Over the last 10 days the weather in Iowa has been favorable for the production of soybean white mold mushrooms. Those are the mushrooms that cause white mold disease to show up in soybeans. However, the level of occurrence of white mold disease in soybeans is likely to be less than last year, says Iowa State University Extension plant pathologist X.B. Yang. He and his ISU associates—SS Navi and Linus Li—ISU provide the following update.

Soybean White Mold Mushrooms

“On July 7 we visited fields near Clear Lake in north central Iowa with crop consultant Dan Muff and found abundant white mold mushrooms in a continuous soybean field, which had a closed canopy with wonderful growth,” says Yang. “The density of the mushrooms was very high, 3 apothecia/square foot, and soybean plants are likely to be infected. The early showing in such a large number in this field was due to early planting, good growth and a higher number of sclerotia from last year. According to Dan, this field had very bad white mold infestation and this year the field was planted April 29.” 
 
“The level of soybean white mold occurrence this year is likely to be less than last year, because last year we had a record cool July,” says Yang. “Statistically, the chance to have two years in a row with record cool temperatures is unlikely, although early July in this season has been cool and wet in Iowa. Because white mold can continue to attack soybean in August, the weather conditions in part of August are also factors that impact the outbreak of soybean white mold.”

White mold mushrooms aren’t very big. They are already showing up in some soybean fields as of July 7 in Iowa. Photo credit: SS Navi and XB Yang

The risk of white mold varies from field to field, in addition to flowering weather conditions. “The field we visited was planted April 29, had early canopy closure and was loaded with sclerotia, representing a high-risk situation. If white mold risk is high in your field, consider using a foliar fungicide to protect your soybeans,” advises Yang. For more on this topic, see the previous article on white mold control for chemical information.
 
“We also found many white mold mushrooms in a cornfield,” says Yang. “Crop rotation with corn to promote white mold mushroom production is a good measure to reduce white mold risk for next year’s soybean crop in the same field. However, white mold spores produced in a cornfield can travel up to 45 yards, with most infected plants in areas adjacent to the cornfield, according to a study from our lab in 1995.”

The study was done in two locations, one in Humboldt and one near Ames. For the field at Ames, spores traveled further. When white mold spores land on soybean plants that are flowering and environmental conditions are right, they can infect soybean plants, explains Yang. The effects on soybean yield by spores from cornfields are yet to be determined. These infected plants, however, produce sclerotia, which are the source of future infections. This presents a challenge: how to balance management practices of using corn rotation to reduce white mold risk.