Sunday, November 27, 2022

Disease Watch: Tar Spot

A relatively new disease to the United States, tar spot has quickly become an agronomic and economic concern for corn farmers. 

Tar spot was first found in the Midwest in 2015, and it was thought to only be a minor cosmetic issue. In 2018, farmers realized it was much more serious when a widespread outbreak led to significant yield losses. Now it’s known that the disease can cause yield loss up to 50 bushels per acre. 

The disease was found in Georgia in both 2021 and this season, but it was found too late in the season to cause yield loss, according to University of Georgia plant pathologist Bob Kemerait. 

“It was easy to find tar spot disease in late-planted corn in some counties, along with southern rust and southern corn leaf blight,” Kemerait says. “Tar spot came in too late to hurt yield, but growers need to know it is here.”

Will Tubbs, market development specialist for Corteva Agriscience, who is based in Iowa, says, “Factors such as hybrid tolerance, previous tar spot infection, timing of infection, environmental conditions, overall plant health and fungicide applications can all factor in together to impact yield losses in corn.”

This example of tar spot was found on corn in the black layer stage in Sumter County, Georgia, in a field of continuous corn with volunteers that survived into the winter because of the warmer weather.
Photo provided by UGA Extension

University and industry researchers are still learning more about this disease. 

Tubbs says, many experts predict that its footprint will likely expand in the future due to its ability to reproduce and disperse so rapidly.

“Tar spot is likely going to appear on some level each year for the foreseeable future, but it will be difficult to predict how severe infection will be until symptoms appear in our fields,” he says.

Tar spot is caused by the fungus Phyllachora maydis and manifests as small, raised stromata on plant leaves. The stromata look like small, black spots that turn into black oval or circular lesions. These symptoms start on the lowest leaves, spreading to upper leaves, leaf sheathes and the husks of developing ears. Eventually, the disease can cause premature plant death.

Kemerait says, while the future impact of tar spot for corn production in Georgia is not known, producers should be prepared to scout fields for this disease and prepare for timely fungicide applications to reduce risk of substantial yield loss. CS

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