After worrying about whether southern rust would repeat this season, many Corn Belt farmers are getting a break.
Near Walker, Iowa, Benton County farmer Trent Kuhn says his bigger concern than disease pressure this season has been the impact of weather.
“Considering all the rain we’ve had, our corn crop is looking pretty decent,” Kuhn says. “There for a while, the ground was just so saturated the soils just needed oxygen so the roots could breathe.”
The same stretch of hot, dry weather helping Kuhn’s cornfields recover is also slowing disease development in Iowa, especially tar spot.
“We’ve had reports of tar spot, early on in the growing season,” says Alison Robertson Iowa State Extension plant pathologist. “That tar spot we’ve seen in central Iowa at very, very low levels is just going to sit now because it’s going to be too hot for it to do anything.”
That’s true in central Illinois, too. According to Farm Journal Field Agronomist Ken Ferrie, his recent conversations with farmers there have focused more on protecting yield potential.
Because disease pressure is not at extreme levels like it was in some areas in 2025, Ferrie says central Illinois growers can return to a more traditional fungicide strategy in corn. He says to focus on making applications that protect the top leaves, which drive grain fill and final yield.
“We can go back to thinking about normal fungicide applications, meaning keeping the fungicide in the upper canopy, protecting what we call our high-dollar leaves above the ear,” he explains.
Keep Scouting Despite Slow Disease Development
Even though tar spot development has been slow, Ferrie warns growers not to become complacent. He encourages them to keep scouting for it, as it could still be an issue this season. Southern rust has also not occurred for the most part but still could. In the Midwest the disease is primarily driven by three factors: spore transport by southerly winds, hot and humid weather, and susceptible host crops
“If southern rust does flare, it’ll be in the upper canopy this year, not below the ear like it was last year,” Ferrie says. “So, we’ll want to keep the fungicide higher in the crop. That’s also very different from how we typically want to manage tar spot. Usually, we want fungicide lower in the plant canopy to address it,” he adds.
To date, the Crop Protection Network has verified southern rust in only five southwest/western Missouri counties. The disease has not been identified in states north or east of there.
Aphids Emerge as New Concern
Another issue farmers in central Illinois need to consider now is aphids. Ferrie says some growers are considering pre-tassel insecticide applications, particularly in susceptible hybrids.
With roughly 60 days remaining until black layer in many May-planted fields, Ferrie encourages growers to continue scouting, verify sprayers are reaching the upper canopy of the crop with fungicide and continue to monitor for southern rust development.
Timing Is Everything for Fungicide Applications
University of Missouri Extension Plant Pathologist Mandy Bish agrees that protecting the ear leaf should remain farmers’ priority, but she cautions against spraying too early.
While tar spot appeared early in Missouri this summer, Bish says disease pressure remains relatively low in the state. However, she adds that crop development has been uneven because of weather extremes and that means a one-size-fits-all fungicide timing doesn’t work well.
“We really want to protect the ear leaf of the corn. That’s our goal with our fungicide applications,” Bish says.
Rather than reacting to the first lesions of tar spot that they see, she recommends farmers monitor fields until the disease reaches roughly one leaf below the ear before making an application.
“With tar spot, it really is about monitoring until we get to seeing it maybe one leaf below the ear leaf. You really want to hold off to spray between tasseling and through that R3 growth stage,” she advises.
Single Application Outperforms Two Passes
That timing recommendation comes after many growers sprayed too early last season, she says, only to have fungicide protection fade before southern rust arrived in the state.
Bish adds that research continues to support a single, well-timed fungicide application in Missouri instead of a two-pass program.
“That single pass, usually a little bit later than VT — maybe nearer to the R2 stage — has given us our best performance,” she says.
Economics Drive Decision-Making
While agronomists are focused on best timing to address disease pressure, farmers continue needing to balance biology with economics this season.
In southern Illinois farmer and broker Sherman Newland says he has already finished fungicide applications on his corn, and the crop is through pollination. He isn’t planning to spend the money on a soybean application.
“I am not planning to spray fungicide on my beans this year to help keep my costs down,” he says.
His decision is based on recent experience with not getting any ROI from treating soybeans.
“The last four years we haven’t got rain in August. I don’t care what you do, spray fungicide or not, if it doesn’t rain in August you won’t get your money back in soybeans.”
Newland’s approach underscores the reality many growers face this summer: successful disease management isn’t just about protecting the crop. It’s also about knowing when an input is likely to pay — and when it isn’t.
Hear more of Ken Ferrie’s agronomic insights and recommendations on treating corn diseases in this week’s Boots In The Field podcast, available at the link below:
