By Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio Soybean Council and Soybean Check-off
The weather in 2023 allowed for soybeans to maximize their yield potential in many parts of the state. In other areas farmers experienced a challenging growing season for soybeans. “Much of the state started off dry. Dry weather is typically not good for soybean disease development; however some pathogens produce structures to help survive these conditions. That dry period was followed by a lot of rain during flowering which created a humid environment for the diseases to show up,” said Dr. Horacio Lopez-Nicora, Assistant Professor, Soybean Pathologist and Nematologist with The Ohio State University. “Parts of Ohio experienced disease pressure from white mold to the extent that it dramatically impacted yields. For disease a to occur we need three conditions to be met at the same time. In plant pathology we call this the disease triangle. Those components include a favorable environment. We need a virulent pathogen, in this case the pathogen that causes white mold. We also need a susceptible host, which was the soybean crop.”
“White mold is a fungal pathogen and it causes the white mold growth that is observed on the stem on infected plants,” said Dr. Wade Webster, Assistant Professor, Soybean Pathology, North Dakota State University. “One of the challenges of this particular pathogen is that it is able to infect a wide range of crops and weeds.”
White mold infection is highly dependent on the weather conditions. “White mold needs moisture, especially during the flowering period. It also needs cool conditions at night (50 to mid-70’s),” said Webster. “Once we reach canopy closure, the moisture that is in the canopy is maintained, even if it is dry outside and has not rained for several days. The plants respire and breath out water moisture that remains under the canopy of the crop. The canopy reduces the wind flow and prevents the evaporation of that moisture and maintains the high humidity. It also provides shade keeping the temperature cooler inside compared to outside the canopy. There are also certain environmental light conditions we see with canopy closure that can lead to the apothecia forming leading to disease development.”
White mold can survive in the soil for up to 10 years as sclerocia. “These are very small, hard black structures,” said Webster. The sclerocia form little cups that are flat on the surface called apothecia. The apothecia are the source of the spores that cause the white mold infection. When the spores are released around the flowering period they will land on and attack the flower tissues, often when the flowers are starting to dry up. This occurs around full bloom (R2-R3 growth stage).”
Once infection occurs, it takes 2-3 weeks for symptoms to develop and be observed. “The first symptom of white mold is the white fungal growth/mold mass that is seen on the stem. Farmers may also observe a bleaching of the stem,” said Webster. “There is also the development of new sclerocia on the outside of the stem. If the white mold development is severe enough the plant will begin to shut down, wilt and prematurely die. This leads to reduced grain fill and lower yields.”
The white mold development information is from a program conducted by Lee Beeres, Assistant Professor and OSU Extension Educator, Trumble County along with Dr. Horacio Lopez-Nicora, Assistant Professor, Plant Pathologist and Nematologist at The Ohio State University, and Dr. Wade Webster, Assistant Professor, Soybean Pathology, North Dakota State University.