White mold (also known as Sclerotinia stem rot) is a significant fungal pathogen of soybean plants in the North Central region of the United States. This pathogen overwinters via small black structures known as sclerotia and can survive in the soil for five to seven years. Under cool, wet conditions, mushroom-like structures (apothecia) bloom from the sclerotia in the top 2 inches of the soil surface. The apothecia produces spores that land on soybeans to infect and colonize the plant. The disease development is favored by cool temperatures around 75°F and humid conditions at flowering when soybean canopy closes and restricts air flow.
White Mold Identification
Early signs of white mold are the tan, disk-shaped fungal structures blooming on the soil surface; however they can often be confused with bird’s nest fungus. Symptoms of white mold often occur in patches throughout the field and appear as brown lesions on the stems, and further infection causes wilting, lodging, and plant death. White, cotton-like fungal growth is on the stems and the dark black bodies of sclerotia may be present. Stems of dead plants will be bleached in color, and when the stem is split lengthwise, the sclerotia is often found inside.
Field records can be indicative of the probability of this disease occurring if weather conditions have been cool and moist or if the field has a history of white mold. This practice can also be helpful when selecting seeds that are less susceptible to the disease, or choosing management practices.
Tillage can be utilized to manage the sclerotia in the soil so that these structures do not remain near the soil surface where they can germinate. Infected fields should be worked last and equipment should be cleaned to avoid spreading the sclerotia. Crop rotation is also beneficial as non-host crops, such as corn or cereals, would terminate the cycle of creating more fungal structures for overwintering.
High populations of soybeans (greater than 175,000 plants per acre) and narrow row spacing is associated with increased white mold disease. Reduction in population or using wider rows (greater than 20 inches) may be beneficial in reducing white mold occurrence, but be sure populations maintain yield potential.
Research has recommended fungicide applications must be applied during soybean flowering (R1) to be most effective, but some can be applied up to the R3 growth stage. Efficacy can also be impacted by the penetration of the fungicide into the canopy where infection started. Biological controls, such as the fungus Coniothyrium minitans, colonizes and degrades the sclerotia in the soil and should be incorporated in the soil three months before white mold develops.