Algal disease (alga spot, red algal...
 Phytophthera foot rot

Title: Some common diseases of Tahiti lime in Florida
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00066888/00001
 Material Information
Title: Some common diseases of Tahiti lime in Florida
Series Title: Plant Pathology Fact Sheet PP-24
Physical Description: Book
Language: English
Creator: Pernezny, Ken
Marlatt, R. B.
Affiliation: University of Florida -- Florida Cooperative Extension Service -- Department of Plant Pathology -- Institute of Food and Agricultural Sciences
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 1982
Spatial Coverage: North America -- United States of America -- Florida
 Record Information
Bibliographic ID: UF00066888
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 by the source institution and holding location.

Table of Contents
        Page 1
    Algal disease (alga spot, red algal disease)
        Page 2
    Phytophthera foot rot
        Page 3
        Page 4
        Page 5
Full Text

Plant Pathology Fact Sheet

Some Common Diseases of Tahiti Lime in

Ken Pernezny, Professor of Plant Pathology, Everglades Research & Education
Center, and R. B. Marlatt, Professor of Plant Pathology, (Deceased) Tropical Re-
search & Education Center, Homestead, 1982, Revised November 1999.

Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean


Tahiti lime is a seedless citrus grown in
Florida for fresh market and processing. Most
of the commercial acreage is located in the
southeastern part of the state, especially in Dade

Several diseases are important limiting
factors in lime production, if not adequately
controlled. These include melanose, alga spot,
greasy spot, scab, and Phytophthora foot rot.
Sooty mold is a common disorder found on
limes that greatly resembles a disease problem.
The diagnostic characteristics, descriptions of
disease development, and measures for control
are included in this fact sheet. Due to frequent
changes in the availability and use restrictions
for specific agricultural chemicals, consult the
University of Florida Extension Service for spe-
cific, current fungicide recommendations.


Melanose (caused by the fungus
Diaporthe citri) can at times cause significant fi-
nancial loss, especially if there is substantial
fruit blemishing, resulting in downgrading of
unsightly fruit to juice production. Distinct
melanose symptoms occur on leaves, green
wood, and fruit. On leaves, melanose lesions
begin as small, dark brown to black sunken
spots. As the leaf tissue ages, the spots may be-

come raised (Fig. 1).

Symptoms on young, green twigs are
quite similar to those on leaves except that they
tend to be distinctly raised.

On fruit, lesions begin as light brown,
circular spots that later become brown to black
raised pimples (Fig. 2), imparting a sandpaper-
like feel to the fruit. If infections become nu-
merous, symptoms appear as large areas of
dark, rough scar tissue on the rind.

Spores of the melanose fungus are pro-
duced in fruiting structures (pycnidia) that de-
velop only in dead wood. The spores are em-
bedded in a sticky matrix and, for all practical
purposes, are released only by splashing rain-
fall, after which they can infect young leaves,
green twigs, and fruit. Subsequent lesions on
living tissue do not serve as inoculum sources,
since no fruiting structures are formed in them.

Since dead wood is the primary source
of melanose spores, removal of dead wood is a
significant control factor. It is important to note
that serious outbreaks of melanose usually oc-
cur after freezes that result in a lot of dead wood
in groves.

Copper fungicides are important in controlling
melanose, especially those applied during the
rainy season.


Algal Disease
(Alga Spot, Red Algal Disease)

This disease is caused by a parasitic alga,
Cephaleuros. This organism has a wide host
range among tropical and subtropical trees, but
is a particularly serious problem on Tahiti lime.
Lesions on leaves are roughly circular, raised,
and greenish-gray in color. The alga will even-
tually produce rust-colored microscopic
"spores" on the surface of the leaf spots, giving
the spots a reddish appearance (Fig. 3).

The alga may also attack branches, and
bark splitting may occur. If wood infections
become severe, girdling and death of branches
may result. The appearance of the masses of
red "spores" on the wood (Fig. 4) is highly di-

Algal disease became much more preva-
lent in Florida with the advent of organic fun-
gicides (such as benomyl and the dithiocarbam-
ates). When copper sprays are included in the
disease control program, it is usually well con-
trolled. Again, copper sprays in the rainy sea-
son, especially in June, are most effective.

Greasy Spot

Greasy spot, caused by the fungus
Mycosphaerella citri, is a major disease of Tahiti
lime. Severe defoliation and subsequent long-
term loss of tree vigor can result if it is not ad-
equately controlled.

