Sclerotinia Stem Rot of Tomatoes in Greenhouses
Disease Information Note 4 (VDIN-004)
Charles W Averre, Extension Plant Pathologist
stem rot (timber rot) in North Carolina greenhouse tomatoes occurs erratically
during moist, cool periods in the spring. Distribution of diseased plants
in a greenhouse is random. Plants of all ages are susceptible. The disease
is important because 5 to 10% of the bearing plants may be killed and
the fungus can survive several years in the soil.
and Signs of Disease
The disease can be recognized by a soft, watery rot with white, moldy
growth on stems, petioles, and leaves of tomato plants. Often initial
infection occurs in the axes of branches or where a supporting string
may be tied to the base of the plant. These points accumulate nutrients,
plant refuse, and moisture on which the fungus becomes established. Infection
may start on leaves in contact with the soil and gradually grow through
the petiole to the stem and eventually girdle it. If conditions remain
moist, a large amount of cottony, moldy growth can be seen on the dead
tissue. As this growth progresses, hard black, irregularly shaped bodies
called sclerotia form on the surface or in the pith of the stem; they
are diagnostic for the disease. Sclerotinia range from 2 to 10 mm inch
in length and tend to be about 2 to 3 times longer than thick. They are
white to pinkish inside. After the infection has apparently dried up,
the line of demarcation between healthy and diseased tissue is very sharp.
Often the diseased tissue is a light, straw color.
Agent and Disease Cycle
The disease is caused by the fungus, Sclerotinia sclerotiorum,
and it attacks over 170 species of plants. Vegetables, especially susceptible
include bean, cabbage and other crucifers, carrot, cucumber, eggplant,
Irish potato, lettuce, pepper and squash. Many other crops such as clover,
soybean and peanuts are susceptible.
The fungus overwinters (or oversummers) as sclerotia in the soil and may
survive up to 7 years in dry soil. However, if the soil is maintained
warm and moist, following a dormant period when the moisture and temperature
are suitable, the sclerotia in the greenhouse or the field germinate by
resuming vegetative growth, or by forming as many as 35 small mushroom-like
bodies, called apothecia (Figure 1). The apothecia produce enormous numbers
of spores that are blown about and cause primary infections. They are
the only infective spores in the disease cycle. Once the fungus is established
it continues vegetative growth as long as there is sufficient moisture.
Disease is dependent on high moisture and cool temperature (60-70 degrees
F is ideal), but disease will progress slowly at 36 degrees F and as high
as 80 degrees F. However, for the sclerotia to germinate and form apothecia,
the temperature must be below 70 degrees F; for spores to cause infection
relative humidity must be above 90%.
The control of this disease, as with many soil-borne diseases, requires
a continuous good management program all year:
All infected plant parts should be removed from the greenhouse as they
appear on plants. At the end of the season, plants should be removed
promptly and the greenhouse thoroughly cleaned. During the summer, the
soil should be cultivated and kept moist and free from weeds as these
might harbor the fungus and make soil fumigation more difficult. Livestock
manure and plant mulches should not be used unless disinfected by heat
or gas. Fields surrounding the houses should not be cultivated with
Soil disinfestation: Greenhouse soils should be fumigated or
heated to make sure that all disease-causing germs have been killed.
This is a good practice even if the stem rot is not a problem since
other diseases and pests are also controlled. Use a broad spectrum soil
fumigant. Heat the soil with stem to 180 degrees F for 30 minutes.
Moisture control: During the growing season, the greenhouse soil
should be maintained as dry on the surface as possible. This can be
done by irrigating heavily, but infrequently in holes next to tomato
plants. Frequent, irrigations that wet plants must be avoided. The greenhouse
atmosphere must be maintained as dry as possible by continuous, forced
air circulation through a "poly tube" and by introducing cold air while
heating. Removing lower leaves from the plants will enhance air movement
and help prevent infection through the leaf tips.
Fungicides: Botran 50WP at the rate of 0.5 to 1 lb/100 gal, Botran
75WP at the rate of 1.0 lb/100 gal water sprayed to the lower 18 inches
of the plants and to the soil, and chlorothalonil 20% smoke generators
released at the rate of one 3.5 oz can per 1000 sq ft should help reduce
new infections. Follow the recommendations on the label of the container.
Solarization: After sanitizing the greenhouse in late spring,
close up the greenhouse during a hot and sunny week in the summer. Keep
soil in greenhouse moist.
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Carolina Agricultural Chemicals Manual
For assistance with a specific problem, contact your local
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of specific chemicals are based upon information on the manufacturer's
label and performance in a limited number of trials. Because environmental
conditions and methods of application by growers may vary widely, performance
of the chemical will not always conform to the safety and pest control
standards indicated by experimental data. All recommendations for pesticide
use were legal at the time of publication, but the status of registration
and use patterns are subject to change by actions of state and federal
regulatory agencies. 02/91/500
North Carolina Cooperative Extension Service
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