White mold of soybeans is a common disease caused by the fungus Sclerotinia sclerotiorum. It can cause major seed yield reductions when soybeans are planted in infested soil and there is a dense plant canopy with prolonged periods of wet weather (a major factor in disease development). The disease is rarely observed during dry periods in July and August. Besides seed yield reductions, the disease also results in reduced seed quality and seed contaminated with the black sclerotia of the fungus. Sclerotinia overwinters as sclerotia in soil.
The sclerotia germinate to form small mushrooms called apothecia that produce spores termed ascospores. The ascospores infect senescing flower tissue and then infect the stems of the plant; disease is closely tied to flowering.
Symptoms usually are observed after the canopy has closed. Dead plants are generally the first symptom observed. An inspection under the canopy will reveal a cottony, white mycelial (fungus threads) growth on stems, leaves or pods. Lesions develop on main stems and side branches. Stems appear bleached and sometimes shredded from advanced decay. Large sclerotia form in and on diseased tissue. Seeds in diseased pods are usually shriveled and may be infected by the fungus or replaced by sclerotia.
When a field with white mold is harvested the seed is almost always contaminated with sclerotia. Yield losses usually occur when incidence of disease is 15% or greater. Yield losses can range from to 1.3 to 3.7 bu/A for every 10% increase in disease incidence. The pathogen has an extensive host range of over 370 plant species and causes diseases on a wide variety of crops such as sunflower, dry bean, canola, alfalfa, buckwheat, lupine, mustard, potato, Jerusalem artichoke, safflower, lentil, flax, field peas and many vegetables. There are also many common broadleaf weed hosts such as marsh elder, lambsquarters, pigweed, Canada thistle and wild mustard. The fungus that causes white mold on soybean is the same one that causes white mold of sunflower, dry beans, canola, and other crops.
The most important controls for white mold of soybean are to choose less susceptible cultivars, avoid planting on soils heavily infested with Sclerotinia, and to maintain open rows so air movement through the crop reduces plant wetness. Cultural practices, such as wider rowspacing, which reduce environmental conditions favoring disease are helpful. Orienting the rows toward the prevailing wind, for example, will help dry the crop following precipitation. Under very prolonged rainy periods or in protected areas such as along shelterbelts where humidity is higher, disease may develop even in an open canopy.
Soybean fields should be monitored for disease incidence. Check the seed hopper at harvest for the presence of sclerotia. As disease begins to increase in a field, the rotation time to non-susceptible crops such as small grains and corn should be increased. Crop rotation will reduce populations of sclerotia in soil, but will not entirely eliminate the pathogen. Do not plant highly susceptible crops such as drybeans and sunflowers during the rotation. If you rent land, find out the disease and cropping history before making planting decisions.
Although common soybean cultivars adapted for this region are susceptible to white mold, some cultivars are less susceptible than others. Information on cultivar susceptibility is available from the NDSU Extension Service. Do not use seed from a white mold infected crop. Seed quality could be low, and sclerotia may be introduced into the field along with the seed. Also, maintain good control of broadleaf weeds, since they can be hosts of Sclerotinia. When growing soybeans under irrigation avoid practices that favor a dense canopy and free water on the plant during flowering, since these will create ideal conditions for disease development.