Alyssa Koehler, Extension Field Crops Pathologist; firstname.lastname@example.org
Grey leaf spot (GLS) has remained the primary disease showing up in corn. If GLS lesions move into corn that is already past R3, yield potential should not be significantly influenced. If you have corn that is at VT/R1 and over 50% of plants have lesions on the second leaf below the ear leaf or higher, a fungicide may be considered, depending on the expected yield potential of the field.
Over the past week, downy mildew has been present in soybeans. Downy mildew appears as pale-green to light yellow spots that can turn brown to dark brown as they mature (Figure 1). If you flip the leaf over, the underside is covered with tufts of pale to grey sporangiophores (Figure 2). Under the microscope, these sporangiophores are branched and spores called sporangia are visible (Figure 3). This disease does not typically influence yield. Continue to monitor the weather and scout for foliar diseases like frogeye leaf spot through R6 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. Pale yellow lesions of Downy Mildew on Soybean
Figure 2. Underside of a soybean leaf with Downy Mildew
Figure 3. Sporangiophores and sporangia of Peronospora manshurica, causal agent of soybean downy mildew
A great new Disease Severity and Defoliation Training tool is available from the Crop Protection Network https://severity.cropprotectionnetwork.org/. This tool has features to practice rating foliar diseases and insect defoliation of soybean and corn as well as ear rot of corn. Ten question quizzes allow you to train your eyes for more accurate ratings (Figure 4).
Figure 4. Screenshot from the Crop Protection Network Disease Severity and Defoliation Training tool for soybeans