"White Mold" a Problem in
  Some County Soybean Fields

       Sclerotinia stem rot, or white mold as it is often called, is a disease that occurs when soybeans are grown in rotation with susceptible crops and environmental conditions are favorable. During 1997, Sclerotinia stem rot has been observed in many soybean fields throughout the area and state. Ed Oplinger, Extension Soybean Specialist, reports severe infestations in the UW variety performance trials at Galesville and Hancock. The disease is caused by a fungus that survives for many years in the soil as resistant black fungal bodies called sclerotia. When the environment is favorable (humid, cool conditions during flowering), sclerotia germinate resulting in air-borne spores that infect any host crop. Yield reductions can be significant with severe disease infestations.

       Disease symptoms on leaves usually appear during the early stages of pod development, however, stems express symptoms 1-2 weeks prior. The foliar symptoms are chlorosis and wilt, with tissues between major leaf veins developing a gray-green cast while vein tissues remain green. In time, leaves become tan, curl, and die but remain attached to stems.

       Foliar symptoms of Sclerotinia can be mistaken for those caused by Phytophthora root rot, brown stem rot, and stem canker late in the season. Sclerotinia infects the old flower parts adhering to the tips of very small pods. The fungus then progresses through the pod tissue and into the stem at a node. Stem lesions develop at nodes and appear gray or "water-soaked". These lesions become 2 to 12 inches in length and are often covered by white, fluffy mycelium (mold growth). At harvest, diseased stems are characterized by poor pod development, a white appearance, and an abundance of sclerotia or black bodies on diseased plant tissues.

       Soybean varieties differ in their susceptibility to Sclerotinia. Recent varietal ratings for the disease at Galesville ranged from 1 percent to 80 percent infection. Early maturing varieties tend to have more resistance, however, the amount of disease development seems to be more strongly related to environmental factors and cultural practices. Moderately resistant varieties can show significant disease severity at 7-inch row spacings, especially if weather conditions are favorable for disease development. Nevertheless, producers should consider variety selection as a key line of defense for future years in fields with a history of white mold problems. Soybean variety ratings for Sclerotinia can be found in the UW Soybean Variety Performance bulletin.

       The incidence of Sclerotinia stem rot is somewhat enhanced by narrow row planting which makes for a more humid environment under the crop canopy. However, the potential yield loss from growing soybeans in wide rows precludes this cultural control practice from being practical. Many common crops and weeds act as alternate hosts and help to perpetuate the survival of Sclerotinia in soils. These include: alfalfa, pea, clover, sunflower, pigweed, velvetleaf, ragweed, and lambsquarter. Growers are advised to harvest heavily infested fields last to avoid spreading the black sclerotia to other fields via harvest equipment.

       No fungicides are specifically registered for control of Sclerotinia stem rot of soybean, however, Benlate and Topsin-M are registered for control of other soybean diseases and can suppress the disease if applied during flowering. Often, the symptoms are not noticed until it is too late for fungicide application.