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.  White mold 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 (officially-- 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.    For many years, white mold wasn't a disease that was widespread in the upper Midwest.  The early 1990's was when life in the soybean field changed. This corresponded with a time when soybean acres in the state skyrocketed. White mold is said to be a "high yield" disease, favoring well-managed fields with the greatest yield potential.  The disease devastated fields in the mid-1990's and early 2000's.  It came back with a vengeance in 2009.  

What to look for....

     Disease symptoms on leaves usually appear during the early stages of pod development; however, stems express symptoms one to two 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.  

Reducing white mold risk....

     Soybean varieties differ in their susceptibility to Sclerotinia and, realistically, variety selection offers the best defense against the disease.  Growers are encouraged to reference the "2009 Wisconsin Soybean Variety Test Results" bulletin (published in November) for detailed ratings on variety susceptibility.

     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. Growing soybeans in 15-inch rows may prove to be a good compromise. Many common crops and weeds act as alternate hosts and help to perpetuate the survival of Sclerotinia in soils. These include: most bean crops, alfalfa, pea, clover, sunflower, pigweed, velvetleaf, ragweed, and lambsquarter.

     Controlling white mold with fungicides is difficult because it is hard to predict if environmental conditions will favor pathogen infection.  Currently researchers are testing the efficacy of several fungicide products along with the biocontrol agent Coniothyrium minitans, a soil fungus that establishes and kills the sclerotia before they germinate in subsequent crops.  Wisconsin research has also shown that using a winter cover crop (e.g. winter wheat) that is killed with glyphosate at soybean planting time was effective in forcing sclerotia to germinate and release spores far ahead of flowering.

     For more information on Sclerotinia stem rot, growers can view a new UW-Extension video hosted by Paul Esker, UW Extension Plant Pathologist, at: http://www.youtube.com/watch?v=rdc7ac60R0M