled.
Another factor governing infection is varietal susceptibility
of the host. The order of susceptibility among commercial
varieties is as already given above.
Control.-Foot rot may be controlled by 1 of 2 means: by
preventing its occurrence or, once it has become established, by
curing it. Prevention is cheaper and much more satisfactory,
and can be secured easily by adhering to the following simple
1. Trees intended for heavy, wet soils should be budded on
foot-rot-resistant rootstocks (e. g., sour orange, Cleopatra man-
darin). It should be mentioned, however, that sour orange, as
a rootstock, is susceptible to tristeza, a virus disease known to
,occur in Florida and to be a potential hazard. The tristeza-
tolerant Cleopatra mandarin is being used as a rootstock by
growers who do not wish to risk the possibility of future spread
of tristeza in the field. Trifoliate orange is another rootstock
highly resistant to foot rot, but because of its susceptibility to
exocortis, it is not being recommended at present.
2. Trees should be budded sufficiently high to prevent sus-
ceptible trunks from being splashed by spore-laden soil water.
Trees should not be planted so deep as to bring susceptible scion
tissues close to the soil surface. The first main lateral roots
should be as near to the surface of the ground as possible after
soil has settled.
3. Planting sites that tend to become water-logged during
wet weather should be drained. Adequate ventilation is assured
if trees are properly spaced.
4. Trees should be maintained with sufficient vigor so that
canopies are developed that will suppress the growth of weeds

FOOT ROT (ctd.)

Citrus Diseases in Florida 63

around bases of trees. The skirts of trees should be lifted to
permit movement of air around trunks. Besides checking the
Development of foot rot, this operation will also reduce the
possibility of brown rot, a disease of fruits caused by the same
fungi responsible for foot rot.
5. In areas where foot rot is a recurrent problem, trunks
should be given an application of bordeaux mixture (10-10-100).
When combined with clean cultivation and the exposure of crown
roots early in the spring, such an application is adequate to pro-
tect trees through the susceptible period.
6. Certain cultural practices are of especial importance in
the prevention of foot rot. Irrigation should be applied in a man-
ner to avoid puddling. Injuries to the bark should be prevented,
particularly during wet weather. Rank growths of weeds should
be removed, and brush or prunings should not be stacked around
If, despite these precautions or through failure to carry
them out, foot rot should become established, it may be cured
by treatment as follows. Remove soil from around the trunks
and main roots of affected trees, making a basin outward about
3 feet in all directions. Where many trees are to be treated, the
task of removing soil without injuring roots can best be done
by hosing, using one of the leads from a power sprayer. Areas
of diseased bark should be removed with a sharp knife, chisel,
or gouge. Dead roots should be cut flush with the crown roots.
All shavings and severed roots should be burned. Cuts and in-
juries should be treated with a good, safe disinfectant, like ave-
narius carbolineum. Following the application of a disinfect-
ant, all exposed woody surfaces should be painted with a good
waterproof wound dressing to prevent entrance of wood-boring
insects and decay-producing organisms. It is advisable to leave
treated crown roots exposed for several weeks or months, or
even indefinitely, provided there is no danger from freezing.
Aeration provided in this way will promote healing of the bark
and hasten killing of the causal fungi. Frequently, the mere
removal of soil adjacent to the trunk, leaving crown roots ex-
posed, will check further development of foot rot. The exposure
method, however, is not applicable to heavy clay soils, since ac-
cumulation of water in the basins may be fatal to the tree.
When lesions occur at or above the bud union, the diseased
bark should be removed, together with half an inch of healthy
bark from the sides of the canker limits and an inch of the non-

FOOT ROT (ctd.)

64 Florida Agricultural Experiment Stations

invaded bark at top and bottom. Exposed areas should be treated
with disinfectant followed by application of wound dressing, as
previously suggested. If affected areas involve more than a
third of the circumference of a tree, treatment is not considered
advisable; under average grove conditions it is more economical
to pull and replace such trees. Trees that show extensive deteri-
oration of the tops also should be removed.
Affected trees to which sentimental value attaches may be
saved by in-arching, i.e., by the grafting into the trunk above
lesions of 3 or 4 seedlings that have been planted close to the
tree. In time the affected tree will come to rely on the root sys-
tems of these seedlings for its nutrition. Another means of
saving prized trees is to bank soil around the cleaned and disin-
fected foot-rot lesions to a height of 10 or 12 inches above the
cuts, and to keep the mounds well watered. This encourages the
formation of adventitious roots that will eventually replace roots
that have become diseased (Fig. 27).
Growers should inspect all trees in groves twice a year to
locate any foot-rot lesions in order that remedial measures may

Fig. 27.-Self- or scion-rooting of a tree affected by root rot. Soil has
been removed to show extent of root regrowth from scion portion of trunk
above bud union. Scion rooting is one way of saving trees affected by
root rot.


FOOT ROT (ctd.)

Citrus Diseases in Florida

be taken before trees are seriously affected. Foot-rot lesions on
the trunk can be seen long before a decline of the top becomes
See Greasy Spot.

Fig. 28.-Greasy spot on upper and lower surfaces of citrus leaves. Note
greater development of lesions on under surface (lower leaf) and peculiar
restriction of spots to a portion of the leaf blade-two characteristics of
greasy spot frequently observed.

GREASY SPOT (Black Melanose, Greasy Melanose)
Greasy spot is a spotting of leaves resembling to some extent
melanose. Symptoms consist at first of yellowish brown spots
on one side of leaf blades. With time, an affected area becomes
visible on both sides. On undersurfaces, spots usually have a
slightly blistered appearance that in color ultimately turn an
oily chestnut-brown. Spots vary from small dots to areas 1/4
inch in diameter, and masses of them may cover several square
inches (Fig. 28). While some spots approximate the small size


