Sclerotinia stem rot, more commonly known as white mold, is a common and destructive disease. It is sometimes seen across the U.S. and Canada; however, it is more common in southeast Minnesota, northeast Iowa, and NW Illinois. This disease is difficult to manage and resistant varieties are a partial solution to this perennial problem. White mold thrives in these conditions and timing:

• Long wet spells in the summer months
• Long cold spells in the summer months
• During flowering for soybeans in July
• Other less-important factors can include: high plant populations, high fertility, narrow rows, protected fields and parts of fields where plants dry slowly, and lack of air circulation under soybean canopy.

Symptoms of White Mold: According to the University of Minnesota, “Lesions usually develop first at stem nodes during or after flowering. The lesions expand and the tops of the plants become grayish-green and then wilt and die. Infected stems often become soft and watery, and become covered with white moldy growth in moist conditions. Dry, dead stems can develop a bleached, white appearance. Hard, black sclerotia that look similar to rodent droppings develop on or inside infected stems and pods. Scattered dead plants can often be seen standing upright in affected fields, or plants can be killed in patches.”

Causal Pathogen: According to the University of Minnesota, “White mold is caused by the fungus Sclerotinia sclerotiorum. This fungus survives from year-to-year in soil or stems in the form of hard black masses of mycelium (sclerotia). The sclerotia germinate near the soil surface to form small tan-to-gray mushroom-shaped structures called apothecia, which produce ascospores that spread via air currents to infect dead soybean flowers. The infection then spreads into the stems. Seed lots can be contaminated with sclerotia, and seeds can be infected with the pathogen. The host range of this pathogen includes many broadleaf crops and other plants, but not corn or small grains.”

Disease Management: Selection of soybean varieties with some level of resistance to white mold can be helpful in managing this disease. Low plant populations or wide rows may help to reduce white mold severity, but these tactics do not always make a difference. Where irrigation is used, reduce irrigation frequency during flowering . Long-term rotation with corn or small grains may reduce white mold. Foliar fungicides may be helpful, but have generally performed inconsistently and may not be economically beneficial for managing this disease.