No. 18 Article 8/July 24, 2009

Conditions Favorable for Sclerotinia Stem Rot (White Mold) on Soybean

White mold (aka Sclerotinia stem rot), caused by the fungus Sclerotinia sclerotiorum, is a disease of soybean and other broadleaf crops. Historically, white mold on soybean has been observed sporadically in central Illinois, more frequently in northern Illinois, and rarely in southern Illinois. Cool (temperatures below 85°F) and wet conditions, especially when soybean plants are blooming, are favorable for the development of white mold. These cooler-than-normal and wet conditions have been occurring in central and northern Illinois the last couple of weeks, increasing the risk of white mold in these areas.

White mold gets its name from the fuzzy white growth that can be observed on affected soybean plants. This growth is the mycelia of the fungus that causes the disease. Symptoms include wilting leaves, stems that appear to be "bleached," and shredding of the stem tissue. Small black structures known as sclerotia can be found on and inside plants that have been affected by white mold.

Soybean stem affected by white mold (aka Sclerotinia stem rot).

The disease cycle of white mold is complicated, and favorable environmental conditions and soybean growth stages must intersect for the disease to occur. The fungus overwinters in the soil as sclerotia. These sclerotia can survive in the soil for many years. Under wet soil conditions, the sclerotia germinate and form small mushroom-like structures known as apothecia. Airborne spores (ascospores) are discharged from the apothecia and land on soybean plants. Ascospores that land on senescing petals of soybean flowers are the most likely to cause infection. As the soybean flower petals senesce, the ascospores begin to germinate, grow, and infect the stems. If wet and cool conditions continue, the disease continues to develop throughout the plant. Eventually, sclerotia will form on and inside the affected plants. Many of the sclerotia will be blown out of the back of the combine during harvest, adding more "inoculum" back into the field.

Apothecia (mushroom-like structures) growing from a sclerotium. (Image courtesy of J. Venette, North Dakota State University.)

Management of white mold in soybean is difficult, and multiple practices must be integrated to achieve the best control. These management practices include the following:

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