Soybean growers need to be aware that recent weather conditions have been favorable for possible development of sclerotinia stem rot in soybeans. In the following paragraphs, Anne Dorrance, OSU Extension soybean pathologist, outlines the disease potential and steps growers can take to reduce the impact of this disease.
The cool nights and wet conditions over the last couple of weeks are very favorable for infection of Sclerotinia sclerotiorum, the fungus that causes Sclerotinia stem rot or white mold of soybean. This is particularly concerning if you have a field with history of this disease combined with a susceptible cultivar. Note that not every field in Ohio has inoculum. Sclerotia must be introduced into a field with contaminated seed or from another susceptible crop.
Infection is favored by narrow row spacing (7 - 15 in), high plant populations, and canopy closure at or prior to flowering. Apothecia are reproductive structures that look like little mushrooms that are the size of small erasers. Spores released from apothecia land on dead flowers and can infect the plant. Symptoms generally develop 2-8 weeks following infection.
If a susceptible cultivar was planted and the cool nights, rain and heavy dews continue to occur, consider a fungicide application at the beginning of flowering stage (R1) to full flowering (R2). If a resistant cultivar was planted, some disease will develop but it will be below the economic threshold. In most cases, incidence of 20% does not result in yield loss as the plants that are healthy next to the disease plants will compensate. Work with your seed supplier to determine if the variety planted in the historic white mold fields has a good resistance score. Under Ohio conditions, a good resistance score means a fungicide application is not needed. Fungicides are only necessary if the field has a history of the disease and a susceptible cultivar was planted. In addition, if the fields are past flowering (late R2); it is too late for a fungicide application for white mold.
An updated white mold of soybean fact sheet is available on-line and contains more detailed information: http://ohioline.osu.edu/factsheet/plpath-soy-3