Frogeye Leaf spot

Will fungicide resistant frogeye be an issue for Ohio in 2020?

By Matt Reese

With growing concern about fungicide-resistant frogeye leaf spot in some parts of the Corn Belt, farmers in Ohio may wondering if this will be a management issue with soybeans in 2020. This could be especially problematic after the fairly mild winter conditions that may set up potential problems for the 2020 growing season.

“Frogeye leaf spot has now become a recurring problem for soybeans in southern up to central Ohio. High levels of inoculum — lots of leaf spots — in the fall can overwinter in Ohio, so this is especially important for those fields that are continuous soybean,” said Anne Dorrance, Ohio State University Extension plant pathologist. “The first thing is if you had frogeye at the end of the season in 2019, please do not plant the same variety back in that field. I do that to create the best opportunity for our research plots to develop disease for fungicide studies, and since I have that covered, you don’t need to do that. Any frogeye in fields in 2019 — conditions were not as favorable as previous years — means it’s time to choose something with better resistance scores. A resistant cultivar will not develop frogeye, so no yield hit and no added input costs for fungicides if conditions are favorable for disease to develop.”

Varietal resistance has added importance in the case of frogeye leaf spot that is resistant to the QoI class of fungicides (Group 11) that includes strobilurins. This fungicide-resistant frogeye leaf spot pathogen has been confirmed in 19 states, including Ohio. While the fungicide resistance problem is at severe levels in some areas further west and south where the susceptible populations of frogeye have all but disappeared, that is not the situation in Ohio.

“As always, Ohio has been a bit different, but we have very low sample size. We can still find susceptible strains,” Dorrance said. “Last year, frogeye was fairly rare in Ohio and Linda Weber, a graduate student on the project, did find susceptible strains.”

There are fungicides outside of Group 11 available with fair to very good efficacy on frogeye, Dorrance said. To be safe when spraying for frogeye, choose a product that has another mode-of-action, such as a triazole. Dorrance also recommends if the soybeans are sprayed, to leave unsprayed check strips in at least three separate locations in the field and collect the yield off each separately to compare them with fungicide sprayed strips.

OSU Extension summary of data for the last 3 years- R=all isolates from location were Resistant (R), all isolates were susceptible (S) or a mixture of resistant/susceptible (R/S).


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