Plant Health Initiative

White Mold (Sclerotinia Stem Rot)

A sign of white mold infection is white, fluffy mycelium as seen on this soybean stem. Photo credit: Craig Grau, University of Wisconsin

White mold is caused by the fungus Sclerotinia sclerotiorum. The fungus is easily recognized by the presence of fluffy white mycelium (the vegetative body of the fungus) that is the source of the name white mold.

 

White mold has progressed from a sporadic disease to an annual threat to soybean production

White mold was discovered in central Illinois in 1948. Although it eventually became a chronic problem in Michigan, Minnesota, and Wisconsin, outbreaks were localized, and occurred where soybeans were grown in rotation with other susceptible crops.

Beginning in 1990, however, the occurrence of white mold became more widespread in the Great Lake states, and by 1992 was prevalent throughout all the North Central states.

Reasons for the sudden increase of white mold are not fully understood, but could be related to changes in cultural practices that promote a greater canopy density, changes in the genetic base of current soybean varieties, or changes in the white mold pathogen.

Distribution and overall risk level of white mold

Each year, the occurrence of white mold is heavily dependent on weather conditions during soybean flowering and early pod development. Rain, cool temperatures, high relative humidity, and moist soil favor the growth of the fungus if it is present. However, pathogen biology and crop management decisions interact strongly with weather to determine the risk of production losses caused by white mold.

The distribution and overall risk level of white mold in years of conducive environmental conditions is illustrated in the map below. High risk levels indicated by orange, medium risk in yellow, and low in blue. Risk levels are estimated based on disease history and other risk factors as outlined on the page White Mold: Risk Assessment.