It’s more important than ever for U.S. Corn Belt farmers to stay vigilant in seeking out early tar spot of corn to avoid significant yield loss. New data shows that tar spot of corn — which presents as black, tar-like flecks on leaves, stalks and husks that produce infection-spreading spores — continues its march across prime production regions.
Farmers who study its expansion can be proactive in their management throughout the growing season to ensure any problem signs are quickly addressed.
A decade of tar spot challenges
Tar spot is a fungal disease (Phyllachora maydis) that was first identified in Mexico in 1904 and has been active there for decades as well as in Central and South America and in the Caribbean. It’s been 10 years since the first documented U.S. tar spot case in Indiana back in 2015. Corn crops began experiencing substantial yield reduction from the disease three years later in 2018.1
In 2021, the disease expanded beyond Indiana to include Illinois, Michigan and Wisconsin, resulting in losses of 231 million bushels — far beyond the yield losses caused by other diseases that year.2
As of this writing in 2025, the reach of tar spot of corn has only grown, stretching from the Midwest into the Great Plains, the Northeast and the South.3 The disease has been either confirmed or reported in all of Indiana’s 92 counties as well as in Delaware, Florida, Georgia, Illinois, Iowa, Kentucky, Maryland, Michigan, Minnesota, Missouri, Nebraska, New York, North Dakota, Ohio, Pennsylvania, South Dakota, Virginia and Wisconsin.
Bushel losses also have climbed: Experts estimate tar spot claimed upward of 293 million bushels in 2024 in the U.S. But the annual damages from this disease have not been a simple linear progression: In the northern U.S., tar spot claimed the highest losses for corn in 2024, 2022 and 2021, yet in 2023 and 2020 it didn’t even make the top five disease ranking. The variance is due to differences in specific environmental conditions. Tar spot thrives in cool temperatures between 60 F and 70 F, with high humidity and wet conditions.
The disease’s erratic behavior means farmers must stay vigilant in monitoring crop conditions.
Growers can keep tabs on tar spot’s presence in the counties where they operate as well as throughout the U.S. using the map on the Regional IPM Center’s tracking website. The Tarspotter app from University of Wisconsin’s Integrated Pest and Crop Management program also tracks local weather and environmental conditions in order to help growers anticipate disease risk and plan accordingly.4
Tar spot of corn identification
Growers should begin scouting for tar spot between V8 and V10, though the disease most often shows up between corn silking and R6. Black lesions appear on the top and bottom of corn leaves. With time, these are surrounded by brownish areas of dead tissue that spread from the initial spot. And unlike some other diseases, the spots can’t be rubbed off with a finger.
Getting ahead of tar spot of corn
Although research into tar spot of corn is ongoing, a study published in 2024 indicates correctly timed foliar fungicides can help farmers address the problem even when it is severe.5 Yet proactive management can and should be implemented to avoid a crisis situation.
BASF fungicides Veltyma®, Headline AMP®, Priaxor® and Revytek® are all labeled for tar spot control in corn. A regional BASF representative can help farmers identify the right products and management strategy for their conditions. (Pro tip: BASF Technical Service Representative Kurt Maertens addresses fungicide control of tar spot in the BASF video “Tar Spot Management.”)
Hybrids and fertility
As of early 2025, there are no corn hybrids that are fully resistant to tar spot. But farmers can evaluate their hybrid options with a trusted adviser to mitigate the risk of the disease. They can also implement test strips to understand the value they are getting from any fungicides used to manage the condition.6
Experts are available to help you make your decisions. Reach out to your seed retailer, a nearby extension office agent, or a seed company professional like your regional BASF representative.
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Endnotes
- “Tar Spot of Corn: What To Know and New Research,” Purdue University Extension, 2022. https://extension.entm.purdue.edu/newsletters/pestandcrop/article/tar-spot-of-corn-what-to-know-and-new-research/
- “Corn Disease Loss Estimates From the United States and Ontario, Canada — 2021,” Crop Protection Network, 2022. https://cropprotectionnetwork.org/publications/corn-disease-loss-estimates-from-the-united-states-and-ontario-canada-2021
- “Tar Spot,” Corn ipmPIPE. https://corn.ipmpipe.org/tar-spot/
- “Tarspotter,” University of Wisconsin-Madison Integrated Pest and Crop Management. https://ipcm.wisc.edu/apps/tarspotter/
- Ross, Tiffanna J., et al. “Investigations into Economic Returns Resulting from Foliar Fungicides and Application Timing on Management of Tar Spot in Indiana Hybrid Corn,” Plant Disease, 2024. https://apsjournals.apsnet.org/doi/10.1094/PDIS-05-23-0932-RE
- “Managing Tar Spot in Corn,” Michigan State University Extension, 2024. https://www.canr.msu.edu/news/managing-tar-spot-in-corn
