Several pictures last week and over the weekend of leaves with tan centers and purple to burgundy ring around the outside. These are symptoms of both some types of herbicide injury but frogeye leaf spot as well. With frogeye, conidia will form on the underside of the lesion. One of the ways to visualize them is to place the leaves in a plastic bag overnight and look for “whiskers” (the conidia) on the underside of the lesion the next morning (Figure). Leaves are often damp enough so a wet paper towel is not needed for this, if the leaves have become dry, just rinse them off with tap water, blot them dry, and then place them in the plastic bag.
Frogeye has quickly become an annual problem in Ohio over the past 10 years. Primarily due to the planting of moderate to highly susceptible cultivars. We know that it also overwinters in Ohio, so if you had frogeye develop in a field last year, and it is in continuous soybean, and you did not change your variety or get one with a better resistance package to this pathogen – then it might be a very good idea to scout those fields that were planted in 2019.
Fungicides are effective against this disease, however, we have documented the presence of strobilurin resistance strains of this fungus throughout the state. There are still some pockets where strobilurin type fungicides will still work but these are few. Triazole fungicides are all effective towards this pathogen and based on my testing and that of a lot of my colleagues in the North Central Extension Research Soybean Committee, many are highly effective (Table).
Our studies for effectiveness are based on R3 applications of a few lesions every 100 feet in a field. We do know that R5 applications are too late, and most years only one application is needed in Ohio. Only once have we recommended a spray at V6 and that was when there were 3 to 5 lesions on every plant, a very severe case.