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Tar Spot: the no-fun fungus affecting DeKalb County corn

December 3, 2018

Farmer Steve Drendel of Malta had heard about tar spot before, but had never seen it in person – until this year.

This year, Drendel saw tar spot in all of his cornfields. He planted between eight and 10 hybrid corn varieties, and all were affected by the black-spotted fungus.

“Every hybrid was effected, each differently,” Drendel said. “Some lost 30 to 40 bushels per acre, others 10 to 20 bushels per acre. I’ve been aware of tar spot the last three or so years, but this is the first year I visibly saw it in my fields and had yield reduction.”

Eric Shearer, farm manager at Babson Farms in DeKalb, said he and his tenant farmers have seen tar spot for about four years, but that this year “was definitely worse.”

“This was the first time it was noticeable and that we had yield loss with it,” Shearer said. “Our fields went from green to dead in five days.”

Tar spot’s symptoms include black raised bumps or lesions on corn leaves, and in some cases, necrotic fisheyes or dead tissue surrounding the lesions. The embedded lesions resemble a spot of tar on the corn plant. Significant infection can limit photosynthesis, reduce yield or potentially kill the plant.

Russ Higgins, commercial agriculture educator with University of Illinois Extension, said tar spot has been an issue in Mexico and Latin America, where it’s known as tar spot complex and can cause severe yield losses. It is now an issue in Florida and a growing number of states surrounding Illinois.

“Tar spot has historically been found in southern climates and has been associated with high humidity and moderate temperatures,” Higgins said. “One theory is that the spores from the disease traveled north through weather, maybe a tropical storm or a hurricane.”

The disease, new to the continental U.S., was first observed in Illinois near DeKalb in 2015, with additional sightings in Indiana. Since that time, the leaf disease has been detected in northern Illinois each year, but, until now, infections have been sparse and have occurred late in the season, minimizing yield loss.

“Once we were made aware of it, we found it in more and more locations, but never as much as we’ve had this year,” Higgins said. “In 2016 and 2017, we saw it late in the season and didn’t know if it would overwinter in fields. For whatever reason this year, the infections started much earlier and it has been much more prevalent.”

Higgins said that this year, tar spot has appeared in the fields of nearly every DeKalb County corn farmer. It has also appeared in southern Wisconsin, Michigan, Iowa and parts of Indiana.

According to an article written by extension Plant Pathologist Nathan Kleczewski, little is known about tar spot, how the pathogens interact with one another, the epidemiology of the diseases and how the pathogens interact with their corn host. The disease may act different in Midwest production systems because hybrid genetics, production practices and environments differ from those in Latin America.

“It really hasn’t been an issue until this year, so there haven’t been extensive studies or specific trials,” Higgins said. “It’s hard to validate just how we’ve been effected by tar spot. We’d have to compare an effected area to an area without it, and this year that’d be hard to do. It infected just about everything.”

“We still have a lot to learn on the subject, so much is still unknown,” Higgins said. “We don’t know why it took hold in northern Illinois but skipped central and southern Illinois. We don’t know what extent fungicides or tillage will help. We have no idea how it functions here.”

Higgins said one experiment has been done: sweet corn planted in September that emerged and grew two to three inches also was affected by tar spot.

“That means that tar spot can continue infection through the season if given the right conditions,” Higgins said. “We know that it is here and we are positive that it is overwintering. But we don’t know why this year has been so severe. We’ve had a fair amount of moisture this year, but we’ve also had it in recent years as well. We’re not sure what made 2018 so unique.”

Shearer said that he is talking to his tenant farmers to decide how to combat tar spot next year.

“We have to ask ourselves how to fix this, if it can be fixed,” Shearer said. “Will we use fungicide? We have to figure out what it will cost us. Will we have to spray every acre next year?”

Drendel said next year he plans to look to plant hybrid corn varieties and possibly spray multiple applications of fungicide.

“My plan for next year is to plant hybrids that didn’t show as much yield reduction and spray fungicide that will prevent the disease from spreading,” Drendel said. “All corn, at varying degrees, was effected this year. Tar spot was very severe because it had a multiplying effect with other diseases and weather. We have seen how the combination of below-normal rainfall in July and August and the diseases of anthracnose and tar spot have impacted overall corn yields this year.”

Kleczewski said that current models predict the spread of tar spot across the Corn Belt.

“Models predict that tar spot may be more severe in the north Midwest area near Lake Michigan because that region shares a lot of similarities climatically with Latin America, where the disease may have originated,” Kleczewski said. “In Latin America, the best way to prevent tar spot has been through hybrid genetics. Our end goal is that in the next few years, the industry will have better hybrids and we’ll develop tools to determine a window to help growers make profitable and efficacious fungicide applications, hopefully avoiding the epidemic we had this season.”

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