Tar Spot Confirmed in 28 Wisconsin Counties

Tar spot has now been confirmed in 28 Wisconsin counties, raising concern as growers approach harvest. In the most recent USDA Crop Progress Report, 83% of Wisconsin’s corn crop was rated good/excellent. Of the top 18 corn producing states, the Badger State ranked second. Even though the corn crop is progressing well, tar spot is still spreading across the state.

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

He shared that disease pressure seems to be the major concern, as tar spot, along with southern rust and northern corn leaf blight, are showing up in many fields.

“Heavy pressure from these diseases can be found in certain fields throughout the state,” Groth said.

Southern rust is his main concern now, but changing conditions could alter that.

“Cooler weather this week will slow development of the crop some. It may also slow southern rust development, but other diseases like tar spot may ramp up,” he said.

Groth hears late-season diseases as a big topic of conversation across Wisconsin as growers hope they set their crops up to withstand diseases and maintain the yield potential they had going into the month.

Confirmed 2025 Wisconsin Cases

Tar spot has been found in 28 counties this season according to the Crop Protection Network:

• Ozaukee County – Aug. 20
• St. Croix County – Aug. 13
• Barron County – Aug. 12
• Washington County – Aug. 09
• Crawford County – Aug. 6
• Vernon County – Aug. 6
• Monroe County – Aug. 6 
• Buffalo County – Aug. 6
• Pepin County – Aug. 5
• Dunn County – Aug. 5
• Sauk County – July 31
• Waupaca County – July 31
• Waushara County – July 30
• Dane County – July 30
• Brown County – July 29
• Racine County – July 25
• Shawano County – July 25
• Jefferson County – July 25
• Walworth County – July 17
• Iowa County – July 15
• Columbia County – July 15
• Grant County – July 14
• Lafayette County – July 14
• Dodge County – July 11
• Marquette County – July 11 
• Green County – July 8
• Kewaunee County – July 8
• Rock County – July 3

Wisconsin Corn Progress and Condition

“Corn is well into the denting stage in southern Wisconsin now,” Groth said. “Northern Wisconsin has a wider range of corn conditions, but the majority of acres are likely into the dough stage. Conditions are still generally favorable for the crop.”

Growers are looking ahead to harvest across the state.

“The crop is past the point of additional management,” he said.

The USDA reported 4% of the Wisconsin corn crop had reached the mature growth stage, 38% had reached the dented growth stage, and 80% had reached the dough growth stage. The five-year average is 3% for the mature growth stage, 41% for the dented growth stage, and 83% for the dough growth stage.

“Overall, many growers are looking forward to a really good crop, especially in southern Wisconsin,” Groth said. “The cooler weather over the last week and into this week has slowed crop development some and will give growers a little extra time to prepare for harvest.”

The USDA rated Wisconsin corn as follows: 1% very poor, 4% poor, 12% fair, 57% good, and 26% excellent.