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Central Illinois corn farmers defend against new 'tar spot' crop disease

Corn tar spot
University of Illinois farmdoc
A corn leaf showing few small black tar spot stromata.

A pathogenic crop disease has officially arrived in central Illinois, with the potential to rival the dreaded corn rootworm as a top corn yield-robber.

Tar spot (Phyllachora Madis) is a fungus that affects corn leaves, forming black specks that cannot be easily removed. So-called “fisheye” lesions — brown or tan lesions with darker borders — can also develop. Robbed of their natural, leafy protection, kernels may not reach full growth potential, stifling yields.

In 2018, Purdue University reported a yield-reducing epidemic of tar spot occurred in northern Indiana and in surrounding states. Likely first arriving via northern Indiana, Wisconsin or Iowa, tar spot crept into northern Illinois around three years ago. Moist springtime conditions the past couple of years allowed the fungus to migrate downstate as far as Peoria, where it apparently is there to stay.

“There is some tar spot beginning to surface in area corn fields of Peoria County,” Patrick Kirchhofer, manager of the Peoria County Farm Bureau, said in mid-July. “Tar spot is a fairly new disease in Peoria County as it was first discovered three years ago. It is probably here to stay, because it is an airborne disease and resides in plant residue. Agronomists are still learning about the disease.”

Above-normal wet weather in springtime in central Illinois along with heavy dews were conducive to plant diseases such as gray leaf spot and tar spot during the first part of the growing season, Kirchhofer added.

The fungal disease can overwinter in crop fields, making tillage and crop rotation a near-must for farmers in areas prone to the fungus. In addition, studies show that crop fields with heavy irrigation use can foster more tar spot fungal growth than in non-irrigated fields.

Keeping a keen eye on the spread of tar spot in Illinois is plant pathologist Chelsea Harbach, a commercial agriculture educator for the University of Illinois Extension. In addition to issuing an appeal to farmers to report the disease’s presence in corn fields, Harbach is committed to updating incidences of tar spot throughout the state via the U of I farmdoc website.

“If you’ve been a victim of high incidence and/or severity of corn tar spot in your fields, you know how limiting this disease can be on corn yield,” said Harbach, in an Extension blog posted July 11. “As this disease can be seriously yield-limiting AND the pathogen is relatively new yet in Illinois, we have been monitoring the spread of this pathogen through corn IPMPipe.”

Crop seed companies are investing heavily in research to develop traits that defend against corn tar spot. Though the research is ongoing, university studies have shown that certain commercial corn hybrids are better at restricting tar spot growth than others.

“In 2015 we were caught off guard by the presence of this disease, so none of the hybrids that we had available to us commercially had any level of resistance. Breeders had to be quick to screen germplasm to find good candidates for breeding resistance into our commercially available hybrids,” said Harbach. “Along with developing host resistance, integrated pest management (IPM) must be kept in mind to avoid overusing different sources of resistance. The best thing farmers can do is to scout their fields early and often.”

Carefully selected corn hybrids defended with strategically timed applications of fungicides provide the best known protections against tar spot, according to Dr. Damon Smith, associate professor and Extension plant pathologist for the University of Wisconsin-Madison (UWM). Smith presented the latest research conclusions regarding tar spot to farmers and applicators during the Illinois Fertilizer and Chemical Association’s 2022 convention, held in Peoria last January.

“Tar spot has been isolated as one (solitary) fungus, but as recently as 2018 we weren’t sure. Since then we’ve learned a lot,” said Smith. “For all intents and purposes here in the Midwest, we’re dealing with just one organism. But it can move quickly, and it’s a significant problem that is taking a lot of yields.”

However, the tar spot problem in central Illinois appears to have largely abated — at least for this growing season — with recent above-average temperatures, warmer evenings and sporadic drought replacing the cool, wet overnight hours of spring and early summer.

“Based on reports from the farmer-leaders I’ve talked to in central Illinois, tar spot isn’t a big concern for most Illinois corn farmers in 2022. Tar spot thrives in wet conditions, and the drought-like weather we’ve experienced this growing season has made for an unfavorable climate for tar spot to cause much of a problem so far. We will keep watching and working with experts to make sure farmers have the resources they need if and when tar spot becomes a concern,” said Rodney Weinzierl, executive director for the Bloomington-based Illinois Corn Growers Association, on July 29.

With its hardy resistance and capability to overwinter in crop residue, tar spot will be something Illinois crop pathologists including Harbach will be monitoring for decades to come. To read more about Harbach’s research into tar spot’s prevalence in Illinois, check out her Extension blog Corn tar spot disease monitoring in Illinois: University of Illinois Extension.

Tim Alexander is a correspondent for WCBU. He joined the station in 2022.