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 Symptoms
 Epidemiology
 Control
 Figures






Title: Selerotinia diseases of vegetables and field crops in Florida
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Permanent Link: http://ufdc.ufl.edu/UF00066886/00001
 Material Information
Title: Selerotinia diseases of vegetables and field crops in Florida
Series Title: Plant Pathology Fact Sheet PP-22
Physical Description: Book
Language: English
Creator: Pernezny, Ken
Purdy, L. H.
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: 2000
 Subjects
Spatial Coverage: North America -- United States of America -- Florida
 Record Information
Bibliographic ID: UF00066886
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
    Symptoms
        Page 1
    Epidemiology
        Page 2
    Control
        Page 3
    Figures
        Page 4
        Page 5
Full Text



Plant Pathology Fact Sheet


Selerotinia Diseases of Vegetable and Field

Crops in Florida
Ken Pernezny and L. H. Purdy, Professor, Everglades Research and Education
Center, Belle Glade; and Professor, Emeritus, Department of Plant Pathology,
respectively. University of Florida, Gainesville. 10-81, Revised December 2000


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


The fungus, Sclerotinia sclerotiorum,
causes a profusion of Sclerotinia diseases on
more than 360 different host plant species.
There are at least 61 different common names
that have been used for different Sclerotinia dis-
eases, and these range from banana fruit rot to
wilt. Some common names for Sclerotinia dis-
eases of important crops in Florida are white
mold (beans), watery soft rot (cabbage), drop
(lettuce), stem rot (potato and tomato), and nest-
ing (post-harvest disease of bean).

Sclerotinia diseases of plants that grow
in Florida have a history of more than 80 years.
Lettuce drop was reported to be present in
fields of lettuce in 1896 near Gainesville where
certain fields were almost completely de-
stroyed. Since that time Sclerotinia diseases have
been reported on many crops in the state and
have been particularly damaging in bean, let-
tuce, cabbage, potato, and tomato. Although
Sclerotinia is known to attack peanut and soy-
beans, it does not appear to be a pathogen of
economic importance on these field crops at this
time.

Symptoms

The best indicator of Sclerotinia disease
is the presence of small, black sclerotia (resis-
tant structures) of the fungus (Fig. 1). Sclerotia
vary in size and may be spherical, flattened on
one or more sides, elongated, and almost any


other shape imaginable. They usually are about
3-10 mm long x 3-7 mm wide, with a black out-
side covering and usually a white interior. Scle-
rotia form on the surface of certain plant parts
as well as inside the stem cavity of tomato and
potato. A ring shaped sclerotium may develop
around the stem of tomato fruit when they are
infected by Sclerotinia. Sclerotia enable the fun-
gus to survive from season to season and are
the source of inoculum to infect crops.

Another common indicator of Sclerotinia
diseases is the presence of white, cottony-like
mycelium. of the fungus when the environmen-
tal conditions are favorable, i.e. cool and moist
(Fig. 2).

Symptoms of Sclerotinia are often differ-
ent for different crop plants. The disease white
mold in beans usually does not appear until
after blossoming begins. When flower petals be-
come senescent, die, and fall from the flower,
they may be invaded by the fungus. These fun-
gus-invaded petals serve as an essential, inter-
mediate form of organic matter that allows the
fungus to advance into the plant itself. The dis-
ease often appears first in leaf axils where
flower petals often lodge after falling from the
flower. The disease advances into the stem, as
water-soaked spots that increase in size, girdle
the stem, and kill it above the point of infec-
tion. The disease can also enter the bean plant
through leaves or pods that lie on the soil sur-
face where sclerotia or infected plant parts act


PP-22







as sources for infection; the pods pictured in
Fig. 3 were infected in this way.

Lettuce drop usually begins when wrap-
per leaves still attached to the plant fall to the
soil surface in the normal growth processes and
contact a sclerotium that germinates to initiate
infection. This fungal growth progresses
through the leaf into the base of the lettuce stem,
where the fungus destroys the stem causing the
plant to collapse or drop (Fig. 4). The entire let-
tuce plant may be invaded by the fungus, as
evidenced by the cottony-white fungus growth
on plant surfaces that is often followed by the
production of sclerotia when environmental
conditions are favorable.

