Powdery Mildew (Soybeans)

Erysiphe diffusa

Soybean leaves infected by Powdery Mildew
Soybean leaves infected by Powdery Mildew. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
Powdery mildew is a common leaf disease for soybeans and is caused by a fungus. It can easily be identified by its white, powdery coverage of the plant's leaves, stems, and pods.
The fungus thrives in low temperatures and low humidity, usually between 64 and 75 degrees F. Temperatures higher than 86 degrees F will stop any powdery mildew from developing. Introduction can be from long range and sporadic, from either sucking insects or wind-blown spores.
A clear indicator of powdery mildew is the white patches of mycelium and conidia, which develop on the cotyledons, stems, pods, and upper leaf surfaces. Actual symptoms on the plant itself are interveinal necrosis (death), small specks on the leaf that are dead, and crinkling of the leaf followed by defoliation. At times, later in the season, plant leaves simply become yellow and drop off.
Yield losses have been measured to be anywhere from 0 to 10 bushels per acre, depending on location and level of resistance. Soybeans planted later are at a higher risk of susceptibility to yield loss than those planted early.
Planting soybeans earlier is the best way to avoid infection of powdery mildew. Rotation is not an effective method of control due to the remote and sporadic introductions of the windblown spores.
There are fungicides labelled for use against powdery mildew that can be effective. Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.
Most current soybean varieties are highly resistant to powdery mildew. It can be difficult to develop more resistant varieties due to the low or nonexistent pressure by the fungus. Be sure to choose bean varieties that have some level of resistance.