Frogeye Leaf Spot - A Soybean Disease to Be Aware Of

A soybean disease that is causing more concern in Nebraska is Frogeye Leaf Spot. The past 10-15 years frogeye leaf spot sometimes caused issues here in southeast Nebraska, but it is showing up more in southeast Nebraska and the chances are great that it is resistant to the Qol (Group 11) fungicide which includes Strobilurins. Quadris is a popular fungicide that Frogeye Leaf Spot has been found to be resistant to. In my April issue of this newsletter, I addressed this issue of frogeye leaf spot in soybeans.  In 2019 ten counties in Nebraska have identified soybeans that have been infected with Frogeye Leaf Spot resistant to fungicides.

In 2020 Extension Educators across Nebraska sampled fields for frogeye leaf spot to determine how widespread resistance is to QoI or group 11 fungicides. Out of 375 samples from 128 fields in 48 counties, they all were resistant to group 11 fungicides. This is major resistance to a fungicide. This doesn’t been that this fungicide cannot be used, but it should be used in combination with other fungicide groups if you want to control frogeye leaf spot. Recommendations for control of frogeye leaf spot include: 1) plant resist varieties, 2) use longer crop rotations and 3) use fungicides with active ingredients from 2-3 effective classes and modes of action. The Crop Protection Network has a good publication on the efficacy of various fungicides on control of different soybean diseases, including frogeye leaf spot. You can find this publication at this site: . It shows still good control of frogeye leaf spot, with combination of fungicides. Fungicides such as; Fortix SC, Preemptor SC, Delaro325 SC, Lucento 4.17 SC, and Revytek had good to very good control of frogeye leaf spot. In trials conducted in 2020 -2021 at (HAL) the Haskel Ag Lab at Concord, NE and SCAL, South Central Ag Lab, at Clay Center, combinations of the different classes of fungicides decreased incidence of frogeye compared to group 11 alone. Group 11 fungicide treatment still had significantly less disease than no fungicide treatment. In 2021 at HAL, there were no significant differences in soybean yields between untreated and fungicide treated soybeans, all yielding between 65- 70 bu/ac. At SCAL there was a significantly higher yield from soybeans treated with the fungicide Delaro Complete, 88 bu/ac vs 81 to 84 bu/ac for the other fungicide treatments, with the untreated soybeans yielding 82 bu/ac.

As soybeans are flowering and moving into their reproductive stage, it is important to scout fields for infection of frogeye leaf spot and other soybean diseases. While it may not be necessary to treat every field with a fungicide, soybean fields and/or soybean varieties that have shown susceptibility to disease issues in the past may benefit from a fungicide application.  Much research has been done on fungicide timing and applications in the R3 (pod set) growth stage have shown the best results. This may extend into the R4 stage if we have dryer conditions followed by rain later (shift in when the diseases develop).                  

If you have questions about diseases in soybeans or fungicides, feel free to contact me at , (402) 274-4755 or (402) 274-9639 (cell).  

Gary Lesoing
Extension Educator
Nemaha County
July 2022