Late Season Treatment not Advised for Frogeye Leaf Spot in Soybeans
August 25, 2011
Frogeye leaf spot is more common than usual this year in several areas of Nebraska. Some fields scattered over the eastern third of the state have fairly severe levels of infection, but most have minor levels.
Infection can occur at any stage of soybean development, but most often occurs after flowering, typically in the upper canopy. Initial symptoms are small, dark spots on the leaves. Spots eventually enlarge to a diameter of about ¼ inch and the centers become gray to brown and have a reddish purple margin. Individual leaf spots can coalesce to create irregular patterns of blighting on the leaf.
As the soybean leaf matures, it becomes less susceptible to infection. That's why damage is generally confined to the upper canopy. In many fields this makes the damage appear much worse than it actually is as all the symptom is on top.
Early in the season when soybeans were at the R3 to early R4 growth stages and frogeye was present, some fields were treated with a fungicide. If you are seeing frogeye develop now, I would not recommend a fungicide to manage the disease.
Frogeye leaf spot is well controlled with any of the strobilurin fungicides labeled for use in Nebraska. Be aware that in other states isolates of the fungus causing frogeye leaf spot in soybean have been found to be resistant to strobilurin fungicides. Resistance was identified in southern Illinois, Kentucky, and Tennessee. We have not observed this resistance in neighboring states, but you should be aware of the possibility, given this is an airborne disease.
Frogeye leaf spot can survive on residue. In future years, if significant development occurs, consider a fungicide application and use of resistant soybean varieties to manage the disease.
Extension Plant Pathologist