I must admit that I am a bit surprised at how slow frogeye leaf spot, a foliar leaf spot of soybean, was to show up in fields this summer. Early indications were that it was going to get a good early start. But scouting of fields throughout the state – it was hard to find. It took me 90 minutes before a crop walk in western Ohio in July to find 2 spots! Last week a couple of calls came in, where folks found large round gray lesions on the new leaves. Based on the size of the lesions – it was clear that this was a susceptible cultivar. These lesions were also only on the new leaves on plants in the midst of pod fill. This is actually more typical of when we would see frogeye, late in the season and at these late growth stages at these low inoculum levels, it does no damage. A few spots on a few leaves on the top of the plant, scattered throughout the field will not cause yield loss. More importantly, the disease cycle for 2017 is finished as no more new leaves or very few new leaves will develop on the plant, and these are the susceptible parts of the plant. So no fungicide is warranted at these late growth stages. Where we have measured substantial yield losses is when there are lesions, throughout the canopy, mid-to-upper canopy leaves all have many leaf spots.
We are still monitoring these fields to measure the amount of fungicide resistance to the strobilurins we have in Ohio populations of the fungus, Cercospora sojina, which causes this disease on soybean. So if you do come across some lesions, please send them to the lab. These lab tests are paid for by the soybean check-off through Ohio Soybean Council.