66 Florida Agricultural Experiment Stations

of true melanose pustules, it is unlikely, because of the much
larger average size of greasy-spot lesions, that the two diseases
will be confused. Greasy spot appears to develop after leaf tis-
sues have matured, and in this respect also differs from mela-
nose. Still another point of difference is in the lack of a sand-
papery sensation felt when greasy spot lesions are rubbed with
a finger.
Severe attacks of greasy spot cause a premature dropping of
leaves. Instances are known in which 80 percent of the foliage
was lost because of this disease.
The cause of greasy spot is not known with certainty. There
is evidence to suggest that the trouble results from infesta-
tions of rust mites, though some newer findings seem to impli-
cate a fungus belonging to the genus Mycosphaerella. In recent
experiments it was found (a) that greasy spot did not develop
on potted plants, despite the fact that they were heavily infested
with rust mites, and (b) that greasy spot would form at times
on leaves that were kept free of rust mites.
Grove trials indicate that good control of greasy spot is pos-
sible with a neutral copper (0.4 to 0.5 lb. of metallic copper in
100 gals. of water) or an oil emulsion spray applied between July
1 and August 7. On oranges, a July oil or oil-parathion combi-
nation is suggested because summer applications of copper ac-
centuate formation of cork in surface wounds (see Star Mel,
anose) that in turn may lead to a lowering of grade in the pack-
inghouse. If, however, the crop is to be processed so that ex-
ternal appearances are of no importance, control measures might
be limited to a single copper-wettable sulfur spray applied in July.
Promising results have been obtained recently with zineb, a
non-copper-containing fungicide, at the rate of 2 pounds per 100
gallons water. A single application during July appears to give
good control.
For further developments in the field of greasy-spot control
see the latest Spray and Dust Schedule.

The word gummosiss" as used in Florida is a better descrip-
tion of a condition than it is the name of any particular disease.
As used in the former sense, the word is appropriate, for the
suffix "osis" means "full of". As the name of a disease, -how-
ever, it is poor because many different diseases and injuries can
cause gumming. Originally, as used by Fawcett in 1907, gum-


Citrus Diseases in Florida 67

mosis was applied to a certain disease (probably that which today
is called Rio Grande gummosis) but in the course of years it has
become so corrupted as to lose its relevancy. In the sense now
used by Florida growers, the term gummosiss" most often refers
to brown rot gummosis (Phytophthora foot rot).
Gum formation may be normal and harmless. Gumming, in
fact, is generally beneficial in that it floods tissues ahead of cer-
tain invading parasites, thus slowing down their progress.
Some diseases that produce gumming are Phytophthora foot
rot, Diplodia collar rot, psorosis, exocortis, Rio Grande gummosis,
and anthracnose. Other factors that cause gumming are over-
fertilization with nitrogen, injuries resulting from feeding of
rodents and insects, and strong concentrations of spray chemi-
cals and fertilizers when brought in contact with bark of roots
and trunks. Exanthema, a deficiency disease resulting from in-
adequate amounts of copper, also will lead to flows of gum. The
causes of still other types of gumming are not known.
Because the word gummosiss" as used in Florida does not
denote a specific disease, it is suggested that the reader refer to
other sections of this bulletin where above-mentioned diseases
are described in detail.
Certain basic precautions are common to the control of most
gumming diseases. These are:
1. Avoid placing manure and other organic matter against
the base of trees.
2. Spray and dust only when weather conditions are favor-
3. Keep excess moisture from covering trunks and crowns
for long periods.
4. Disinfect and cover with waterproof dressing ail bark in-
juries and pruning wounds.
5. Make periodic inspections of trees to locate and treat
lesions before extensive damage occurs.

HEART ROT (Wood Rot, Concentric Canker)
The woody tissues of a tree are the natural breeding ground
for many rot-producing fungi. Normally such fungi are pre-
vented from gaining access to the wood by the layer of bark
that covers a tree. However, if this layer becomes broken, then
decay-producing organisms (which are ever present in the air

HEART ROT (ctd.)

68 Florida Agricultural Experiment Stations

and on surfaces of a tree) are quick to become established and
to initiate infection. It has been estimated that a single conk
of the shelf-fungus, Fomes applanatus (Fr.) Gill., liberates 30
million spores daily.
Breaks in the bark result in many ways. Pruning is prob-
ably the most frequent manner in which wood becomes exposed.
Other common entryways are made by cultivation injuries, wind-
splitting at the crotches, parasitism of bark-rotting organisms,
feeding of rodents, and sun scalding.
Trees of any age may become affected, but more often the
disease is to be found in older trees, which have had a longer
history of pruning and injuring. Heart rot is found especially
in trees that have been severely pruned, headed back, top-worked,
frozen back, sunburned, attacked by foot rot, or otherwise in-
jured or diseased.
The symptoms of heart rot are difficult to detect early in the
course of the disease. One of the first signs to appear is the
presence of numerous wood-eating ants around the trunks of
affected trees. In more advanced stages, the rot within the
tree becomes evident on the outside by a killing and settling of
the bark covering affected areas (Fig. 29-A). Often the pattern
of dead bark is circular with sunken bark arranged in concen-
tric bands. Heart rot is best revealed, however, by windstorms
and hurricanes; strain-weakened limbs break more easily. Much
of what is commonly called hurricane damage is actually the
culmination of many years of infection by wood-rotting fungi.
Coupled with structural unsoundness of affected trees is a
gradual decline in vigor. Heart rot is one of the diseases of
citrus that may lead to dieback in the tops.
The progress of heart rot can be observed if large, incom-
pletely-healed pruning stubs are split and the path of rotting is
traced. In its first stage, heart rot causes a discoloration of
the wood but affected tissues remain firm. Later, secondary
organisms invade these areas, imparting either a hard, firm
type of decay or a soft, punky type. Boring insects, ants, and
termites may accelerate the destruction.
Once wood-rotting fungi have gained access to woody tissues,
they grow throughout the heartwood of the tree (Fig. 29-B).
Spread is usually slow; in large trees the disease may be present
for many years before serious effects are noticed. Affected trees
do not necessarily die (in fact many live to be as old as the
healthiest trees in the grove) but their productivity is impaired.

HEART ROT (ctd.)

Citrus Diseases in Florida 69

r ig. LZ.-Heart rot of citrus trees. (A) External view, showing the
concentric sinking of bark over wood that has been rotted by Fomes ap-
planatus. Infection took place through pruning stub at point of knife.
(B) Internal view, showing involvement and destruction of the heart wood.
(C) Fruiting structure of Ganoderma sessilis, another of the fungi capable
of causing heart rot. (Photographs A and B through courtesy of Dr.
J. F. L. Childs.)

HEART ROT (ctd.)

