Frogeye leaf spot is easily recognized by the circular to angular spots on the upper and lower leaf surfaces. These spots are usually surrounded by dark, reddish-brown margins and can be found throughout the canopy. If these spots are numerous enough to cover 30% of the leaf area, leaves will quickly wither and fall off of the plant prematurely. This is a serious disease and can cause substantial yield loss, so keep a close eye on your fields.
Many, if not most of our soybean varieties have some resistance to the disease. If the variety is labeled as “resistant”, you’ll usually not experience the disease. However, I have seen and continue to see significant disease on varieties will “moderate resistance”, especially in non-rotated fields.
The main question that I’m getting now is “How late is too late to spray for frogeye leaf spot?” First, check the fungicide label that you intend to use. Applications with some fungicides must cease when the soybean reach the R6 stage. Others may have a 14 to 21 day pre-harvest interval. The R6 stage will last nearly 3 weeks and the R7 stage will last another week to 10 days; so the pre-harvest interval is not my major concern. Regardless, make sure that you follow the label.
Assuming that there are no label concerns with fungicides, my inclination is to spray the field, especially if yield potential is good. My concern with not treating is that if weather conditions are favorable, the infected areas will produce more spores and cause additional infection. Keep in mind that at R5, only about 25% of the yield has been made; at the start of R6, only 50% has been made. So, from this standpoint, there’s still lots of yield to lose if the problem were to get worse.
If you decide to spray, I highly recommend a combination product such as Priaxor, Quilt XL, Quadris Pro, or Stratego XL. There are reports of failures to control frogeye with only strobilurin fungicides (Headline, Quadris, Approach). We have not documented this, but using a combination product may help prevent. In addition, a strobilurin + a triazole (or carboxamide in the case of Priaxor) should provide better control.