In tomato and potato the Sclerotinia dis-
eases begin usually about the time of flower-
ing. Early stages of these diseases are similar
to the disease in bean. Infections usually start
in the leaf axils or joints of the plant where fallen
flower petals lodge. Spores of the fungus light
on these flower petals, germinate, invade the
petal, and subsequently advance into the stem.
Water-soaked spots are usually the first symp-
tom observed, and these are followed soon by
further invasion of the stem, girdling, and death
of the upper part of the stem that turns a
bleached light gray, causing the stems to re-
semble bones of animals that have been left in
the sun. The fungus can also enter plants at the
soil line or at other points where plant parts
touch the soil or other plants that are diseased.
Plants in large portions of the field may become
diseased and die, producing large, more or less
circular, areas of dead plants (Fig. 5). The hard,
black sclerotia of the fungus are often formed
inside the stem cavity and tend to assume the
shape of the cavity; that is, they are definitely
longer in one dimension than in the other (Fig.
6). Some may be almost tubular.

The Sclerotinia disease of cabbage may
be similar to the disease in lettuce, and, indeed,
may be referred to as drop of cabbage, but wa-
tery soft rot is a better name. The disease be-


gins usually on the leaves where growth cracks
or other damage occurs. Spores of the fungus
land on these places, germinate, invade the leaf,
and grow into the head and stem. Eventually
the entire plant may be invaded by the fungus
and sclerotia often form over the surface of the
head.

Almost all Sclerotinia diseases are field
diseases, but when they occur in post-harvest
situations they are very damaging. When beans
are shipped in hampers or other containers in
which diseased pods are included, a situation
called nesting may develop. The fungus grows
from the diseased pod to other adjacent pods
and produces the cottony-white fungus growth
and sclerotia. A mass of diseased pods is cre-
ated that is stuck together by the fungus growth,
resembling a nest (hence, the name "nesting").

Epidemiology

Sclerotinia is a fungus that prefers cool,
moist weather, causing diseases of great inten-
sity when the temperature ranges from 60 70F
(15 210C). High humidity with dew formation
supports the spread and increases the intensity
of disease.

Small, mushroom-like structures called
apothecia develop from sclerotia and bear in-
fectious spores (Fig. 7). When the environmen-
tal conditions change suddenly, these spores
are ejected into the air and carried to healthy
plants, where they germinate and produce dis-
eases described here. Spore ejection will occur
after sclerotia in soil have been wet or soil mois-
ture is supportive of plant growth (e. g. after
several rains or irrigations).

When in the soil, sclerotia may germi-
nate to produce a fungus growth that may in-
fect certain plants directly, without first grow-
ing on nonliving organic matter, such as senes-
cent or dead leaves of the host plant, or dead
leaves and plant parts of other plants. In cer-
tain situations sclerotia may germinate and the







resulting fungus growth remains on the dead
leaves for a short time, after which the plant
stem is invaded.

Control

Consult the University of Florida Plant
Disease Control Guide and current labels for
specific legal uses of fungicidal chemicals used
in the control of Sclerotinia diseases.

Four to five weeks of flooding of fields
that have a history of Sclerotinia diseases may
reduce the numbers of viable sclerotia, thereby
reducing the amount of disease in succeeding
crops. Seedbeds for celery transplant produc-


tion are flooded before seeding (Fig. 8). This
practice, and the application of the practice to
other locations, may reduce the incidence of
Sclerotinia diseases as well as the incidence of
other diseases.

Sprinkle irrigation has been reported to
enhance the development of certain Sclerotinia
diseases, mostly those that are initiated by
Sclerotinia spores. Recycled irrigation water may
move sclerotia to fields where sclerotia are not
present. The suppressing effect of plastic mulch
on Sclerotinia diseases, the density of plant
canopy, and other cultural practices must be
evaluated to establish conclusively their influ-
ence on Sclerotinia diseases in Florida.


Figure 1. Sclerotinia sclerotiorum growth in Figure 2. Cottony, white, fungus growth on
Petri dish culture. Note large, black sclerotia tomato plant with Sclerotinia stem rot.
(resistant structures, characteristic of this
pathogen, in both culture and in the field).



























Figure 3. Sclerotinia infection of bean pods.


Figure 5. Roughly circular area of Sclerotinia
infection in field of direct seeded tomatoes,
showing severe wilting and plant death.


Figure 4. Lettuce drop, showing collapse of
outer leaves.


Figure 6. Black sclerotia inside tomato stem.



























Figure 7. Mature apothecia (mushroom-like
sporebearing structures) growing out of a
Sclerotinia sclerotium (photo courtesy of
Peter Adams, USDA, Beltsville, Md).


Figure 8. Flooding- a common practice for
Sclerotinia control in celery fields.




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