70 Florida Agricultural Experiment Stations

The fungus chiefly responsible for heart rot in Florida is
Fomes applanatus (Fr.) Gill. Another fungus suspected of
being able to invade healthy wood is Ganoderma sessilis Murrill
(Fig. 29-C). Other fungi also are present in rotted wood, but
these are secondary invaders that live on dead tissues. Some of
these that have been isolated from cases of heart rot in Florida
are Daldinia concentrica (Bolt. ex Fr.) Ces. & De Not., Xylaria
polymorpha (Pers. ex Fr.) Grev., and Ganoderma spp.
If the breaks, cuts, or injuries in the bark are small, the
natural defenses of the tree are usually adequate to check in-
vading fungi. If, on the other hand, cuts or wounds are larger
than approximately 1 inch in diameter, they should be treated
with wound dressing. Wounds that have been neglected and
have become the centers of rotted wood may be gouged to re-
move discolored tissues; the cleaned area should then be painted
with wound dressing. Performing surgery in treating large
areas of rotted wood is of questionable value; trees affected
severely are more advantageously replaced than treated. Filling
of cavities with concrete does not pay; it is better to paint such
cavities after cleaning and to leave them exposed for continual
Much can be done to prevent heart rot by pruning according
to certain rules:
1. Cuts should be made flush with branches or trunks, thus
avoiding stubs which are slow to heal.
2. If dead branches are pruned, the cuts should be made
through live wood to promote rapid healing.
3. To prevent tearing of bark, branches should be notched
from below before they are cut through from above.
4. All wounds larger than 1 inch in diameter should be
promptly painted with an antiseptic pruning paint, and this pro-
tective coating should be maintained periodically until healing
is complete.
Pruning away the lower branches of trees insures ventila-
tion around trunks and reduces the incidence of bark rot, one
of the troubles that may eventually lead to heart rot. Removal
of dead branches will stop the progress of heart rot if pruning
cuts are made well within live tissues, and the cut ends are
treated with wound dressing.

See Rio Grande Gummosis.


Citrus Diseases in Florida 71

Juvenile spot is an abnormality peculiar to grapefruit trees.
Rarely it may be found also in orange and tangerine trees.
Juvenile spot is so named because it has been found only on
trees that are less than 5 years old. The trouble is of little im-
portance, despite the rather alarming appearance presented by
affected foliage. At worst, juvenile spot produces a scare; spots
are often mistaken for those of bacterial canker.
Either surface of a leaf may
be spotted, depending on the side
that is uppermost. Thus, if a
leaf is abnormally twisted so
that the lower surface happens
to be skyward, it is that side on
which spotting appears. Spots
consist of circular chocolate-
brown platelets ranging in diam-
eter from 1,32 to 3/8 inch, with
centers composed of a dry gum-
my substance sometimes con-
centrically r i n g e d (Fig. 30).
Surrounding the brown area is
a narrow yellow halo. In the
middle of each young spot is a
heaped-up knobby mass that dis-
appears with age. Older lesions
are more or less sunken at the
center as if punctured by insects
or produced by the collapse of
stomatal cells.
The cause of juvenile spot is
unknown. It has not been found Fig. 30.-Juvenile spot on grape-
fruit leaf.
possible to reproduce this spot-
ting by dusting leaves with various fertilizers, including mix-
tures used in groves showing juvenile spot. Lesions have been
examined repeatedly for presence of bacteria and fungi, but
none has been found. Neither have inoculations with diseased
tissues reproduced the disease. The pattern of distribution of
juvenile spot in groves suggests that the trouble is not infectious.
Occasionally, severe spotting will cause leaf drop but, in
general, juvenile spot results in no lasting disturbance and for
that reason measures for control are considered unnecessary.


72 Florida Agricultural Experiment Stations

LEPROSIS (Florida Scaly Bark, Nailhead Rust)
Leprosis affects fruits, leaves, twigs, and branches. When
lesions occur on fruit, the disease has also been called nailhead
rust, and when on limbs, Florida scaly bark.
Leprosis at one time commanded the attention of the entire
Florida citrus industry. Between 1900 and 1920 the disease
assumed such importance that quarantine measures were en-
acted to curb its spread. Under terms of Public Notice dated
March 12, 1917, the State Plant Board prohibited movement of
all citrus trees or parts thereof from 120 towns in 9 of the
leading citrus-producing counties. Despite such energetic meas-
ures, the disease continued to spread until by 1926 it was known
to be present in 17 counties.
Rather suddenly during the late 1920s, leprosis ceased to be
a problem, and in the years that followed, it was seen less and
less frequently. Today, the disease is practically extinct, and
can be found only in a few neglected groves on the East Coast.
Here, however, leprosis produces the same damage to fruit,
leaves, and twigs that occurred during its epidemic phase in
the Twenties, and demonstrates that, though nearly extinct,
leprosis has lost none of its destructive capabilities. Reasons for
the sudden disappearance of leprosis and for regarding the dis-
ease as still a threat to citrus production are presented in the
discussion under cause of leprosis.
Leprosis is primarily a disease of sweet orange trees, though
a few lesions may rarely be found on other varieties, including
grapefruit, mandarin, sour orange, and rough lemon. Seedling
sweet orange trees appear to suffer more from attacks of leprosis
than do budded sweet orange trees.
The geographical distribution of leprosis includes, besides
Florida, citrus-growing areas in South America, Mexico, the
Philippines, Java, Japan, China, India, Egypt, and South Africa.
In Argentina, Brazil, and Paraguay, thousands of acres of citrus
have been destroyed, and the yearly toll continues.
Symptoms.-The principal symptoms of leprosis are the char-
acteristic lesions that occur on fruits, leaves, shoots, and bark.
On fruits, spotting is so conspicuous that the disease can be
diagnosed while driving past affected groves. The chestnut-
brown spots contrast vividly with the yellow color of maturing
fruit, and this striking affect has aptly been termed nailhead
rust (Fig. 31-A). Spots range in size from pinpoint specks to
platelets 1/a inch in diameter. When first detectable several


Citrus Diseases in Florida

Fig. 31-Leprosis or nailhead rust on (A) fruit, (B) leaf, and (C) twigs.
Fig. 31.-Leprosis or nailhead rust on (A) fruit, (B) leaf, and (C) twigs.



