White Mold in Soybean

White mold, favored by a cool and moist growing environment, may be found in soybean fields this year. The sclerotia or fruiting structure produced by the disease can remain viable in the soil for many years and can result in elevator discounts if present in marketed seed.

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Figure 1

Figure 1. Soybean stem affected by white mold, also known as Sclerotinia stem rot.

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White mold or Sclerotinia stem rot is caused by the fungus Sclerotinia sclerotiorum. The fungal fruiting bodies known as sclerotia overwinter and germinate when soils are moist for several consecutive days and soil temperatures are at or below 60° F.1 Row shading enhances the germination and development of apothecia (a mushroom-like structure) as sunlight hinders development. Plant infection occurs when spores are released from the apothecia and attach to flower petals that are beginning to wilt from age. Infection appears as a white cottony growth and development of sclerotia on infected plants (Figure 1). Severe infections cause plant wilting, lodging, and death.

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Disease development for 2015 depends on plant and environment factors throughout the remainder of the growing season. Cool temperatures (65 to 75° F), rainfall or irrigation, and a closed canopy favor disease development. Warmer temperatures (> 82° F) and a dry environment reduce the opportunity for development.

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Some studies show that white mold can be managed by avoiding fall tillage in infected soybean fields and no-tilling corn the following year. Tillage buries the hard textured sclerotia, which can survive in the soil for up to seven years.2 The disease does not infect corn; therefore, under favorable conditions, sclerotia which germinate and grow under the corn canopy reduces the inoculums and potential incidence in future soybean crops.

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Labeled foliar fungicides may prevent white mold and are usually applied at R1 growth stage or before disease symptoms develop. However, depending on the planting date and if weather conditions are favorable for the pathogen, application may be more effective at canopy closure.1 Two applications may be necessary for disease prevention. Soybean seed products that have demonstrated white mold tolerance should be considered if planting into fields with a history of white mold. Reducing planting populations, increasing row widths, avoiding high-nitrogen fertilizer applications, and controlling weeds are additional measures to consider and practice to reduce the potential for white mold development.