Sclerotinia Stem Rot of Tomatoes in Greenhouses
Vegetable Disease Information Note 4 (VDIN-004)
Charles W Averre, Extension Plant Pathologist

Sclerotinia 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.

Symptoms 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.

Causal 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%.

Management

The control of this disease, as with many soil-borne diseases, requires a continuous good management program all year:

Sanitation: 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 susceptible crops.
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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Recommendations 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