Powdery mildew of Soybeans

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.