Farmers looking to decide when to apply a soybean foliar fungicide can get their biggest ROI from the application by determining the crop’s growth stage, says Matt Duesterhaus, Crop-Tech Consulting research agronomist in Illinois.
A growth stage begins when 50% or more of the soybean plants are in or beyond a specific point of development, according to Iowa State University Extension.
But before you decide when to apply fungicides, university Extension says to evaluate product and application costs, which can run as high as $30 to $40 per acre. Penn State Extension says the realized yield response needed to cover costs may not justify the need for a foliar fungicide, if the risk of disease is not there.
Farmdocdaily, based at the University of Illinois, provides some potential scenarios for soybean growers (and corn growers, too) to evaluate based on yield and prices:
Farmdocdaily adds that while you can figure out the cost of applying foliar fungicides, consider that their application introduces two uncertainties – particularly important to know in a low-price year:
1) the yield response from using fungicides, and
2) the additional drying cost associated with higher moisture due to fungicide use. To learn more, check out the Farmdocdaily article Fungicides Use in a Lower Price Environment.
When Soybeans Make Big Yield Moves
Back to application timing, Duesterhaus says from R2 to R4 is when most farmers are staging soybeans for fungicide applications (as well as insecticide and foliar fertilizer). This period of development typically occurs in later July in the Midwest.
During these growth stages, you start to see some of the biggest yield moves in soybeans – both up and down.
“These pod development stages are about finalizing the potential for soybean yields,” Duesterhaus says. “From R2 to R4, we need to do everything we can to protect the beans so they can hang onto those flowers as pods. Then, in August, we see how we can fulfill the potential out there.”
A Different Recommendation For White Mold
If you have an issue with white mold in soybeans, Mike Staton, Michigan State University Extension, says to evaluate your crop and be ready to apply a foliar fungicide at R1 (one open flower per plant on 50% of the plants).
Staton adds that a second fungicide application made approximately 10 days after the first application may improve white mold control if the weather is predicted to remain cool and wet or humid.
Sporecaster, a relatively new phone app from University of Wisconsin, was created to suggest if a fungicide should be applied for white mold.
The Crop Protection Network has also released its 2025 Soybean Foliar Fungicide Efficacy ratings to help farmers make product selections for all disease issues in soybeans.
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