White Mold - Disease Symptoms, Development, and Management Considerations

White mold, also known as sclerotinia stem rot, invades a wide range of plant hosts and can survive in soils for many years. White mold development is most prevalent in areas that experience wet, cool conditions during the summer—such as Minnesota, Wisconsin, Michigan, New York as well as Canada. This disease can also be a serious threat to soybean fields in the northern growing areas of Iowa, Illinois, Indiana, Ohio and Pennsylvania.

The first symptom of white mold infection is a water-soaked stem lesion originating from a node. If the lesion remains wet, it becomes overgrown with a white, cottony mass of mold. The disease can then spread directly from plant to plant by contact with this moldy tissue. Eventually, sclerotia (small, hard black structures) are formed within the moldy growth and inside the soybean stem.

At the field level, leaves of infected plants turn yellow and gradually wilt and die, but often remain attached to the stem past maturity.

White mold persists in soybean fields over time by production of survival structures called sclerotia. Sclerotia are dark, irregularly shaped bodies formed within the cottony growth on the outside and inside of the soybean stem during the fall. Sclerotia survive in plant debris and soil, giving rise to apothecia, which produce spores. Spores colonize dead plant tissue/flowers before moving into the soybean plant.

For white mold to develop in a soybean field, wet, cool conditions are required throughout the white mold disease cycle. This includes germination of the sclerotia in the soil, spore release, infection of soybean flowers by spores and spread of white mold from plant to plant.

Tolerant Varieties: At this time, there is no know genetic resistance to white mold, but there are clear differences in variety tolerance. Growers should select varieties that rank high in yield and high in white mold tolerance across environments where white mold is a problem.

Rotation: Rotation with a non-host crop is an effective means of reducing disease pressure in a field. Non-host crops include corn, sorghum, and small grains. Susceptible crops to avoid in a rotation include alfalfa, clover, sunflower, canola, edible beans, potato and others. Because sclerotia survive for up to ten years in the soil, rotation is only a partial solution.

Weed Control: White mold has over 400 plant hosts, including many broadleaf weeds such as lambsquarter, ragweed, pigweed and velvetleaf. In addition to acting as host to the disease, weeds can also increase canopy density, which favors disease spread.

Production Practices: Early planting, drilled/narrow rows and higher plant populations increase yields but may also increase white mold incidence. In areas with lower white mold levels or drier climate, production practices which increase yield but also increase white mold levels may still be highest yielding. In areas with higher white mold levels and a cool, wet climate, some change in production practices may be necessary