This year we seem to be seeing more frogeye leaf spot on soybeans than we usually do here in Pennsylvania. There may be several reasons for this. Because the fungus that causes frogeye survives in plant residue, it may have overwintered better this year than in previous years. Places with extended snow cover could have protected the fungus by insulating it. Then, warm and humid conditions through the first half of the bean season would have encouraged growth, sporulation and infection. The earlier infection starts in the field, the more opportunity the fungus has to infect more bean tissue throughout the season. Once a frogeye lesion is detected in your canopy, the fungus within will create spores if the weather remains warm and humid. These spores can then move through wind and rain splash to new leaves and cause new lesions, or blow to other fields during storms.
The disease is fairly easy to recognize once you know what to look for. Discrete, roundish lesions with light centers and dark reddish-purple margins are apparent on leaves (Fig. 1). Upon close inspection with a hand lens, dark fungal sporulation may be seen emerging from the center of these lesions. These are the spores that may then blow to other leaves to cause new lesions (Fig. 2).
It is important to pay close attention to timing when it comes to frogeye leaf spot. This means we have to consider the timing of bean growth stage, arrival of disease, and fungicide application. The fungus that causes this disease is only capable of infecting new leaves--it can't get into a fully expanded leaf. Because of this fact, an infection that starts late (say, R4 or R5) will not have the opportunity to spread very much since most of the leaves will be already formed and, therefore, resistant.
Typically, we don't see this disease starting early enough or becoming severe enough to warrant a fungicide application in PA. However, this year that might be a tougher call. If your beans are still early, scouting by the R1 growth stage will give you the notice you need to plan for a fungicide application if enough disease is present. Getting a fungicide on will have to happen by R3 for it to be effective. After R3, we don't see any yield benefit compared to not treating. There is no formal spray threshold for this disease, although OSU plant pathologist, Anne Dorrance, has reported her best yield response when a fungicide is applied when 1 lesion is seen per 25 sq ft at R1/R2. This is probably a too low for our typical disease pressure in PA. You should consider the stage of your crop, the price of beans, the cost of application and the purported level of resistance of your bean variety. There can be quite a bit of difference in response to fungicide application among varieties, and if you are growing one with high levels of resistance to frogeye, you most likely won't see a benefit from application.
If you do decide to apply a fungicide, please follow the labeled rates for frogeye leaf spot management. Applying at too low a rate will fail to control your problem, and will increase the likelihood that the fungus will develop resistance to the fungicide. There is widespread resistance of this fungus to the strobilurin fungicides in the Midwest already--we don't want to encourage that to develop here. This chart can help you identify a good product for frogeye and other soybean diseases.