Consider ROI before applying fungicides

COLUMBIA, Mo. – Before applying fungicides this season, growers should carefully consider the potential return on investment (ROI), says University of Missouri Extension state plant pathologist Mandy Bish.

“We have a habit of assuming ‘yield increase’ and ‘return on investment’ are interchangeable,” says Bish. “That is not always the case.” In years where disease pressure is low, the cost of application may outweigh the benefits, she says.

As of early March 2026, Missouri cash bids for No. 2 yellow corn are hovering around the $4 per bushel mark. With low prices like this, corn growers must pay extra attention to the timing of fungicide applications, says Bish.

Tar spot vs. southern rust dilemma

Tar spot has been confirmed during June in Missouri over the last three years. Bish urges growers not to panic at those early sighting. The fungus favors mild temperatures (64-73 F) and humidity below 90%, and it typically slows its growth in July.

The same July temperatures that restrict tar spot growth can create a favorable environment for southern rust when moisture is present. The last two years southern rust has moved north, creating challenges in some parts of Missouri.

“If we spray too early for tar spot, we risk having no residual protection if southern rust moves into the area,” says Bish.

Most university research shows a positive ROI when treating tar spot–infected fields with a single application between VT/R1 and the R3 growth stages. Morgan Goodnight, a Ph.D. candidate at Purdue University, indicated that growers need to prevent at least 7.5 bushels per acre of yield loss to pay for a single fungicide application. If a second pass is made, the economic threshold increases to 15 bushels per acre.

The Crop Protection Network, a project of land-grant universities, says late-season-spraying rarely benefits yields. Once corn reaches the R5 (dent) or R6 (black layer) stage, spraying is not beneficial; applying a fungicide this late in the season will not “cure” already-diseased plants. The degree of yield protection will depend on the level of disease in the crop, susceptibility of that hybrid to tar spot (and other diseases), and upcoming weather conditions.

Applications to soybean

We rarely see such aggressive foliar diseases in Missouri soybean, where farmers apply fungicides to approximately 65% of soybean acres, says Bish. By contrast, in 2004, before soybean rust entered the United States, fungicides were applied to only 1% of Missouri soybean fields.

“Across 66 on-farm strip trials, the average yield increase following a fungicide application was 1.8 bushels per acre,” Bish notes “Realistically, with current prices, a grower needs to see more than 3 bushels per acre to approach a positive ROI.”

Bish isn’t asking farmers to take her word for it. She would like farmers to test the practice for themselves.

“I want farmers to confirm they are seeing the yield bump they believe they are getting,” Bish says. “Remember to factor in labor. Even if you aren’t paying someone else to spray, your time has a clear cost.”

Growers interested in setting up their own on-farm strip trials to test efficacy can contact their local MU Extension field agronomist for assistance. For real-time disease tracking, farmers can also use the Crop Protection Network’s forecasting tools and the national corn IPM maps.