More Than a Spot

Tar spot, Image by Martin Chilvers, Michigan State University

No amount of elbow grease can remove the aptly named corn disease called tar spot, but researchers are devising better ways to monitor it.

The disease first surfaced in the U.S. in 2015, when farmers and plant pathologists found the black, tar-like speckles on corn leaves in fields in Illinois and Indiana. It had a banner year in 2018, and infestations spread through counties in six states in the north-central U.S.--Illinois, Indiana, Iowa, Michigan, Ohio and Wisconsin.

Now scientists from the University of Wisconsin and Michigan State University have teamed up to produce a tar spot risk-assessing app, called Tarspotter. The app is not public yet, but a large group of industry and academic scientists are using it this year, hoping to validate its models so it can be available for growers to use next year.

So far, the app has put numerous places in the Midwest at high risk for tar spot infestations, such as northern Illinois, northeast Iowa and most of Wisconsin, says University of Wisconsin Extension plant pathologist Damon Smith.

Cool and humid temperatures and above average rainfall favor the disease. No-till cornfields with a previous history of tar spot infections are most at risk, Smith notes. Late-planted fields could experience more damage from the disease because infections will have more time to build to economic levels.

Researchers have found that fungicides with multiple modes of action were more consistently effective against tar spot in past years than single mode-of-action products, he added.

See more on tar spot in corn management from the Crop Protection Network here

See more on the Tarspotter app and current risk for the disease here

Follow the latest from Pamela Smith, DTN Crops Technology Editor, by visiting the Production Blogs at or following her on Twitter: @PamSmithDTN.


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