74 Florida Agricultural Experiment Stations

months after fruit has set, lesions are yellow-colored; as they
enlarge they become brown-colored and are outlined with narrow
yellow halos. Centers of spots are usually sunken and at times
cracked. Penetration of lesions is no deeper than into the
albedo. Badly spotted fruits drop prematurely, after which
the tree may appear to have escaped attack completely. Spot-
ting causes sufficient damage to disqualify affected fruits from
Leaf spots due to leprosis are similar in appearance (Fig.
31-B) to those produced on fruits. Under Florida conditions,
lesions occur predominantly along the leaf margins and are
visible on both sides of the leaf. From 1 to 8 or more lesions
may develop per leaf, often resulting in premature leaf drop.
On immature shoots, lesions are first visible several months
following emergence of shoots from buds, and the appearance
of spots (Fig. 31-C) is similar to those on fruits and leaves. On
mature shoots, however, spots have a faint target-board appear-
ance, turn yellow-brown to red-brown in color, and coalesce to
form areas of scaly bark. Affected areas grow apace with di-
ameter of the wood on which they occur, and give rise eventually
to large circular persistent cankers with excrescent resinous
scales that are much thicker than healthy bark (Fig. 32). If
left undisturbed, scales pile up to form cone-shaped coverings
over cankers. Affected areas continue to expand and eventually
girdle branches and trunks. Girdling, however, does not en-
tirely interfere with passage of food and water across affected
areas because conductive tissues are not completely destroyed.
Old leprosis cankers may be found often today on trunks and
in limbs high in trees that were infected years ago. These per-
sistent lesions are of no importance in the spread of leprosis
either in affected trees or to adjacent ones. It is these areas
of excrescent, sloughing bark that have suggested the name
scaly bark for the stage of leprosis affecting woody tissues.
Bark scaling is the most characteristic symptom of the disease,
because wood lesions accumulate and persist long after affected
leaves and fruits have disappeared.
Old leprosis lesions often resemble those due to psorosis. The
two are readily distinguishable, however, on the basis of scale
characteristics. Leprosis scales are resinous, reddish brown, and
much thicker than normal bark. Psorosis scales, contrariwise,
consist of bark itself and are, therefore, much thinner than the
resinous eruptions of leprosis. Difficulty arises, however, in
separating old leprosis scaly bark from psorosis scaly bark when-


Citrus Diseases in Florida

ever scales of either are no longer present. In such cases, the
diagnosis is leprosis if old bark lesions are found in the branches
of trees at a certain uniform level throughout the grove and if
affected limbs show a certain swelling or increase in diameter
at the site of shelling. Psorosis is indicated, on the other hand,
if lesions occur at heights that vary from tree to tree, and if
the diameter of limbs is unchanged wherever shelling occurs.

Fig. 32.-Old leprosis lesion on the trunk of a sweet orange tree. Note
thickness of scales, which distinguishes leprosis scaling from that caused
by psorosis. Whitish incrustations are lichens.


76 Florida Agricultural Experiment Stations

Cause.-For many years it was thought that leprosis was
the work of a fungus, namely Cladosporium herbarum var. citri-
colum Farl. Quite suddenly in the late 1920s, without any spe-
cial measures having been taken, leprosis began to disappear.
When recent evidence showed leprosis to be associated with false
spider mites of the family Phytoptipalpidae, it became obvious
that innovations in grove care, including rust mite control initi-
ated during the Twenties, must have been responsible for the co-
incidental, if unwitting, control of leprosis.
In Argentina, the mite found in association with leprosis has
been identified as Tenuipalpus pseudocuneatus Blanchard; the
species in Florida has been tentatively determined to be Brevi-
palpus australis (Tucker). That these species of false spider
mites are involved in leprosis has been proved both by caging
mites on healthy sweet orange seedlings with subsequent de-
velopment of symptoms and by controlling leprosis in groves by
means of miticidal sprays. The association, however, between
false spider mites and leprosis does not necessarily mean that
mites are the primary cause of this disease and still leaves un-
settled the question as to whether false spider mites produce
damage by some toxic property of their saliva or whether they
act as vectors of a virus or some other agent. Budwood taken
from leprosis-affected trees will not disseminate the disease.
This evidence strongly suggests that the cause of leprosis is
not a virus.
Control.-Leprosis causes two types of damage: spotting of
fruits and weakening of bearing wood. Consequently, two dif-
ferent approaches to the control of this disease are necessary.
Current infections, which are responsible for fruit spotting, may
be prevented by the application of a late dormant spray of
wettable sulfur, 10 pounds in 100 gallons of water, or alterna-
tively, DN Dry Mix, 2/3 pound in 100 gallons. These materials
will also prevent the initiation of twig cankers, but will not elim-
inate cankers already present. Removal of existent cankers
requires pruning, and this is recommended in order to stimulate
the growth of new wood to take the place of limbs weakened by
eruptions of the bark.
Recent findings showing leprosis to be associated with mites
and not with fungi account for the fact that applications of bor-
deaux mixture, as previously recommended, have never been
particularly effective in controlling leprosis.


Citrus Diseases in Florida 77

Lichens frequently occur on 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 for mutual benefit.
Lichens appear in a variety of forms. Some are scaly in-
crustations on bark or leaves (Fig. 32). Others are somewhat
leaf-like with upturned margins. Still others are much-branched
stringy structures that hang from twigs and branches. Their
colors are even more varied, ranging from the blood-red color of
Chiodecton sanguineum (Swartz) Vainio to the grays of Physcia
spp. As a rule, lichens live superficially and obtain food ma-
terials from the air and from dead bark on which they grow.
Lichens are abundant in damp, shady locations and are fre-
quently conspicuous on neglected trees. Since lichens are sus-
ceptible to either copper or oil sprays, their presence in a grove
is good evidence that trees have not been sprayed recently with
these materials.
Lichens are considered harmless as far as direct injury pro-
duced by their growth is concerned, but they detract from the
well-kept appearance of trees. Since lichens do not penetrate
bark deeply nor establish parasitic relationships with the tree,
it is supposed that any damage they cause is limited to mechani-
cal interference with respiration and transpiration processes
through lenticels or breathing pores of the bark.
Lichens can be eradicated easily by spraying affected parts
with bordeaux mixture (5-5-100) or oil emulsion (1 3/5 percent).
Best results are obtained by spraying when lichens are dry.


