Soybean Disease Update: White Mold

By Dylan Mangel

While concerns about white mold are present across northeast Nebraska, reports of disease are being received from fields in Polk and Seward counties. The disease, also called sclerotinia stem rot, infected these fields and many others earlier in the season during flowering. It is only during the flowering stage that soybeans are susceptible to white mold infection. Unfortunately, this means the time for management is behind us. Initial infection timing can also be estimated based on which node of the plant the infection grows from.

Cool, wet conditions are best for white mold infection. Temperatures around 75°F are ideal. However, the disease can spread in temperatures up to 90°F. Irrigated fields often have sufficient moisture to provide good conditions for the disease to spread.

By mid-August, disease symptoms can be seen on individuals or pockets of dead or dying plants. These plants will have a white cottony fungal growth on the stems and hard black fungal growths called sclerotia. Sclerotia are the overwintering structures of the pathogen. These growths can be found both on and inside of the stems and upon harvesting will fall to the soil where they can survive for several years.

Management

Resistance

Soybean varieties vary in their resistance to white mold. Selected a rated variety for these areas.

Fungicide Application

Unfortunately, it is now too late for fungicide treatments. Fungicides are only effective during plant flowering.

Cultural Practices

Due to the long survival of the sclerotia, crop rotation is only partially effective. Narrow rows favor early canopy closure, which creates an environment conducive for infection. In some areas, more disease is being observed in narrow rows. Modifications to irrigation during flowering can be effective if cool (ideal) temperatures are present. After flowering has passed, irrigation should focus on ensuring the plant has the proper amount of water. Limiting water will only add more stress to the plants.

Diagnosis and Plant Disease Information

To protect your future investment, always take the time to do proper pathogen identification. If you are uncertain of the cause of damage in your field, I encourage you to have it identified at the University of Nebraska Plant and Pest Diagnostic Clinic.

More information on these and other soybean diseases can be found in the Soybean Section of Plant Disease Management in CropWatch.