Cause and symptoms

Title: Greasy spot of citrus
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00066868/00001
 Material Information
Title: Greasy spot of citrus
Physical Description: Book
Creator: Kucharek, Tom
Publisher: Gainesville : Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 1979
Funding: Florida Historical Agriculture and Rural Life
 Record Information
Bibliographic ID: UF00066868
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.
Resource Identifier: oclc - 51337637

Table of Contents
    Cause and symptoms
        Page 1
        Page 2
        Page 3
Full Text

Plant Pathology Fact Sheet

Greasy Spot of Citrus

Tom Kucharek and Jack Whiteside, Professor and Extension Plant Pathologist, Department of Plant
Pathology, Gainesville; and Retired Professor and Plant Pathologist, Citrus-REC Lake Alfred,
respectively; IFAS, University of Florida, Gainesville. 1979; Revised November 2000

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

Cause and Symptoms

Greasy spot is a major foliar and fruit
disease on citrus in Florida. It causes prema-
ture leaf drop beginning in the fall and contin-
ues through winter and spring. As a result,
yields of the following crop are reduced. Fur-
thermore, cold damage has been observed to
be more severe on severely defoliated trees.
Rind blemish from this disease causes down-
grading of fruit intended for the fresh fruit mar-
ket and this can be particularly severe on grape-

Greasy spot is caused by the fungus,
Mycosphaerella citri. Infection of leaves and fruit
occurs primarily in June, July and August by
spores that originate on previously infected,
fallen, decomposed citrus leaves. Only after
successive wetting and drying of the fallen cit-
rus leaf litter will the spores be formed and re-
leased. Spores are most abundant during the
earlier part of the summer's rainy season. As
the rainy season progresses, the number of
available spores decreases because of further
leaf decomposition and an insufficiency of fur-
ther leaf drop during the summer to replenish
the supply. Only spores that land on the
undersurface of citrus leaves are capable of
causing infection, as the infection sites (the
stomates) are confined to the underside of the
leaf. After landing on the leaf, provided that

there is free water on the leaf or the relative
humidity is near 100%, the spores germinate.
The resulting germ tubes are capable of infect-
ing the leaf or fruit rind immediately. However,
a more significant feature with this fungus be-
havior is that under conditions of high mois-
ture combined with high temperature the germ
tubes can continue their growth and form a
microscopic branching mycelial growth on the
leaf surface. This increases the chances for in-

Only if a high density of penetrations
occur will symptoms eventually develop. The
superficial growth increases the chances of this
occurring. The incubation period (infection to
symptom expression) can be as little as 4 to 6
weeks on lemon leaves but it usually exceeds
four months on leaves of orange and grapefruit.

Leaf symptoms begin as slight blisters on the
underside of the leaf (Figure 1, leaf on left) with
a yellow mottle at that point on the upper-side
of the leaf (Figure 2, leaf on left). In time, the
blistered areas become dark orange to brown
to black and have a greasy appearance, (Fig-
ures 1 and 2). On fruit, pinpoint black specks
occur between the oil glands and delayed col-
oring of the rind often occurs at those points
(Figures 3 and 4). On grapefruit this same dis-
ease is referred to as "pink pitting" (Figure 5).



Greasy spot is controlled effectively with
copper fungicides or oil provided spray tim-
ing is correct and the placement of the material
is on the underside of the leaf. A single appli-
cation should be made in June or July. Because
of the superficial growth of the fungus on the
surface of the leaf and the long period of time
(2-3 weeks) for deep penetration by the fungus
into the leaf, the single application not only pro-
tects the leaf from future infections but also kills

the superficial fungus growth already present.
It is therefore unnecessary to apply the spray
before the first spores are likely to reach the leaf
as is the case with most other fungal diseases.
Where greasy spot has been severe in a grove,
an additional application should be made in
August! Where fruit is intended for the fresh
market, oil is not effective in reducing symp-
toms in fruit. Also, when copper is applied late
it can cause blackening of windscar and other
existing corky tissue. A detailed spray program
is available in the Florida Citrus Spray Guide
(Circular SP-43).

Figure 1. Early and late symptoms of greasy Figure 2. Early and late symptoms of greasy
spot on underside of leaves, spot on upperside of leaves.

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Figure 3. Close up of infected grapefruit rind
showing delayed coloring between oil

Figure 5. Late symptoms on grapefruit rind-
pink pitting.

Figure 4. Delayed coloring between oil
glands on grapefruit

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