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Farmers need to scout for tar spots on corn leaves



Iowa Correspondent


WASHINGTON, Iowa — Last fall, tar spot was confirmed in Florida, Illinois, Indiana, Iowa, Ohio, Michigan, Wisconsin, and most recently in Minnesota – and may also pose a threat to this year’s growing season, according to agronomists and plant pathologists.

“This disease first made its appearance in the U.S. in 2015,” said Rebecca Vittetoe, Iowa State University (ISU) Extension field agronomist. “With it being a relatively new disease, there is much to learn.”

Although tar spot, which is a fungal pathogen (Phyllachora maydis), is still a fairly new corn disease in Iowa, 86 of the state’s 99 counties have now confirmed its presence, as of Dec. 23, 2019, as it continued to spread through a significant swath of western Iowa.

“There are just 13 counties in western Iowa (mainly along the western border of the state) where tar spot has not been confirmed,” Vittetoe said.

Alison Robertson, ISU associate professor of plant pathology and microbiology, said, “tar spot spread further than ever in 2019, north, south, east and west.”

Referring to an official map on, which is used to actively track tar spot, she said, “It did not cause as severe losses in 2019 as it did in 2018. It came in a little later (after tassel) as opposed to pre-tassel in 2018.”

While tar spot has been reported in western Iowa, Robertson said the map shows it is not present along the western border. 

“In western Iowa, more of the disease we found was only one to a few spots on one to a few plants in a field,” she said, “so [it’s] not very severe.

“However, the pathogen is now present in those states (referring to the map), so we’ll have to see how it plays out over the next few years,” she added. “Is this something we need to worry about and manage, or is it just another disease we see on occasion?”

Vittetoe said one thing the map doesn’t show agronomists and plant pathologists is its severity.

“It just shows us that tar spot was found in what counties,” she said. “For a county to light up red on the map, we just had to find one tar spot stroma on one leaf in one corn field in the county.”

In Illinois, Nathan Kleczewski, University of Illinois extension agronomist, said, “We would expect less P. maydis inoculum (spores) to overwinter, simply due to fewer corn acres planted and less corn infection in 2019. This could result in lower inoculum loads and potential disease in 2020.”

In Indiana, Darcy Telenko, Purdue University assistant professor of botany and plant pathology, said, “We have currently confirmed active tar spot fields in 20 counties (as of Oct. 1, 2019).”

In fields potentially infected with tar spot, Robertson said farmers need to look for small, black, raised spots on the leaves of corn plants, which can occur anywhere in the canopy.

Vittetoe said research is being done to better understand tar spot and how to best manage it.

She said residue management and crop rotation may help to reduce the inoculum present in the field: “However, residue management may not be an option in some fields,” she added.

“Hybrids do vary in their susceptibility to this disease,” Vittetoe said. “Noting what hybrids farmers saw tar spot in this year, and visiting with their seed dealer can help them select hybrids that are more resistant to this disease.

“Scout corn fields. If tar spot is observed, a fungicide may help reduce the disease, depending upon when the disease is observed (in the growing season) and the timing of the fungicide application.”

Damon Smith, University of Wisconsin plant pathologist, is working on developing and validating a prediction model called the Tar Spotter App to better predict the risk of tar spot, she added.