Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Downy mildew is showing up in Delaware soybean fields with the increased humidity and shower activity. Downy mildew is a common disease of soybean that occurs wherever soybean is grown. Fortunately, however, it rarely affects soybean productivity in Delaware. The downy mildew fungus, Peronospora manshurica, is biotrophic, which means it can only grow in association with the soybean plant. Because of this very close relationship with the soybean plant, it is capable of rapid genetic change in response to genetic changes in soybean. Thirty-three races are described for P. manshurica and the number of described races will likely increase as research continues.
Downy mildew appears on the upper surface of young leaves as pale green to light yellow spots which enlarge into pale to bright yellow spots. The spots look slightly gray and fuzzy when viewed from below, especially during periods of high relative humidity. (See the following photos) Younger leaves are more susceptible to downy mildew than older leaves. Occasionally when conditions are very favorable for disease and the variety is susceptible seed infection can occur which produces a dull white crusty coating of spores on the seed. Fungicide control is rarely needed.
Downy mildew on soybean (upper surface)
Downy mildew (lower surface)