corn tar



He spotted it on his farm for the first time last year. “This year,” Trempealeau County Farm Bureau President Shane Goplin said, “it was worse.” Still, it didn’t compare to some farms near Galesville. “They were harvesting corn in mid-September because it was just dying,” Goplin said.

Tar spot, a new fungal disease that can cause significant yield loss for corn, is becoming more widespread in Southeast Minnesota and western Wisconsin. “We didn’t have much of it until this year,” University of Wisconsin (UW) Extension Agricultural Educator for Trempealeau and Jackson counties Steve Okonek said. “It’s really bad this year.”

Named for the black flecks that cover infected leaves, the disease can also take the form of “fish eye” lesions. Unlike corn rust, the black tar spots cannot be scraped off, according to the Crop Protection Network. Farmers should be on the lookout for tar spot going forward, experts advised. There are steps they can take to protect their fields, but because the disease is so new in North America — it was first spotted in Indiana and Illinois in 2015 — research on management and resistance is still in early stages.

In southern Wisconsin, Okonek said, “Some of the yields I’ve heard down around that area, they’ve lost close to 100 bushels per acre … That takes it from a profit to a loss real fast, and a large loss per acre financially. The people who have the fields that are severely infested, it’s going to hurt big time.”

University of Minnesota Plant Pathologist Dean Malvick had a lower, though still significant, estimate for the potential yield loss: “Some people think it could take 25 percent of yields in certain conditions or even more.”

“Tar spot has been known in Latin America for decades,” said Malvick, who documented the first case of tar spot in Minnesota in 2019 in Fillmore County. How did it pop up in Indiana and Illinois in 2015? “That’s still the million-dollar question,” he responded. “There’s lots of guesses and hypotheses.” Perhaps infected corn or leaves were brought to the Midwest from Latin America, or maybe the disease simply spread on the wind, he said. In any case, the disease has been spreading outward since then.

In Latin America, tar spot thrives in cool temperatures (60-70 degrees F), high humidity, and periods when leaves are wet for hours from rain, fog, or dew, Crop Protect Network reports. Exactly how the disease behaves in North America is still not completely understood, Malvick said. There have been reports of major outbreaks despite the weather not matching the conditions tar spot normally prefers, he noted.


What can farmers do?

What can farmers do about tar spot? “The main option is probably going to be fungicides, unfortunately,” Malvick responded, “which of course brings in an expense that may not normally be part of a farm’s expenses.” According to Malvick and the Crop Protection Network, there is not yet a playbook for which fungicides are best or when to apply them. “We have a lot of learning to do, and that extends to the fungicide question: When? How much? How many applications? What products work best in what places?” he explained.

“With tar spot being relatively new, it takes a while for industry and university to get ramped up and come up with fungicides and other treatments to manage this,” Okonek noted.

For now, Malvick recommended’s fungicide guides. One of its publications lists fungicides’ efficacy ratings against tar spot:

Okonek recommended the UW Extension app Tarspotter (, which attempts to predict the risk of tar spot outbreaks based on local weather forecasts in order to guide fungicide application decisions.

Winona County Farm Bureau President Glen Groth proactively applied fungicide on some of his fields that were damaged by hail, and he reported seeing less tar spot compared to untreated fields. “That would be your best bet, but it’s not necessarily a cure-all,” he said.

There are not yet any corn varietals in North America specifically bred for tar spot resistance. “I truly expect to see some resistant varieties in the future,” Okonek said. “Now how long it’s going to be, I don’t know.”

Some existing corn hybrids have some degree of resistance to tar spot, though Malvick said the data on that topic is not very strong yet. While certain companies are rating their corn as partially resistant, he said, “That may or may not be solid information.” The research needs more time, he explained, adding, “I think in the next couple of years, we’ll have a much better idea of what hybrids work better.”

The Crop Protect Network also recommends tilling under crop residue and rotating crops. Those recommendations are based on the fact that infected crop residue can cause new outbreaks the following year.

However, Malvick was skeptical that tillage or rotation would be the best safeguards. “[Tar spot] has been recorded and found in a lot of fields where it’s never been seen before, including fields that were in soybeans there the year before,” he said. He noted that the disease appeared in the U of M’s research corn plots in St. Paul, with no other corn fields for miles around. “I have strong suspicions it can spread considerable distances,” he said. “Until we know more, rotation — we would think it would be helpful, as well as tillage,” he explained. However, Malvick said he might not choose to implement those practices himself if they came with significant costs.

Asked if he would consider more tillage on his farm, Groth responded, “Not if I don’t have to … Any time you do tillage, you’re disrupting any soil health gains you’ve made.”

Tar spot will be another disease farmers need to pay attention to in the future, and as researchers learn more, hopefully the tools for managing it will improve, Okonek said.