White Mold on Tomato

White mold is a fungal disease that can infect more than a hundred different plant species, including field crops, garden vegetables, herbaceous ornamental plants, and a number of common weeds. A tomato plant showing signs and symptoms of white mold was recently submitted to the Iowa State University Plant Disease Clinic.

White mold, caused by species of the fungus Sclerotinia, is a serious disease that often results in death of plants. Infection by the fungus is favored by cool, moist conditions. Diseased tissue initially shows a water-soaked appearance. As described by the common name of the disease, white mold can be seen on the infected plant tissue when conditions are humid, and is usually most evident on stem parts. Small black sclerotia, about the size of sunflower seeds, can be seen on or in the diseased tissue, often embedded in the cottony, white fungal strands of the fungus. The dying tissue tends to show a bleached, dried appearance.

Sclerotia are the tough survival structures of the fungus, making it a challenge to control the disease. Because the white mold fungus produces survival structures and can infect so many plant species, rotation is often of limited usefulness. Since infection is favored by abundant moisture, practices that improve air circulation, such as wide spacing at planting, staking of plants, and preventing water splash on leaves and stems can help reduce conditions favorable for disease development. Resistant varieties are available with some plants, but not currently with tomato. Fungicides are labeled to help control the disease in certain crops, but are not usually practical for use in the home garden. Sanitation, the careful removal of infected plants, is the most important management practice for home gardeners. Since sclerotia can survive in the soil for several years, removal of infested soil may be feasible only if the area affected is small. Many weeds are susceptible to white mold, so control of weeds in and around the garden is helpful in managing the disease.

White mold on tomato
White mold on tomato
Black sclerotia in stems of infected tomato vines. Photo by Paula Flynn.
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