This fungal disease appears just after cool, wet periods. Already this year at the Plant Clinic, we have seen the disease on tomatoes. Many ornamental crops could be affected, including begonia, daisy, delphinium, hydrangea, marigold, pansy, and zinnia. Vegetable crops commonly affected are tomato, green bean, and pepper. Report on Plant Disease No. 1008 lists hundreds of plants that are susceptible to this disease.
White mold is caused by Sclerotinia, a fungus that thrives in cool, wet weather. Because conditions have been nearly ideal for this disease in many parts of Illinois over the past month, we should be expecting this problem. The fungus remains in a resting state (sclerotia) in the soil for many years. With ideal conditions, the fruiting bodies (apothecia) form on the sclerotia and spores are released into the air. A period of wet weather is then required for infection to follow this spore release.
Symptoms of white mold are bleached areas on the stems and at the leaf axils. These areas appear almost like animal bones dried in the sun. In cool, wet weather, a white fluffy mold develops on the bleached areas, but this mold will not be seen in dry weather. Within 7 to 10 days, sclerotia form inside the stem. They are large, black structures, almost like rabbit droppings, but they are found inside the stem—and occasionally on the outside as well. The sclerotia fall to the ground when they are dry, and the cycle continues. The disease does not need to be confirmed in a lab; symptoms are quite diagnostic.
Control options for this disease are limited. The home grower can try to keep plant density low so that air movement helps dry plants out sooner. There are no rescue treatments for commercial growers, but some fungicides may prevent the disease from spreading. Fungicide applications may help on a preventive basis in areas where this disease is a problem every year. Rotation to a nonhost crop for at least three years is suggested. For chemical options consult the 1998–1999 Illinois Commercial Turfgrass and Ornamental Pest Management Handbook or the Homeowners’ Guide to Pest Management.