Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
You may be seeing some spots and lesions on the unifoliates or first 1-2 trifoliates in full season soybeans. At first look you may think these look like Frogeye leaf spot (FLS) lesions. We have seen FLS early in no till fields of continuous beans in the past, and variety plays a big role in symptom development and expression. However, there are some chemicals that can cause similar looking symptoms in soybeans. Here are some clues that can help you determine if you are not dealing with FLS: 1) There an obvious pattern to the symptoms; 2) The entire field is affected; 3) Nearly all plants show symptoms of disease somewhat uniformly, it’s everywhere.
If you really want to see if you have FLS, look at the undersides of the leaves. On the underside of leaves infected with FLS you will see dark spots and if you are lucky you may even see long, silvery spores with the aid of a hand lens. This makes the lesion look as if it has whiskers (Figure 1). Regardless, keep an eye on the field and see if the symptoms progress or stay the same.
Figure 1. Closeup of Frogeye leaf spot lesion. Note purple boarder and grey/black mass at center of lesion. If viewed from the side, this black area will appear fuzzy or will have “whiskers.”