Don’t get comfortable.
That’s the message emerging from cornfields across the Midwest as southern rust makes its return — even if this year’s disease pressure looks nothing like last year’s, at least so far.
In eastern Iowa, Landus Cooperative agronomist Dan Bjorklund recently found active southern rust in a field of corn still roughly 30 days from maturity. He would consider treating that particular field (His comments were made prior to the devastating hailstorm that went through parts of eastern Iowa over the weekend.).
In Nebraska, University of Nebraska–Lincoln Extension plant pathologist Tamra Jackson-Ziems is finding the disease in at least a dozen counties. Yet Jackson-Ziems says growers shouldn’t confuse the disease’s arrival there with a reason to automatically spray.
“Right now we’re not seeing widespread southern rust,” she says. “It’s still very patchy and seemingly not severe in any of these fields.”
In total, farmers in at least eight Midwest states – Iowa, Illinois, Indiana, Minnesota, Missouri, Nebraska and Wisconsin – have confirmed the presence of the disease with the Crop Protection Network with varying degrees of severity reported.
The Distinction That Matters
Southern rust, caused by Puccinia polysora, is hardly an unknown visitor to Nebraska. Jackson-Ziems says she and her colleagues have confirmed its presence in the state in 20 of the past 21 years.
What matters now is what the disease does next — and whether there is enough growing season left for it to threaten meaningful yield.
“This is a warning for us to get out, that it’s time to start scouting more intensively to monitor for this disease, because our corn is still young enough that it could potentially be impacted,” she says.
Corn that is “still filling grain” has something to protect, adds David Hula, world corn-yield record holder.
Speaking from York, Neb., during a Pioneer Century of Innovation event last week, Hula says he saw scattered southern rust. While he does not expect a repeat of the dramatic 2025 outbreak, he believes some cases warrant treatment.
“If we know the corn’s green, unlike the southern rust episode we had (in Nebraska) last year, it’s continuing to try to make kernels and fill those kernels out,” Hula says.
Which Fields Deserve Closest Attention
That leaves growers watching several variables at once: crop stage, disease development, field conditions and weather.
Jackson-Ziems has a short list of fields she would put at the top of the scouting rotation and potential treatment.
Pivot-irrigated acres are one.
“If your field is pivot irrigated, we’re adding moisture to that canopy,” she says. “It does increase the chance of more disease, whether that’s southern rust or tar spot.”
Then there are cornfields that started the season late.
Corn planted behind schedule — or replanted after an early-season problem — may still be at a susceptible growth stage when disease pressure builds.
Those acres, Jackson-Ziems adds, “are more likely to be impacted if diseases begin to develop more strongly as we approach the end of the season.”
The Weather Wild Card
And then there is the wild card farmers can’t control: Mother Nature.
Southern rust favors warm conditions. Tar spot, by comparison, is more comfortable when nighttime temperatures cool down. Both diseases can become more concerning as the season progresses, but weather will determine how aggressively they develop.
That is exactly what Bjorklund is watching in eastern Iowa.
On Aug. 5 (last Wednesday), Bjorklund was walking the eastern leg of what he calls his corn safari when Landus’ Sky Scout system pointed him toward a southern rust hot spot.
The corn was planted in May and was only in the late-milk stage by the middle of last week.
Bjorklund estimates the crop still needs about 30 days — essentially the rest of August — to reach maturity.
He says a field with only a few weeks of grain fill remaining presents a different fungicide decision than a field with a month of yield still hanging on the stalk.
“I’m concerned because I think there’s active pustules here that are going to be able to keep inoculating this field,” he notes.
Don’t Assume The Threat Is Gone
That makes one common assumption especially dangerous: that a later-than-normal arrival of southern rust means the threat has essentially passed.
“Everybody says, ‘Hey, it didn’t come in early and we’re fine,’” Bjorklund says. “I’m not so certain…. Be careful, eastern Iowa. You’re not safe yet,” he adds. “There’s too much going on here after pollination to just leave to chance. We’ve got to protect this yield.”
Jackson-Ziems repeatedly cautions growers against making the arrival of southern rust an automatic spray trigger.
“It doesn’t mean that we need to treat automatically,” she says.
Growers who already made an initial fungicide application have another reason to pause before reaching for the sprayer again.
“Keep in mind, if you’ve already made a fungicide application, it’s most likely that you won’t need to do so again (at this point),” she says.
Making The Treatment Decision
For untreated fields, the decision becomes a matter of risk and return.
Jackson-Ziems recommends considering disease levels, crop stage, weather and field conditions before deciding whether a treatment has enough yield to protect to justify its cost.
“If you’re thinking about making a fungicide application, consider the stage of the crop,” she says.
The historical “sweet spot” for disease control and return on investment has generally been from early tasseling through the milk stage, or R3.
That is a much different message than “southern rust is here, so spray.”
It is also why field-by-field decisions matter.
A field with active disease and weeks of grain fill ahead may warrant a closer economic look than a field nearing maturity. An irrigated field may carry different disease risk than a dryland field. A late-planted field may have more vulnerable tissue remaining than an earlier field approaching the end of grain fill.
The question is not simply whether the pathogen is present.
It is whether the disease is developing fast enough, early enough and aggressively enough to justify the cost of intervention.
When The Numbers Might Pencil Out
Hula is a proponent of spraying a fungicide late-season when the numbers pencil out. That calculation is increasingly difficult this season, given current grain prices.
But Hula says he sees an opportunity in the uncertainty surrounding this year’s disease pressure. He suggests making some calculated applications and using the results as information.
“Go out and put a fungicide on a portion of your crop just to see if it’s going to pay, so you can plan for next year,” he advises.
Hula says fungicides can at least break even in some late-season situations and potentially deliver a larger return when disease or heat stress threatens grain fill.
“Fungicides, at worst case scenario, you’re breaking even,” he says. “The goal, of course, is to make two, three times your dollar.”
Jackson-Ziems offers another way to sharpen that calculation: use the Crop Protection Network’s Return on Investment tool.
The tool allows growers to work through disease risk, treatment costs and expected returns, including comparisons among products.
The goal is not to predict the future perfectly. It is to make the best decision possible with the information available in the field.
Make Some Preparation For Tar Spot
There is one more distinction worth making as growers walk fields this August.
Southern rust and tar spot may show up in the same general timeframe, but their histories in a field are very different.
Tar spot can overwinter.
Southern rust cannot.
“Consider what you’ve got in your fields, and whether you’ve had a history yet of tar spot,” Jackson-Ziems says. “That one will overwinter. Southern rust cannot overwinter from last year, so it doesn’t matter if you had southern rust last year or not.”
That makes last year’s southern rust outbreak less useful as a predictor of where the disease will show up this year — or in 2027.
Instead, growers need to pay attention to what is happening now.
Where is the disease showing up? How much of the field is affected? How quickly is it developing? What growth stage is the crop in? And what does the weather forecast suggest about the next few weeks?
Those questions are more useful than simply knowing southern rust has been confirmed nearby.
And if the diagnosis is uncertain, Jackson-Ziems says growers don’t have to make the call to spray by themselves. Extension and other local agronomists can help.
“If you need more information or if you need help identifying a disease on the sample, please reach out to us in Nebraska Extension, or bring a sample to the UNL Plant and Pest Diagnostic Clinic, or check in with your local county Extension professionals,” she says.