In addition to the disease mentioned under Anthracnose,
'there is a more serious one caused by a related fungus that
affects Key lime. Lime anthracnose attacks young tissues rather
than, as in the case of anthracnose, those that are mature, sen-
escent, or injured. While anthracnose of orange and grapefruit
is a secondary trouble, that of lime is definitely primary and
Lime anthracnose occurs principally in southern Florida and
on the Keys, where at times it causes serious damage to buds,
blossoms, young fruits, and shoots of the lime variously known


Citrus Diseases in Florida 77

Lichens frequently occur on 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 for mutual benefit.
Lichens appear in a variety of forms. Some are scaly in-
crustations on bark or leaves (Fig. 32). Others are somewhat
leaf-like with upturned margins. Still others are much-branched
stringy structures that hang from twigs and branches. Their
colors are even more varied, ranging from the blood-red color of
Chiodecton sanguineum (Swartz) Vainio to the grays of Physcia
spp. As a rule, lichens live superficially and obtain food ma-
terials from the air and from dead bark on which they grow.
Lichens are abundant in damp, shady locations and are fre-
quently conspicuous on neglected trees. Since lichens are sus-
ceptible to either copper or oil sprays, their presence in a grove
is good evidence that trees have not been sprayed recently with
these materials.
Lichens are considered harmless as far as direct injury pro-
duced by their growth is concerned, but they detract from the
well-kept appearance of trees. Since lichens do not penetrate
bark deeply nor establish parasitic relationships with the tree,
it is supposed that any damage they cause is limited to mechani-
cal interference with respiration and transpiration processes
through lenticels or breathing pores of the bark.
Lichens can be eradicated easily by spraying affected parts
with bordeaux mixture (5-5-100) or oil emulsion (1 3/5 percent).
Best results are obtained by spraying when lichens are dry.


In addition to the disease mentioned under Anthracnose,
'there is a more serious one caused by a related fungus that
affects Key lime. Lime anthracnose attacks young tissues rather
than, as in the case of anthracnose, those that are mature, sen-
escent, or injured. While anthracnose of orange and grapefruit
is a secondary trouble, that of lime is definitely primary and
Lime anthracnose occurs principally in southern Florida and
on the Keys, where at times it causes serious damage to buds,
blossoms, young fruits, and shoots of the lime variously known


78 Florida Agricultural Experiment Stations

as the Key, West Indian, or Mexican. The only other lime or
citrus variety known to be attacked is the Dominican Thornless.
Lime anthracnose is known also in other parts of the world.
It is of particular importance in the West Indies; in Trinidad it
is a limiting factor in the lime industry.
Symptoms.-Lime anthracnose may affect fruits, leaves,
blossoms, buds, and young shoots. When shoots become in-
fected, wilting occurs that may extend downward from the tips
for several inches. At times affected shoots may be ringed or
girdled, as if by insects, and hang lifeless or drop away.
Young leaves develop dead areas along margins or at tips,
and as a result become distorted after further growth. Un-
opened buds may be blasted and fall before opening, or once
open, petals may turn brown and stigmatic surfaces of pistils
become attacked, leading to a drop of fruit recently set.
On Key lime fruits, the appearance of anthracnose lesions is
such as often to cause alarm, especially if compared with written
descriptions of citrus canker. The two diseases are strikingly
similar in appearance. They differ, however, in that anthracnose

Fig. 33.-Lime anthracnose on young fruits of the Key lime, showing
prominent corky spots and cracked, misshapen fruits.


Citrus Diseases in Florida 79

lesions are sunken and produce a distortion and splitting of the
fruit (Fig. 33); also in that the thick, corky excrescences are
confined to the fruit. In contrast, canker lesions cause no split-
ting or distortion of fruit, and they are to be found on leaves
and twigs as well as on fruit.
Cause.-The fungus responsible for lime anthracnose is
Gloeosporium limetticolum Clausen, as contrasted with Colleto-
trichum gloeosporioides Penz., the cause of anthracnose on
orange, grapefruit, and other varieties. The Colletotrichum fun-
gus is found on many varieties of citrus, including Key lime,
but the one causing lime anthracnose is restricted to Key lime.
The two fungi differ, too, in that the Colletotrichum is a weak
parasite or a saprophyte, whereas the Gloeosporium is a definite
parasite and does not depend on lowered vitality or wounds for
Climatic factors provide the chief contributing conditions for
outbreaks of lime anthracnose, fungus development being fa-
vored by humid weather. As in scab, melanose, and leprosis,
only young tissues are susceptible, mature ones being immune.
The fungus probably lives over from year to year on dead twigs,
and spores reach susceptible tissues in rain or dripping dew. Suf-
ficient moisture is prerequisite for germination of spores and
Control.-Lime anthracnose can be controlled with applica-
tions of copper fungicide, commonly with bordeaux mixture
6-6-100 or its equivalent in neutral copper compounds. Certain
difficulties arise, however, in spraying Key lime trees. In this
variety new foliage and blossoms are produced over an extended
period. It is necessary, therefore, to apply a number of sprays
if all susceptible tissues are to be covered and the disease con-
trolled. In general, 2 or 3 applications at intervals of several
weeks during the flushing period are required. The addition of
oil emulsion, 1/ to 1 percent, will facilitate the sticking of bor-
deaux mixture or neutral copper compounds to young foliage.
Since high humidity is an important factor in the develop-
ment of lime withertip, considerable control can be obtained by
locating new groves in relatively dry areas. The cutting down
of high cover crops around trees will promote better air drain-
age and reduce high humidities favoring infection.


See Lime Anthracnose.


80 Florida Agricultural Experiment Stations

Melanose occurs throughout the citrus-growing sections of
Florida and the Gulf states and has been reported from most
other citrus areas of the world. Melanose was first observed at
Citra, Florida, about 1892, and was described and named by
Swingle and Webber in 1896. With growth of the citrus in-
dustry, melanose has become widespread and prevalent. It is
today the most important
cause for lowering the grade
of fruit. In fact, melanose,
together with the associated
Phomopsis type of stem-end
rot, impairs the fresh-fruit
market value of more citrus
than all other parasitic dis-
eases combined.
All commercial varieties
of citrus grown in Florida
are attacked by melanose
but grapefruit is somewhat
more susceptible than
orange. Melanose is rarely
important in young groves
S but ordinarily increases pro-
Low- gressively with advancing
-i age of trees. In general,
l- those conditions that lead to
accumulation of decaying
wood (see under Cause) may
be considered predisposing
factors for the development
of melanose.
Symptoms. Melanose
affects the foliage, twigs,
and fruits of citrus trees, at-
tacking these parts only
Fig. 34.-An early stage of mel- while they are immature.
anose on grapefruit leaf, showing yel- n irst v
low-bordered spots. On Leaves.-First visual
symptom of melanose on
leaves is the appearance of minute, round pinpricks that with
time become larger, water-soaked, sunken, and dark-centered
with yellowish halos (Fig. 34). As leaves age, these spots be-


Citrus Diseases in Florida

come raised, giving affected areas a sandpapery feel. At this
stage, spots vary in size from mere dots to 1/32 inch in diameter
(Fig. 35) and assume a color ranging from amber to dark brown.

