When a plant or part of the plant (leaf, flower, fruit) rapidly collapses and dies this is called blight. Many bacteria, fungi, and viruses can cause blights. Most blights are favored by certain weather conditions, such as hot and humid or cool and moist
White mold is caused by the fungus Sclerotinia sclerotiorum. White mold is common when spring and fall are unusually cool and wet. Under these conditions, plentiful spores are produced and can be carried by air currents to ornamentals. Moist weather is required for stem infection. A small spot appears on the stem, but usually, the disease is not noticed until the plant wilts. At this time a fluffy white mold is produced on blighted stems, and often black, hard sclerotia are found inside stems and on stem surfaces. These sclerotia somewhat resemble black sunflower seeds; they have a thin black outer layer, and inside they are solid and white.
Scerotinia white mold on candytuft. Photo: Ellen Hartranft
White mold can attack a wide variety of plants but is most serious on composites (aster, sunflower, chrysanthemum, daisy, etc), legumes and crucifers (such as Iberis). The white mold fungus is commonly found on legume forages (alfalfa, clover, vetch, etc.), and is often more of a problem in rural areas located near pastures and forage fields.
Signs of Sclerotinia stem rot (on a legume). Photo: Daren Mueller, Iowa State University, Bugwood.org
Control
One management option for white mold is to space plants so that they dry after irrigation and rains; closely planted crops maintain a moist environment at the soil line that favors infection. During the growing season, it is very important to remove infected, wilting plants so that the fungal sclerotia are not dropped into the garden to serve as a source of more infections. Fungicide applications can effectively reduce losses, but should be combined with sanitation and plant spacing for best results. Consult your local extension office for the fungicides currently registered for white mold on ornamentals.
The cornerstone for control of all blight diseases is sanitation both during the growing season and in the fall. Wilted and blighted plants and plant parts should be promptly removed from the production area. Do not compost material killed by southern blight or white mold because the sclerotia of these fungi may survive composting. In the fall, all plant debris should routinely be cut at ground level and removed. This material may be composted.