Long-term Studies: Do Insurance Applications of Foliar Fungicides Provide Benefits in Corn and Soybeans?

By Bruce Potter 

Insurance applications of foliar fungicides to soybean – do they provide a benefit?

To help determine the yield and economic benefits of insurance applications of foliar fungicides applied to rotated soybeans, studies funded by the MN Soybean Research and Promotion Council, have been conducted since 2016 at three University of Minnesota Research & Outreach Centers (ROCs) located across southern Minnesota. These conventionally tilled study sites were not selected based on the expectation of a particular disease (e.g., Sclerotinia white mold, frogeye leaf spot). In 2021, two foliar fungicides (Miravis® Neo and Delaro® 325) were applied to three soybean varieties (1.5, 2.0, 2.3 RM) at the R3 growth stage. Varieties and fungicides varied by year. The fungicides were compared to an untreated control with respect to their effects on soybean disease and yield.

An early-season drought affected all three sites but moderated at different times during the season. Higher yielding sites had an earlier onset of rain. Stem and foliar disease pressures were at the lowest levels since we initiated these multi-site studies in 2016. While varieties varied in yield, no significant yield differences (p= 0.10) among the untreated check and the two fungicides were observed.

Before 2021, in trials conducted from 2016-2020, one or both fungicides had a significant ( p=0.10) positive yield response in 10 of 13 studies (77%). When the results from 2021’s drier early and midseason weather are combined with previous years, only (62%) of the studies had a significant yield response.

These data show that foliar fungicides can help maintain soybean yield in some southern Minnesota environments and provide economic benefit if used selectively. However, consistent, profitable yield responses are unlikely to be obtained when, as in this study, applications are not targeted to specific environments and diseases. They do not provide evidence that insurance applications of foliar fungicides will compensate for bad weather, drought, or poor agronomic decisions. The 2021 data do not counter the hypothesis that grain moisture and harvestability influence yield responses to foliar fungicides when disease pressure is low.

What about corn?

Similar fungicide studies in rotated corn, funded by the MN Corn Research and Promotion Council, have been conducted at several University of Minnesota ROCs since 2017. During 2020 and 2021, a corn-corn rotation was added at Lamberton and Rosemount. In 2021, two foliar fungicides (Miravis® Neo and Delaro® 325) were applied to three corn hybrids at the VT stage. Both sites experienced reduced yields due to drought stress. The corn-corn rotations yielded less than rotated corn at both locations and the continuous corn yields at Lamberton were very low.

In the corn-corn rotation, hybrids differed in yield at Lamberton and overall but not at Rosemount (p= 0.10) but the highest and lowest yielding hybrids differed by location. Fungicide-treated plots yielded numerically lower at both sites and overall. None of the four corn-corn site years showed a yield benefit for fungicide application. Not unexpectedly due to maturity differences, hybrids differed in moisture at both locations and overall.

In the corn-soybean rotation, hybrids differed in yield at both locations and overall. Fungicides did not significantly affect yield at either site, with fungicide-treated hybrids yielding numerically lower at the severely drought-stressed Lamberton site. Fungicide treatments yielded numerically higher at Rosemount. Overall, there was a significant yield difference among the untreated and fungicide-treated plots, but this differed among sites and hybrids. Across the three hybrids, one fungicide averaged 4.2 bushels less and the other averaged 5.0 bushels more than the untreated hybrids. Since 2017, only two of the twelve soybean-corn site-years (17%) have shown a significant (p-0.10) response to fungicide.

Similar to the results obtained from soybean studies, these data indicate that insurance fungicide applications to corn are unlikely to provide a yield benefit when in the absence of a controllable, yield-limiting fungal disease (e.g., tar spot, northern corn leaf blight).

Foliar fungicides can be targeted toward known or expected diseases based on field history, weather, or symptoms with some expectation of yield protection. However, in many cases, fungicides are applied before the type and severity of the disease are known. Analyses correlating yields of these longer-term studies with factors such as planting date, seasonal rainfall seasonal temperatures may provide clues to help growers increase the probability of positive economic returns for their fungicide applications.