~diS~gf ~
; ip& 'YC~~.. Y aj

r r-*

i ~~U le.
't* : d 7
? .

Fig. 35.-Portion of a grapefruit leaf showing character of mature
melanose spots. (Twice natural size.)

With age, halos are lost. Melanose lesions occur on either sur-
face of the leaf and may be grouped in different ways, ranging
from isolated pustules to aggregated masses forming scar tissues
of various shapes. In cases of severe attack, leaves may become
twisted and otherwise distorted (Fig. 36), lose their green color.
and drop prematurely from the tree.


Florida Agricultural Experiment Stations

Fig. 36.-Distortion of foliage on young grapefruit shoot as a result of
severe melanose infection.


Citrus Diseases in Florida 83

On Twigs.-Melanose on twigs (Fig. 37) appears at first very
much as it does on leaves, but the minute corky outgrowths ulti-
mately become more raised. If
infection is severe, twigs may be A
completely covered by scar tis-
sue and die.
On Fruit.-Melanose lesions
on fruit resemble those on
leaves. At first, spots are circu-
lar, light brown, and sunken;
later, they become distinctly
raised and reddish-brown to al-
most black (Fig. 38). The sand-
papery feel produced by the in-
dividual raised spots serves to
distinguish melanose from rust-
mite injury and from surface
stains caused by anthracnose.
Melanose spots may be arranged
in various patterns. "Tear-stain
melanose" (Fig. 39) results
from infections initiated by the
washing of spores over fruits by
drops of rain or dew. In other
cases, aggregations of spots may
form crusts of scar tissue, pro-
ducing "shark-skin melanose."
"Mudcake melanose" develops
when areas of scar tissue crack
into more or less irregular pat-
terns (Fig. 40). Fruits heavily
affected are usually smaller and
weaker, and, while still young,
are the first to drop if the tree
encounters adverse conditions.
Cause.-Melanose is caused Fig. 37.-Melanose lesions on
by the fungus Diaporthe citri twigs of grapefruit. Note along
sides of twigs the extent to which
(Fawc.) Wolf, the same organ- pustules are raised. It is this
ism that produces Phomopsis marked elevation that gives mel-
anose-affected tissues a sandpa-
rot, one of the stem-end decays pery feel.
of citrus fruits. Two types of
spores are formed by this fungus: sexual spores (produced in
minute, flask-shaped structures called perithecia, and borne on


Florida Agricultural Experiment Stations

..ti _..

4 4

W. ;*VIC
t A

,rr' *' se

Fig. 38.-Melanose on grapefruit, showing raised, brown- to
black-colored pustules.

decaying twigs and branches on the ground), and asexual spores
(produced in pycnidial bodies similar to perithecia, and borne on
dead twigs in the tree). Spores of the asexual, or Phomopsis
stage, are the chief source of infection. These "seeds" of the
fungus are disseminated by dripping water and carried onto
newly emerging sprouts and developing fruits. Infection of
susceptible tissues is accomplished within 36 hours after spores
have been deposited, but spots do not become visible to the naked
eye until 4 to 7 days later. Spore production has not been en-
countered in mature citrus fruits in the grove, nor are spores
produced in melanose spots occurring on living parts of the tree.
It is difficult to detect or isolate the fungus in mature melanose
Since dead wood in the tree is the principal source of infec-
tion, pruning of such wood is an important measure in the con-


Citrus Diseases in Florida 85

trol of melanose. Also important are measures that can be
taken to prevent the formation of dead wood. The principal
causes for the dying of twigs in Florida, in addition to mela-
nose, are droughts, freezes, windstorms, waterlogging, root rots,
foot rot, infestations of scales and mites, starvation, and pruning
of roots due to deep cultivation. In general, any factor that
reduces the vigor of a tree and results in dead wood will lead to
an intensification of attack by melanose.

.* _

.. 3? 4

@ ,:: ,,... _

Fig. 39.-"Tear-stain" melanose, resulting from run-off that washed
spores of the causal fungus down the fruit. A similar tear-stain pattern
may be produced by rust mites, but the latter injury is smooth to the touch
and not sandpapery as is the case with tear-stain melanose.

Pycnidia may be found at any time of the year but usually
are not plentiful in winter. With the advent of spring, their
development is accelerated and, about the middle to the latter
part of April, pycnidia are found in abundance. At this stage,
many of the fruiting structures are already filled with spores
while others are only in the first stages of spore development;
thus, successive crops of spores are produced during the time
growing parts of the host are susceptible. The length of time
that twigs have been dead has an important bearing on the pro-
duction of melanose spores. Twigs that have been dead for but
a few months are most likely to harbor the organism, although


Florida Agricultural Experiment Stations

twigs may serve as a source of inoculum well into the second
year following death.

Fig. 40.-"Mudcake" melanose on grapefruit, resulting from the
cracking of thickened melanose scar tissue.
Another important factor necessary for the development of
melanose is moisture, in the form of either rain, heavy dew, or
fog. Spores of the causal fungus develop in the fruiting bodies
where they are imbedded in a gelatinous matrix that swells
when moistened. Spores are forced out of the microscopic
mouths of the pycnidia in minute, tendril-like masses, after
which rains or the drips of water from dew or fog wash them
over young leaves, twigs, and fruits. Thus, given frequent
rains, heavy dews, or fogs, fruiting bodies on dead wood of rela-
tively recent origin may continue to infect growing tissues for
as long as they remain susceptible.


