Powdery mildew in soybean is caused by the fungus Microspaera diffusa. The disease is easy to identify by white, powder-like patches that form on cotyledons, stems, pods, and leaves. Some varieties may develop chlorotic leaves, and in some cases cause defoliation. This early defoliation is usually caused by another disease, but powdery mildew compounds the problem.
The fungus develops under cool temperatures and low humidity. You will normally see this disease when temperatures are 64ºF to 75ºF. When temperatures are higher than 86ºF, powdery mildew will not develop. Since cool temperatures during the growing season are not common in Iowa, this disease is not seen that often. The last time we saw it significantly in Iowa was in 2004.
Management of Powdery Mildew
Powdery mildew is frequently more severe in late-planted soybeans.
Although there is no soybean variety that is completely resistant to powdery mildew, some varieties are much more susceptible than others. However, it's hard to evaluate or breed soybeans for resistance to powdery mildew because disease pressure is so low in the midwest.
Crop rotation is not effective because the wind-borne fungus can spread over large distances. There are fungicides, such as Topsin M, that are registered for powdery mildew and are effective in controlling the disease. However, because the disease is not common and generally does not cause yield loss, foliar fungicide treatment is not usually recommended.
Powdery mildew of soybean - printable pdf file from the University of Wisconsin