Southern rust makes early arrival in Illinois

August 4, 2026 6

Southern rust was confirmed in several Illinois cornfields in July, making it the second consecutive season the disease arrived earlier than expected.

“This is starting to potentially become a trend, and we need to shift our mindset around when we’re applying fungicides to address that,” Mike Probst, technical service representative for BASF in central and southern Illinois, told FarmWeek.

Southern rust does not overwinter in Illinois. In the past, it has spread northward from southern states later in the season. Earlier detection means growers may need to be more proactive with crop protection.

“We’ve seen a lot of growers hold off on fungicide applications until later towards that R3 to R4 growth stage to target late southern rust,” he said. “Intentionally delaying fungicide applications like that is going to open up the window for southern rust to infect before the applications go out.”

According to the Crop Protection Network (CPN), southern rust had been reported in eight Illinois counties as of press time: Williamson, Gallatin, Hamilton, Jersey, Washington, St. Clair, White and Wayne.

The good news is farmers in many southern counties got fields planted early this year, so crops are ahead in terms of maturity.

“I’m really not terribly worried about that corn down there because it’s so far through its life cycle at this point there’s really not a lot of yield left for that southern rust to impact,” Probst said.

But with plenty of potential for disease spread, it could be a different story if southern rust reaches central and northern Illinois.

Growers may need earlier fungicide decisions

“If you’re still in that R2, R3, verging on R4 growth stage of your corn and you haven’t sprayed a fungicide, there is still some protection to be had there, and with the risk of southern rust coming up, I think that is still an economically good decision,” Probst said.

Southern rust produces small, orange to light-brown pustules typically found on the upper surface of corn leaves. The pustules are powdery and can appear in dense clusters.

“The disease can develop rapidly under warm, humid conditions, so early detection is important when evaluating whether a fungicide application may protect yield,” University of Illinois Extension plant pathologist Boris Camiletti said, noting it can often be mistaken for common rust.

Growers who suspect southern rust in their corn can submit a sample to the University of Illinois Plant Clinic to confirm the diagnosis.

Probst said grey leaf spot, northern corn leaf blight and tar spot have also been detected in Illinois fields this season, but in a more typical fashion.

Tar spot also spreading

CPN reports 10 counties across the state have confirmed cases of tar spot, as far north as DeKalb County and as south as Madison County.

Given the spread of planting dates this growing season, it is important to note different crop growth stages when scouting for diseases.

“What we’ve seen traditionally through a good amount of research is later planted corn tends to be at a higher risk for negative impacts from disease and also tends to respond at a higher level to fungicide applications,” Probst said.

As of July 27, USDA rated the state’s corn crop 13% excellent, 46% good, 28% fair, 10% poor and 3% very poor. That was a 4 point drop in the good to excellent categories from the week prior making it the largest single-week drop of the season so far. Soybeans were rated 11% excellent, 46% good, 30% fair, 9% poor and 4% very poor.