As harvest approaches in Wisconsin, corn growers are keeping a close watch on tar spot, which has now been confirmed in 28 counties, according to the latest update from the Crop Protection Network. This late-season fungal disease, along with southern rust and northern corn leaf blight, is contributing to elevated disease pressure across the state-even as the overall corn crop remains in strong condition.

In the USDA's most recent Crop Progress Report, 83% of Wisconsin's corn is rated good to excellent, positioning the Badger State second among the nation's top 18 corn-producing states. Despite this favorable outlook, the rapid spread of tar spot has farmers and agronomists on high alert.

"Disease was the big factor in August," said Nick Groth, agronomic service representative with Syngenta Crop Protection. "Heavy pressure from these diseases can be found in certain fields throughout the state."

Groth noted that while southern rust is currently the primary concern, cooler weather conditions may shift the advantage toward tar spot, which thrives under damp, cool conditions. "Cooler weather this week will slow crop development, but other diseases like tar spot may ramp up," he warned.

Wisconsin's corn crop is maturing steadily. According to USDA data:

4% of the crop has reached the mature stage (vs. 3% five-year average),

38% is in the dented stage (vs. 41% average), and

80% has reached the dough stage (vs. 83% average).

"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." The cooler early September weather is slowing development slightly but offering more time for growers to prepare equipment and storage for harvest.

Despite disease pressure, farmers are optimistic about yields, especially in southern Wisconsin. Groth emphasized that the crop is beyond the point of additional management, with growers now focused on protecting the yields they've built all season.

Confirmed cases of tar spot in 2025 span a wide geography-from Rock and Racine Counties in the south to Barron and St. Croix Counties in the north. The first case was detected in Rock County on July 3, with the most recent confirmation in Ozaukee County on August 20.

Counties Reporting Tar Spot in Wisconsin - 2025

The full list of affected counties includes:Rock, Green, Kewaunee, Dodge, Marquette, Lafayette, Grant, Columbia, Iowa, Walworth, Jefferson, Shawano, Racine, Brown, Dane, Waushara, Waupaca, Sauk, Pepin, Buffalo, Monroe, Vernon, Crawford, Washington, Barron, St. Croix, and Ozaukee.

USDA condition ratings for Wisconsin corn:

1% Very Poor

4% Poor

12% Fair

57% Good

26% Excellent

As the 2025 harvest nears, disease management is top of mind, but Wisconsin growers are entering the final stretch with a strong crop, moderate weather, and cautious optimism-provided diseases don't gain further ground in the final weeks.