Tomatoes are susceptible to a stem decay disease called white mold (also called Sclerotinia stem rot or timber rot) caused by the fungus Sclerotinia sclerotiorum. Death of the stem segment leads to death of the top of the plant. During cool, moist weather, white cottony strands of the fungus are produced on the stems. Splitting the diseased stem open reveals a hollow stem filled with black sclerotia (fungal survival structures about the size of raisins). These sclerotia drop to the ground, can survive in the soil for many years, and can infect future plantings.
Cool, wet, rainy spring weather favors the development of this disease. Remove and destroy diseased plants in the garden, being careful to avoid dropping the sclerotia on the soil. There are no varieties of vegetables resistant to this disease. A long crop rotation avoiding lettuce, beans, and cabbage is recommended, as these plants are also very susceptible to this fungus. This disease should not to be confused with Southern Stem Blight (Sclerotium rolfsii) which is prevalent on tomatoes and other vegetables during hot humid weather.
For more detailed information see: Sclerotinia Stem Rot of Tomatoes in Greenhouses
