How the Disease Spread Across Top U.S. Corn States

Tar spot once again spread throughout the top corn-producing states in the U.S. this growing season. Looking back over the season, 13 of the top 18 corn-growing states confirmed tar spot infections by August. North Dakota has since been added to the list, leaving Colorado, North Carolina, Tennessee, and Texas as the only top corn states not reporting cases.

Below is a map from the Crop Protection Network showing end-of-season data for the 2025 growing season.

States With Confirmed Tar Spot Cases:

• Illinois
• Indiana
• Iowa
• Kansas
• Kentucky
• Michigan
• Minnesota
• Missouri
• Nebraska
• North Dakota
• Ohio
• Pennsylvania
• South Dakota
• Wisconsin

Yield Losses and 2026 Plans

In South Dakota, Madaline Shires, South Dakota State University Extension state pathology specialist, said the state just wrapped up its fourth growing season with tar spot.

“We had our first confirmed report of tar spot in July, and that was about six weeks earlier than we had found it previously, so that was a little bit alarming,” Shires said.

“We started finding it up and down our eastern border with both Iowa and Minnesota in the early- to mid-August,” Shires said. “It moved really slowly, and we were dealing with southern rust.”

But then September rolled around, and much like what Shires saw in 2024, tar spot took off. She was concerned at how quickly it got established, but it was not nearly as widespread as last year when it was found in 46 counties. In 2025, it was found in nearly half that many.

“In general, tar spot mostly causes yield loss when it sets in early in the growing season — around the tassel to silking [stages], because it can make the plants prematurely die,” said Shires.  

“I very much doubt that we had any yield loss this year,” said Shires. “It might have happened in a couple of our southeast counties that border Nebraska, just because it did start there early. We did have a few people last year in 2024 who did think that they had lost 10 to 20 bushels per acre on some of tar spot fields that were particularly severe.”

Shires recommended South Dakota growers who have had bad tar spot for a couple years consider planting tar spot–tolerant or resistant hybrids for 2026.  

She also advocated for early-season scouting for tar spot and timely fungicide applications when disease thresholds are met.

“We did have people who were spraying for one or two lesions in the field, which doesn’t really math out,” Shires said.

Alison Robertson, Iowa State University professor and Extension fields crops pathologist, said tar spot was again present this season, but it was easily overshadowed by the southern rust epidemic.

“We had three weeks of temperatures which were above their average for the high temperature and above the average for the low temperature,” said Robertson. “July was very warm, very wet, [and] we had a lot of spores being blown in by those strong southerly winds, and then we had a susceptible host here. All those conditions just came together, and so we had the epidemic that we did this year— the disease was way more severe than it was in 2024 or than it has been in previous years because of all those factors.”

“There were considerable yield losses, but I would say that 90% of those yield losses were due to southern rust,” Robertson said.

She attributed tar spot for about a 5- to 10-bushel-per-acre loss.

“When it came to southern rust, we were seeing 40- to 60-bushel-per-acre losses — sometimes even 80 — just depending on where in the state you were, how susceptible your hybrid was, and whether you used a fungicide or not.”

Given the significant yield losses caused by southern rust, Robertson expects it to be top of mind heading into 2026.

“As we go into 2026, a lot of farmers are going to be thinking about southern rust, and what I’ve been telling people is that, as far as the southern rust goes in 2026, we start with a clean slate. We do not have any inoculum here.”

Disease Triangle

Robertson wants growers to consider the disease triangle before being overly concerned about any one disease.  

“You have to have the pathogen, the host crop, and the right conditions to get disease,” said Robertson. “You must have all three sides of the triangle; otherwise you won’t get disease. As we go into 2026, we do not have the pathogen for southern rust. When we think about tar spot — it does survive here, and other diseases like gray leaf spot and northern corn leaf blight. It’s important for farmers to remember that there are a lot of different diseases that all require slightly different conditions. It’s going to depend on what the growing season is like next year.”

The disease triangle helps us plan for the growing season and think about what actions may be needed.

She wants farmers to ask themselves, ‘Is the pathogen present, have I had the disease on my farm before, and am I concerned about it?’ If the answers are yes, then she recommends controlling the host through hybrids that have resistance to the disease they’re concerned about.

“You should start with a resistant hybrid,” Robertson said. “You should be choosing your hybrids based on their rating for the diseases that you’re concerned about, because those resistant hybrids are going to do much better against the disease. There’s not a whole lot we can do about the environment, so we just wait and see what Mother Nature throws at us.”

Hot Spots

Iowa

Spotted back in June, tar spot made its return to Iowa. “This is no surprise!” Robertson wrote in a blog post at the time. “Every county in Iowa has a history of tar spot and consequently the pathogen is present.”

First reported in the Corn Belt in 2015, the disease has been confirmed in Iowa since 2016.

By mid-July, it had spread to 26 counties, and by the end of July, cases were reported in 35 counties. The wet weather Iowa experienced this growing season helped fuel the spread of tar spot.

“Though the rainfall is improving drought conditions and helping the corn during tasseling and pollination, there are now parts of the state that could use a break from the rain,” Iowa Secretary of Agriculture Mike Naig said in mid-July.

Later in the month, Wyffels agronomist Eric Wilson added, “Heavy rainfall events are making a perfect environment for disease to take off as we get into reproductive stages for corn.”

Nebraska

Syngenta agronomist Travis Gustafson said small pop up showers and thunderstorms were common across the state to start the month of July. Coupled with the precipitation, overnight temperatures were in the “sweet spot” for disease development, he said.

By mid-August, 33 Nebraska counties had confirmed tar spot infections.

“Tar spot started early in Nebraska — confirmed as early as in V6 corn,” said Tamra Jackson-Ziems, a plant pathologist and Extension specialist at the University of Nebraska–Lincoln.

Jackson-Ziems advised farmers to evaluate disease pressure and stalk strength prior to harvest. Fields with more than 10% lodged stalks in the push test should be harvested first — or even earlier — to prevent losses from stalk lodging.

Indiana

Indiana has a history of the disease in all 92 counties. This year, it was first spotted in June, with five counties reporting cases. By Aug. 4, 32 counties had confirmed tar spot infections.

Wisconsin

By September, tar spot had been confirmed in 28 Wisconsin counties.

“Disease was the big factor in August,” said Nick Groth, agronomic service representative for Syngenta Crop Protection.

“Late-season disease was a big topic of conversation across Wisconsin as growers hoped they had set their crops up to withstand disease pressure and maintain yield potential,” Groth said.