Frogeye leaf spot (FLS) is caused by a fungus (Cercospora sojina) which can overwinter on soybean debris and can be introduced on infected seed. Symptoms usually appear during reproductive growth stages and include circular to angular leaf lesions with a purple to dark brown margin around the tan/gray center. Lesions begin as dark water-soaked spots on younger leaves and as they age the centers become ash-gray to light-brown. Examining the underside of leaves, FLS lesions will have a tan to gray center and if sporulating there will be a dark black area in the center of the lesion where spores are being produced.
Fungicide application during reproductive growth stages can help reduce disease and protect yield, although caution is advised as fungicide resistance to the QoI/Strobilurin group has developed. Links to Fungicide Efficacy Table – mobile version or pdf
The fungus that causes frogeye leaf spot (FLS), Cercospora sojina, has developed resistance to the QoI/Strobilurin fungicide group (FRAC 11). This means if a QoI/Strobilurin fungicide is applied to a field that contained the resistant fungus the disease will not be controlled and yield may be lost.
We are currently screening FLS samples to better understand the pathogen population and assess the risk of fungicide resistance. Producers in Tennessee are encouraged to contribute to this effort by mailing in soybean leaves with frogeye leaf spot symptoms along with the information requested on the ‘Frogeye Leaf Spot Sample’ sheet. Samples will be tested, free of charge, for strobilurin/QoI fungicide resistance and results will be emailed to producers. Funding was provided by the Tennessee Soybean Promotion Board for this service.