As soybean farmers deal with the ups and downs of production, figuring out whether to use foliar fungicides is becoming a bigger decision - especially with crop prices and input costs constantly changing.
University of Missouri plant pathologist Mandy Bish has strip trial research that shows spraying fungicides as a routine practice may not always yield a positive return on investment, making it worth farmers reconsidering the approach.
From 2018 to 2024, the Missouri Soybean Merchandising Council sponsored fungicide strip trials across Missouri. The average yield increase for fungicide-treated soybeans was about 1.8 to 3.0 bushels per acre. Given an estimated cost of $40 per fungicide application and soybean prices around $10 per bushel, Bish says that results in net loss of about $10 per acre.
And it is not just in the Show-Me State. Across 13 states, researchers found 90% of the time, applying fungicides results in only a 2.7% yield increase.
These data-driven findings defy the common belief that fungicide applications are necessary for profitability. Rather, in years when soybean prices are down and inputs are up, Bish recommends farmers consider costs versus the potential benefits and consequences.
How widespread are foliar fungicides?
Bish conducted a survey of farmers and found that about 65% of Missouri's soybean acres receive a fungicide application. However, only about 10% of these acres are scouted before spraying, which raises concerns about the thought behind these applications.
While she found that the most common reason to spray fungicides was for frogeye leaf spot management, the ultimate goal remained boosting yield.
Farmers should consider this point when they think about inputs going into next year, Bish cautions, as applying the additional cost of fungicide may not result in profit. However, some farmers need to prove it to themselves.
"We understand that our MU strip trials have not tested every possible combination of product, tank-mix and agronomic practices that an individual farmer may use," she adds. "This could be an ideal year for farmers to set up their own on-farm trials, if done correctly."
Simply applying a fungicide to the center of the field and leaving the border rows as untreated checks is not going to provide reliable data for decision-making, Bish notes. Instead, conducting multiple, alternating strips of fungicide and no fungicide treatments across the field will provide more valuable insights.
Information on the strip trials, including how they are set up, is available on the University of Missouri Extension Strip Trials website.
When should fungicides be used?
Diseases such as septoria brown spot were in nearly 100% of the strip trials, but they rarely cause yield losses unless they reach the upper canopy. Bish notes that while frogeye leaf spot was present in 30% of fields in 2018, the severity levels were too low to justify fungicide treatment.
"None of those disease indexes are worth spraying fungicides for," she adds.
However, there are specific scenarios where treatments might be necessary.
For example, in areas where aggressive diseases like soybean rust are present, a preventative fungicide application can be crucial to prevent yield loss and spread. However, in Missouri, Bish says diseases typically develop slowly, allowing farmers time to assess first rather than preemptively spraying.
One key point Bish emphasizes is that environmental conditions may help mitigate some aggressive foliar diseases in soybeans.
States such as Iowa, where widespread corn and soybean production creates higher inoculum levels, have greater disease pressure. "When we look at Missouri, we have the Ozarks, we have some forest left, and that may dilute the amount of the pathogen we have," she says.
Fungicide resistance, future considerations
Bish is passionate about good stewardship of fungicides in soybeans because it makes financial sense for farmers, and it protects the technology for future use.
The growing resistance of fungal pathogens to strobilurin (QoI) fungicides, particularly frog eye leaf spot, concerns university researchers.
Using fungicides too often when they aren't needed can speed up resistance, Bish explains, which makes the products less effective down the road.
Instead of relying on fungicides, she suggests choosing soybean varieties with built-in resistance to frogeye leaf spot. That way, these treatments stay effective for when they're truly necessary - like during a major soybean disease outbreak.