Citrus Diseases in Florida 87

Citrus foliage and shoots are susceptible from the time they
emerge from the bud until they harden, which usually takes
place before leaves turn deep green in color. This means that,
ordinarily, leaves are susceptible for 2 or 3 weeks during periods
of good growing weather. Melanose infection of the foliage,
however, is not usually of sufficient economic importance in itself
to justify control measures.
Fruits are susceptible when first formed but become pro-
gressively resistant with age. The approximate size at which
they develop practical immunity from melanose depends on varie-
ties. Tangerines are immune when they reach a diameter of 1
inch, oranges, 11/2 inches, and grapefruit, 21/2 inches. With
respect to age of the tree, melanose generally increases in se-
verity with advancing age. Damage to trees of nonbearing age
,is slight. In bearing trees up to 10 years, the disease is not
likely to become serious if trees are kept in healthy condition.
In older trees, however, there is a distinct increase in the se-
verity of melanose-probably the result of the higher propor-
tion of dead twigs to living twigs.
Severe outbreaks of melanose can be traced to rainy periods
occurring in April or May after melanose spores have developed
in abundance and before tissues have outgrown the susceptible
stage. Sudden showers that come earlier in the season do not
appear nearly so favorable for infection as do cloudy, rainy
periods of 2 to 3 days' duration. Rainy spells late in March
come after most of the spring flush of growth has hardened but
before the fruit from even the earliest bloom has become resist-
ant and sometimes before much of the bloom has set fruit. A
moderate amount of fruit infection almost invariably takes place
during March rains. Ordinarily, however, the bulk of infections
occurs during the latter part of April or the first part of May.
Temperature also plays a role in the development of melanose.
Increases in temperature to 81 F. bring about an increase in the
rate of growth of the fungus, but further rise in temperature
retards growth; at about 950 F. growth ceases entirely.
Control.-As early as 1896 it was known that outbreaks of
melanose could be greatly reduced by copper sprays, but growers
found such applications to be impractical until it was shown in
1912 that by pruning dead branches, spore load could be ma-
terially reduced. Today, successful melanose control is based
on a combination of spraying and pruning.
Complete control of melanose might be achieved with numer-
ous applications of a fungicide, but this is uneconomical for


88 Florida Agricultural Experiment Stations

several reasons: (1) cost of repeated applications will be higher
than the premium received for cleaner fruit; (2) excessive ap-
plications of copper may lead to toxic concentrations of this
element in the soil; and (3) heavy residues on foliage and fruit
increase the need for control of scales and mites. The most prac-
tical program, therefore, is one that, while it may fall short of
complete disease control, will yet incur no great increase in the
expense for insect control. On the basis of experimentation and
many years of commercial use, it has been found that melanose
on fruit can be controlled by a single application of a copper-
containing spray (i.e., bordeaux mixture 3-1.2-100 or its equiva-
'lent in neutral copper at the rate of 3/4 pound metallic copper
per 100 gallons of water) applied within 1 to 3 weeks after set
of fruit. During years of higher than normal spring rainfall,
it is advisable to make applications about a week after fruit set.
If melanose has been severe in the past, or if it is desired to pro-
duce especially bright fruit, a second copper spray may be ap-
plied 3 to 4 weeks after first melanose spray. The usual insecti-
cides also must be applied to overcome build-up of scales and
mites that follows use of copper sprays. Specific recommenda-
tions for the control of melanose and related insect problems are
outlined in the Spray and Dust Schedule of the Better Fruit
Spraying with copper fungicides gives better results than
dusting. Sulfur or lime-sulfur sprays or dusts do not provide
proper melanose control.
There are several secondary benefits that may result from the
application of copper sprays. One is a reduction in the tendency
for fruit to develop ammoniation and for twigs and branches
to develop exanthema. Another benefit is that copper is also
a specific for such troubles as scab, spanish moss, lichens, and
possibly greasy spot. It is questionable, however, if, as claimed
in the past, melanose sprays will provide control of stem-end
rot in the packinghouse.

Mesophyll collapse is a condition brought about by the disor-
ganization of tissues within the leaf. Affected areas may be
scattered over the leaf blade but usually occur within confines
of the midrib and the main lateral veins. In size, collapsed areas
may involve one-third the total leaf surface, and, in shape, be
irregular and rather angular (Fig. 41). Affected areas are


Citrus Diseases in Florida

somewhat wrinkled and translucent and are lighter green than
normal portions of the leaf. Eventually, areas of mesophyll
collapse become invaded by secondary organisms (e.g., Alterna-
ria spp., Cladosporium spp., Colletotrichum spp.), whereupon
affected tissues turn dark-brown to black and die.
At times mesophyll collapse may be widespread and cause
concern. The trouble is induced principally by drought condi-
tions, making it impossible for the tree to obtain water sufficient
for all parts of the foliage. Drying winds produce similar effects,
as do also root rots and water damage that reduce the root sys-
tem of a tree. Contributing causes may be the presence of red
spiders, the occurrence of frost, and the abrasive effects of wind-
driven sand. When any of these factors are under the grower's
control, mesophyll collapse can be alleviated.

See Leprosis.
See Spreading Decline, Slow Decline.

Fig. 41.-Mesophyll collapse, showing angular, translucent areas scat-
tered over leaf surfaces. Affected areas resemble the foliar symptoms of
wood pocket in lemon and lime trees, but in mesophyll collapse the chlorotic
areas are slightly wrinkled and depressed.


90 Florida Agricultural Experiment Stations

The puncturing of oil glands on fruit, whether by insects,
mites, thorns, or other agencies, liberates oil that may cause
burning or spotting of the rind. If oil glands are punctured
while fruits are still green, affected areas remain green after
unaffected portions develop color. Liberated oil may also cause
a slight sinking of tissues between the oil glands, resulting in
glands standing out by contrast.
Weakened areas provide entryways for fungi like Colleto-
trichum, Diplodia and Alternaria, and these may extend and
accelerate breakdown of the rind.
Oleocellosis may be prevented by avoiding conditions that
lead to rupture of oil glands. Prevention is possible to some ex-
tent by not picking fruit while wet, and by guarding against
rough handling of fruit, especially in cool weather when they
are turgid. Oleocellosis may develop also on fruit held in cold
storage; proper ventilation will prevent this type of breakdown.

See Xyloporosis.

PSOROSIS (California Scaly Bark)
Of the virus diseases affecting Florida citrus, psorosis is
certainly the most destructive. First discovered in 1896, the
trouble is now known to occur in all citrus-growing areas of the
world. Varieties showing scaly bark symptoms include sweet
orange, tangerine, grapefruit, and Tahiti lime; of these the first
two suffer most. Sour orange, lemon, and calamondin, and
doubtless many other varieties of citrus, may become infected
with the virus but rarely show scaling.
Symptoms.-The name psorosis is applied to a number of
citrus diseases differing widely in appearance but having in
common a characteristic chlorosis of half-grown leaves. This
chlorosis is found typically in two distinct, but similar, patterns:
(1) as faint flecks or translucent areas between the veinlets and
paralleling them (Fig. 42-A), and (2) as yellow or translucent
areas the margins of which resemble an oak leaf (Fig. 42-C).
Both patterns are best seen by transmitted light and are gen-
erally symmetric with respect to the midrib, i.e., the chlorosis in
one longitudinal half is more or less the mirror image of that
in the other half. This symmetry is characteristic of psorosis


90 Florida Agricultural Experiment Stations

The puncturing of oil glands on fruit, whether by insects,
mites, thorns, or other agencies, liberates oil that may cause
burning or spotting of the rind. If oil glands are punctured
while fruits are still green, affected areas remain green after
unaffected portions develop color. Liberated oil may also cause
a slight sinking of tissues between the oil glands, resulting in
glands standing out by contrast.
Weakened areas provide entryways for fungi like Colleto-
trichum, Diplodia and Alternaria, and these may extend and
accelerate breakdown of the rind.
Oleocellosis may be prevented by avoiding conditions that
lead to rupture of oil glands. Prevention is possible to some ex-
tent by not picking fruit while wet, and by guarding against
rough handling of fruit, especially in cool weather when they
are turgid. Oleocellosis may develop also on fruit held in cold
storage; proper ventilation will prevent this type of breakdown.

