Soybean Disease Update

Alyssa Koehler, Extension Field Crops Pathologist;

The rain and high humidity this week have favored some foliar disease development. Grey Leaf Spot (GLS) is showing up in late-planted corn. In corn that is already past R3, foliar lesions that move in should not significantly influence yield potential. If you have corn at VT/R1 and over 50% of plants have lesions on the third leaf below the ear leaf or higher, a fungicide may be considered depending on the expected yield potential of the field, cost of application, and grain prices. The humid evenings have led to some downy mildew in soybeans. Downy mildew appears as pale-green to light yellow spots that can turn brown to dark brown as they mature (Figure 1). The underside of leaves are covered with tufts of pale to grey sporangiophores (Figure 2). This disease does not typically influence yield. Continue to monitor the weather and scout for foliar diseases like frogeye leaf spot through R5 if you are considering a fungicide application. If disease pressure is low or plants are beyond R5, a fungicide application may not be economical.

Figure 1. Downy Mildew on Soybean

Figure 2. Underside of a soybean leaf with Downy Mildew