Infection of new leaves occurs from in-
vasion by spores released in diseased and de-
composing lime leaves on the grove floor. Since
the decomposing leaves require alternate peri-
ods of wetting and drying for spore release,
most infection probably occurs in the summer.

Fungal invasion occurs through
stomates on the underside of the leaves. The
symptoms begin as small, localized water-

logged areas on the underside of leaves that
subsequently blister. These undersurface areas
then begin to turn orange to light brown (Fig.
5) with yellowing of the upper leaf surface. With
time, infections turn dark brown to black, with
a definite greasy look (Fig. 6). Infected leaves
very often drop prematurely. Fruit infection can
occur in Tahiti lime, but does not seem to be
that common.

Effective control of greasy spot can be
obtained with sprays of copper, benomyl, (re-
sistant strains to benomyl exist) or benomyl
plus oil. When greasy spot levels are low, oil
alone maybe sufficient. Homeowners should
benefit from a sanitation program aimed at
cleaning up and removing fallen, infected
leaves from around the base of their lime trees.
See Plant Pathology Fact Sheet No. 9 for a more
detailed description of citrus greasy spot.


Scab is caused by the fungus Elsinoe
fawcetti. Tahiti lime is not as susceptible to scab
as some other citrus. However, sporadic out-
breaks can cause economic loss. The disease is
most common in groves where Tahiti lime has
been grafted onto rough lemon rootstock.
Rough lemon is extremely susceptible to this
fungus and inoculum can build up on suckers
arising at the base of the tree.

Infections in lime leaves appear as light-
colored, raised areas. Severely infected leaves
are distorted.Infections of fruit appear as
prominent, light-colored, raised areas (Fig. 7)
that throw fruit out of grade.

Cultural controls are very important in
management of scab. If rough lemon has been
used as the rootstock, emerged lemon sprouts
must be promptly pruned out. Use of marcots
(airlayers) will, of course, eliminate the rough
lemon problem. Copper and benomyl sprays
are also recommended.

Phytophthora Foot Rot

Foot rot is a soilborne fungus disease
caused by either Phytophthora citrophthora or
Phytophthora parasitica. The latter fungus is par-
ticularly widespread in the rock soils of Dade
County. These fungi damage trees by invading
woody tissue at or below the soil line.

The most serious aspect of this disease
occurs when Phytophthora invades at the tree
base, producing large cankers (Fig. 8). Trees
become chlorotic, and dieback begins.

Rough lemon rootstock is susceptible to
foot rot; the incidence of the disease is notice-
ably lower in marcot trees and those trees on
Citrus macrophylla rootstock. Under flooding
conditions the feeder roots of lime trees may
be attacked by these fungi, resulting in a root
rot. Severe infection may kill trees.

Control of foot rot is primarily cultural.
It is important to prevent disease entry into
groves by strict sanitation in the propagation
nursery. Injuries to tree trunks should be

avoided, because they can be sites for fungal
entry. Weeds should be adequately controlled
to prevent moisture from building up around
tree trunks.

Sooty Mold

Sooty mold is not, strictly speaking, a
"disease" of lime, since it does not involve the
invasion and colonization of the host by a
pathogenic microorganism. Instead, it is a re-
flection of an attack by pest insects, especially
the citrus whitefly. While whiteflies and related
insects feed on lime leaves and fruit, they se-
crete a thick, sweet, sticky honeydew that is
promptly invaded by the sooty mold fungi. The
result is a black discoloration (due to the color
of the fungi) over leaf (Fig. 9) and fruit surfaces.
An observer can usually identify sooty mold,
because it will rub off on the fingers.

It is thought that sooty mold has little,
if any effect on the general health of the lime
tree, and it is considered of economic impor-
tance only when the black growth builds up
to a point where it cannot be easily cleaned
off fruit at the packinghouse.

Figure 1. Sandpaper-like spots of melanose Figure 2. Raised, black melanose lesions in
on underside of lime leaf. lime fruit.

Figure 3. Algal disease in lime leaf.

Figure 5. Earlier stages of greasy spot leaf

Figure 4. Heavy "spore" production by
Cephaleuros (algal disease pathogen) on bark
of lime tree.

Figure 6. Advanced stages of greasy spot

Figure 7. Typical, light-colored, raised scab
infection on lime fruit.

Figure 8. Prominent cnakers at base of lime
t -ew it Phytophthora foot rot.


Figure 9. Sooty mold on lime leaf, resulting
from invasion of whitefly honeydew by the
black sooty mold fungi.

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