Tar spot disease has invaded Wisconsin corn fields, causing yet another concern in an already-difficult agricultural market. Agronomists, researchers and farmers are worried about the coming harvest and the yield impact of the fungal disease – all while trying to learn as much as possible about it.

“This couldn’t have happened at a worse time, as the current milk price doesn’t leave room for error and there are no do-overs in farming,” said Aaron Hass, dairy farmer near Evansville, Wisconsin.

The disease, which was only a cosmetic leaf disease in Wisconsin beginning in 2015, is now a damaging disease with the potential to destroy entire corn crops. The fungus responsible for the growth and spreading of the disease is Phyllachora maydis. It causes small black tar-like spots on the surface of corn leaves. Those fungal structures can attack both healthy and dead tissue. They are often surrounded by a narrow tan circle. Many fields appear to dry down and turn completely brown before the usual harvest time.

“Tar spot is making it very costly for us between lost tonnage and poor quality,” Hass said. “We will have to supplement all of the lost nutrients and they’re not cheap. Corn silage makes up over half of the forage for our milk cows.”

Damon Smith is a University of Wisconsin-Extension field-crops pathologist.

“In 2016 and 2017 tar spot was identified in Green, Iowa, Grant, and Lafayette counties in Wisconsin,” he said. “In 2018 confirmations have been made in these same general areas, but also have expanded to include reports from as far north as Columbia County over to the eastern side of the state, including Fond du Lac, and other areas of the Corn Belt.”

Favorable weather conditions for the growth of tar spot disease include cool nights between 60 and 70 degrees Fahrenheit and days with humidity averaging more than 75 percent, Smith said. This summer’s excessive rain has increased average humidity levels.

Yield impacts are difficult to determine this early in the fall, especially when so little is known about the disease. Impacts will depend on epidemic timeliness and symptom severity.

Smith said farmers in Latin America, where this issue is prevalent every year, use resistant corn hybrids. That solution hasn’t yet proven successful in Wisconsin because this is the first year of the epidemic. Some Wisconsin farmers who have sprayed their fields with fungicides at tassel have had less damage to their corn, but he said that is not a proven solution either.

Footville farmers Steve and Liz Case and their son, Craig, say they’re grateful damage to their corn crop isn’t worse than it is. The Case family usually sprays all its corn silage because it increases the harvest window by 10 to 15 days.

“We sprayed our corn with fungicide at tasseling time, and I (thought) that I maybe wasn’t going to, but my son talked me into it,” Steve Case said. “I’m glad I did it and I think it was the best money I spent this year. I believe I’ll be spraying every year after this.”

Mike Binsfield, technical agronomist with DeKalb-Asgrow of Dodgeville, Wisconsin, said the weather has been tropical this year. The rain has kept the leaf surfaces saturated.

The disease triangle is

  • a susceptible crop, the corn
  • a pathogen, the fungus
  • a favorable environment, high dew points and humidity paired with cool nights

That triangle signifies perfect growing conditions for that particular fungus, he said.

Agronomists and researchers are working now to learn whether the fungus can overwinter in Wisconsin – and what the best course of action is to prevent it in the future, he said.

“A big windstorm blowing through Wisconsin right now could be devastating because the corn plants haven’t rooted in as deep as normal,” Binsfield said. “The roots haven’t had to travel as far down to find water this year. Farmers need to prioritize their harvest and get started right away.”

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Mary Hookham is a freelance agricultural journalist and photographer from Janesville, Wisconsin. She grew up near Footville on a 140-acre beef and chicken farm and went to high school in Orfordville. She graduated from University of Wisconsin-Whitewater with a bachelor of arts degree in print journalism and a minor in English for writers.