By Linda Geist
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.
Source : missouri.edu