Late Season white mold in some Ohio soybeans

By Matt Reese and Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio Soybean Council and Soybean check-off.

In parts of Ohio, the soybean crop is feeling the pressure from white mold. In the Northeast corner of the state, Ashtabula County, the dry weather and intense heat in early September caused some bean fields to shut down and lose their leaves. Fields there suffered from too much moisture early on and now pressure from white mold are bringing concerns of yield losses. 

“The fields that were stressed from too much moisture never recovered and white mold is terrible,” said Jeff Magyar, Ashtabula County Farmer. “The white mold can be seen in 25% to 30% of the soybean acres just driving by the fields.”

A similar story is being told on the west side of Ohio, in Mercer County, as white mold appeared late in the season. With foggy mornings, white mold moved into some fields and is causing potential yield loss concerns. 

“We had some fields where white mold showed up late and will probably cause some yield loss,” said Kyle Nietfeld, Mercer County Farmer. “It’s not widespread in the county, but you can see patches in fields, especially those in river bottom areas.”

White mold is a fungus, Sclerotinia sclerotiorum. It is easily identified by the white, cotton like mycelia and hard, black sclerotia on infected stems. 

“White mold is a mono-cyclic disease,” said Horacio Lopez-Nicora, soybean pathologist and nematologist at The Ohio State University. “When you see the plant dying, there is nothing you can do. The spread of white mold between plants is limited to only what may be touching the mycelium.”

The initial infection occurs at flowering, between R1 and R3. 

“Spores are released from the sclerocia in the soil and contact the blossoms at the nodes during flowering,” Lopez-Nicora said. “The fungus then colonizes the plant. The leaves will turn gray then brown but remain attached. The black sclerocia form and can cover the stems and sometimes will be on the pods as well. These sclerocia are spread across the field and to other fields on the crop residue during harvest by the combine. They also will mix in with the harvested seed, and if not cleaned, can be spread during planting.”

When the soybean crop develops and canopy closure occurs, the moisture retained under the canopy fosters a favorable environment for the sclerocia to become active. Cool night temperatures and rain or fog favor the disease. Sclerocia can survive dry conditions and then become active when the environment is favorable. 

“It seems counterintuitive, but deep tillage and burying the sclerocia make it more likely to survive compared to shallow tillage or no-till,” Lopez-Nicora said. “Most of our yield enhancing practices such as early planting dates, narrow row width, and even high nutrient level soils favor white mold populations.”

Broad leaf weeds and certain cover crops can also host the fungus. 

“Sunflowers and canola (rape) are both host crops,” Lopez-Nicora said. “Crop rotation is an important management strategy, but also being aware of the weeds and cover crops that can host white mold is also important. There is no high level of resistance soybean varieties available. There are several moderately resistant varieties that can be selected.”

Seed treatments next spring may also be a good management practice for preventing white mold problems. 

“The seed treatment does not change the seed bed environment, but if the seed was produced in a white mold infected field, there may be white mold sclerocia mixed in with the seed, and the seed treatment will protect against the spread if the seed is contaminated,” he said.  

Lopez-Nicora is conducting a fungicide timing efficacy study. 

“From our study, the timing of fungicide application to best protect the flower from being infected by the spores is R1 to R3. Most farmers typically apply fungicide from R3-R5,” Lopez-Nicora said. “After R3, a fungicide application is generally not effective in protecting against white mold.”