Tar Spot Confirmed in 33 Nebraska Counties

Tar spot has spread to 33 counties in Nebraska, according to the latest Crop Protection Network (CPN) map. This is up from 19 counties back in mid-July. In addition to Nebraska, confirmed cases have also been reported in Delaware, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Maryland, Michigan, Minnesota, Missouri, New York, Ohio, Pennsylvania, South Dakota, and Wisconsin.

“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.

The fungus Phyllachora maydis causes tar spot. It produces 1/16–3/4-inch round to irregular diamond-shaped, raised black structures called stromata. These structures form on corn leaves’ upper and lower surfaces. In severe cases, stromata may also be observed on leaf sheaths, husks, and tassels, the CPN said.

Tar spot isn’t the only concern for Nebraska farmers.

“For the second straight year, southern rust has been the greatest corn disease threat in eastern Nebraska fields and continues to increase, especially in the lower corn canopy,” Jackson-Ziems said.

She pointed out that the southern rust fungus is beginning to produce black teliospores instead of the more common orange/tan spores, making it difficult to differentiate from tar spot. 

What Farmers Should Do

Jackson-Ziems said she urges farmers to get out in their fields, allowing them to make timely decisions.

“It’s worth scouting fields, and some fields may benefit from fungicide treatment, especially if at early dough stage or earlier,” she said.

Cool temperatures (60°–70°F) and high relative humidity favor tar spot development. The disease increases when leaves have at least seven hours of free moisture due to rain, fog, or high relative humidity, according to the CPN.

Jackson-Ziems said she has noticed tar spot cases increasing, adding it’s likely they will continue to rise, especially as cooler temperatures set in toward the end of the season.

Looking Ahead

Scout your fields before making a harvest plan.

“Fields with substantial disease should be evaluated for stalk strength prior to harvest,” Jackson-Ziems said. “Fields with more than 10% lodged stalks in the push test should be harvested first or even earlier to prevent harvest losses due to stalk lodging.”

Positive 2025 Nebraska Cases

• Sherman County – Aug. 6
• Custer County – Aug. 6
• Hall County – Aug. 4
• Buffalo County – Aug. 4
• Madison County – July 28
• Dixon County – July 25
• Platte County – July 25
• Fillmore County – July 22
• Knox County – July 18
• Cedar County – July 18
• Wayne County – July 18
• Dodge County – July 17
• Butler County – July 14
• Thayer County – July 10
• Jefferson County – July 7
• Saline County – July 7
• Hamilton County – June 30
• Nemaha County – June 25
• Adams County – June 24
• Colfax County – June 24
• Lancaster County – June 20
• Otoe County – June 20
• Johnson County – June 20
• Richardson County – June 20
• Pawnee County – June 20
• York County – June 20
• Cass County – June 20
• Antelope County – June 17
• Polk County – June 17
• Seward County – June 17
• Saunders County – June 16
• Clay County – June 16
• Pierce County – June 16

Nebraska Corn Condition

According to the USDA Crop Progress report, released Aug. 10, 59% of Nebraska corn has reached the dough growth stage; the five-year average for this point in the season is 62%. Only 15% of the crop has progressed to the dented growth stage. This is 1 percentage point ahead of the five-year average of 14% but behind last year at this time, when 23% of the crop was at the dented growth stage.

The USDA rated Nebraska corn as follows: 1% very poor, 3% poor, 20% fair, 49% good, and 27% excellent.