White mold is caused by Sclerotinia,a fungus that thrives in cool, wet weather. Due to near ideal conditions in many parts of Illinois, this disease should be expected. The fungus remains in a resting state (sclerotia) in the soil for many years. With ideal conditions, fruiting bodies (apothecia) form and spores are released into the air. A period of wet weather is then required for infection to follow the spore release. It is easy to understand why the alternating wet and dry periods experienced in some parts of the state have been ideal for Sclerotinia white mold.

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. Within seven to ten days, sclerotia form: large, black structures, almost like rabbit droppings, found inside the stem (and occasionally on the outside as well).

Control options for this disease are limited. The home grower can try to keep plant density low so that air movement helps dry out plants sooner. There are no rescue treatments available for commercial use. Fungicide applications may help on a preventive basis in areas where this is a problem every year. For chemical options, consult the 1997 Illinois Commercial Turf and Ornamental Pest Management Handbook.