Spotting frogeye leaf spot? Spray or not to spray?

Spotting frogeye leaf spot? Spray or not to spray?

The temperatures and plant stage determine if to spray for frogeye leaf spot.

Late reports of frogeye developing on susceptible cultivars in southern Ohio. 

So the next question is what to do.  During 2005-2008, we were able to measure a mean difference in yield of 5 to 10 bu/A when soybeans were treated at R3 in fields where frogeye was present. 

We have also been able to measure a greater yield difference on highly susceptible varieties when frogeye was present during the early flowering stages. 

However, last year, when frogeye was less than 1 spot per 40’ on a moderately susceptible variety, and the conditions were very dry and warm over the next 2 weeks, we could not measure any yield effect – although we could measure differences in the level of disease.

Conditions for frogeye leaf spot are cooler (below 80 degrees F), heavy dews, and frequent rains. 

Frogeye has a long latent period of 7 to 10 days, which is the time from infection to symptom development and sporulation. 

Interestingly, it will only infect new foliage, not the older foliage in the lower canopy. 

If frogeye can readily be found in the field, indicative a highly susceptible variety then:

If your soybeans are in the later growth stages (R4 & R5 or greater), and the beans are finished flowering and beginning to fill, there is no need to spray. 

What lesions will form will be few and will have little impact on yield.

If your soybeans are at R3, they still have more growth to fill out, it is a high yielding field, rains and favorable conditions for infection, our data indicates that you should be able to measure yield differences of 5 bushels or greater. 

The next question is which fungicide to use. 

There are many populations in the region that are no longer controlled by the strobilurin fungicides. 

Therefore, any fungicide that is used, should not be one of these, and that leaves primarily the triazoles. They are all effective, so price of fungicide and application costs should be a consideration here.

Finally, leave check strips. And leave at least 3 untreated strips and make notes on how much disease was present and the growth stage. This will improve decision making in the future. 

Originally posted by Ohio State University. 

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