Downy Mildew (Soybeans)

Peronospora manshurica

Soybean leaf infected by Downy Mildew
Soybean leaf infected by Downy Mildew. Photo courtesy by Daren Mueller. Licensed under CC BY 3.0.
Downy mildew is a common diease in soybeans but is rarely known to cause severe losses. The greatest issue caused by this disease is related to seed quality, often due to the plant being defoliated or the pods infected.
Downy mildew is caused by a fungus that survives in infected leaves or seeds. Infection usually occurs during flowering, often beginning at the top of the plant with the new leaves. Development may be enhanced during times of high humidity and mild temperatures.
The first symptoms occur in the tops of the plants. Pale-green or yellow spots develop on the tops of the leaves and enlarge into pale to bright-yellow or brown spots. Sometimes the spots look gray and fuzzy if seen from below. The center of each spot turns brown and the margins remain yellow. At times, the spots grow together to form larger infected spots that eventually turn brown. Infected seeds become a pale-white and are covered in spores.
Downy mildew as compared to powdery mildew
Early stages of downy mildew on soybean leaf. Photo courtesy of Clemson University. Licensed under CC BY 3.0.
Overall damage of downy mildew is not nearly as extensive as its physical presence. In fact, there is generally no substantial yield loss, even in high concentrations. Control is necessary in that downy mildew can negatively affect the quality of seed or food-grade beans.
Preventative measures for downy mildew are only important if the quality of seed is imperitive. Proper tillage of infected crop residue can effectively control populations of downy mildew spores. Rotating in nonhost crops can prove to be an effective strategy as well.
Fungicide seed treatment has been known to be effective in combatting downy mildew. Note that commonly used fungicides for soybean are unable to control downy mildew. 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.
There are resistant varieties of soybeans, but it is important to consider resistant varieties for more urgent diseases and pests.