See Xyloporosis.

PSOROSIS (California Scaly Bark)
Of the virus diseases affecting Florida citrus, psorosis is
certainly the most destructive. First discovered in 1896, the
trouble is now known to occur in all citrus-growing areas of the
world. Varieties showing scaly bark symptoms include sweet
orange, tangerine, grapefruit, and Tahiti lime; of these the first
two suffer most. Sour orange, lemon, and calamondin, and
doubtless many other varieties of citrus, may become infected
with the virus but rarely show scaling.
Symptoms.-The name psorosis is applied to a number of
citrus diseases differing widely in appearance but having in
common a characteristic chlorosis of half-grown leaves. This
chlorosis is found typically in two distinct, but similar, patterns:
(1) as faint flecks or translucent areas between the veinlets and
paralleling them (Fig. 42-A), and (2) as yellow or translucent
areas the margins of which resemble an oak leaf (Fig. 42-C).
Both patterns are best seen by transmitted light and are gen-
erally symmetric with respect to the midrib, i.e., the chlorosis in
one longitudinal half is more or less the mirror image of that
in the other half. This symmetry is characteristic of psorosis


Citrus Diseases in Florida

leaf patterns and distinguishes them from the irregular chlo-
roses produced by tristeza and certain other diseases and defi-

Fig. 42.-Leaf symptoms of psorosis. (A) F]
Normal, healthy leaf, for comparison. (C) Oakleaf
transmitted light.

I c
lecking pattern. (B)
pattern. Veiwed with

Flecking and oakleaf patterns, although diagnostic of pso-
rosis, constitute a ready means of identifying infected trees
only during a few days to several weeks in the spring and oc-
casionally in the autumn, when trees develop new flushes of
growth. Chlorotic patterns occur only in immature leaves of
such flushes. Diagnosis is complicated by the fact that some-
what similar patterns are caused by thrips, mites, wind-blown
sand, and malnutrition. Although leaf symptoms develop with
all varieties of psorosis, it is not yet known whether trees ex-
hibiting leaf patterns will eventually develop scaling or other
injurious effects.
Psorosis involves symptoms other than leaf patterns; it is
these other symptoms that characterize the various types of


92 Florida Agricultural Experiment Stations

Fig. 43.-Scaling of bark on trunk due to psorosis A. Characteristic
of this type of psorosis is the awning-like curl of sloughing bark that
remains attached at upper and lower edges of lesion.


.w ~


Citrus Diseases in Flordia 93

Psorosis A, most prevalent of the destructive forms of pso-
rosis in Florida, induces a scaling of the bark of trunks and
branches (Fig. 43). Affected areas are more or less circular,
beginning as small blisters and eventuating in large scaly patches
girdling trunks and limbs. An outer layer of dry bark, ap-
proximately 1/8 inch thick, separates from the living bark under-
neath and curls upward from the edges of the lesion to produce
awning-like effects (Figs. 43, 44). Lesions of psorosis A, unlike
those of similar appearance resulting from gummosis, do not
exude gum except to a very slight extent during active grow-
ing seasons. With progress of the disease, deeper layers of bark
and wood become infiltrated with gum and resin-like materials
that produce a brown discoloration of affected tissues and inter-
fere with movement of food and water. When this stage is
reached, deterioration of the tree is rapid (Fig. 45). The top
becomes sparse, exposing the bark to sunburn, which in turn
hastens the process of decline.

Fig. 44.-Scaling of bark on a branch caused by psorosis A.

Psorosis B, a much more destructive form of the disease than
psorosis A, occurs in Florida to only a minor extent. Because
of the fact that symptoms appear earlier in the life of infected
trees than with psorosis A, and because involvement is much
more rapid and destructive, trees with psorosis B are more read-
ily marked for replacement, thereby reducing the frequency with
which psorosis B is encountered in the grove.
Eruptive symptoms of psorosis B differ from those of the
A form in being preceded by a flow of gum. The form of scaling
is different also; instead of roundish patches, scaling with pso-
rosis B occurs in strip-wise or sheet-like areas. Bark lesions


94 Florida Agricultural Experiment Stations


Fig. 45.-Decline of an 18-year-old orange tree resulting from infection
with psorosis A virus. Scaling usually starts at the trunk, as in this
example; with time it may also develop in the limbs.


Citrus Diseases in Florida 95

spread rapidly along the vertical axis and on one side of the
trunk or branch, and extend into much smaller limbs and twigs
than does scaling due to psorosis A.
Another characteristic of psorosis B is found in leaf symptom
expression. In addition
to the usual flecking and
oakleaf patterns, leaves
ma y show translucent
circles or ring spots of
various si z e s. Similar
ring-like markings may
occur also on fruits.
Ring-spotting symptoms,
however, are of rare oc-
currence under Florida
Blind pocket psorosis
is characterized by chan-
nel-like depressions run-
ning lengthwise of af-
fected trunks (Fig. 46).
The depressions are
steep-sided and narrow,
and often occur grouped
in paralleling series, giv-
ing affected trunks the
appearance of being en-
cased in a mantle of
thick vines. Occasionally
blind p o c k e t psorosis
erupts to form scaly bark
and to cause a flow of
gum; this form is known
as eruptive blind pocket.
Neither blind pocket nor
the eruptive type occurs
with any frequency in Fig. 46. Trunk symptoms of blind-
Florida groves; neither pocket psorosis. The eruptive stage may
or may not occur.
are they associated with
serious decline. Caution, however, should be exercised to avoid
such trees when collecting budwood else these troubles, or one
of the more serious types of psorosis, may become propagated.
Blind pocket should not be confused with folds commonly seen

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