Disease With Unknown Yield Impact Pops Up Across Eastern Corn Belt

Tar Spot on corn with Black raised “bumps” and small necrotic fisheye symptoms. ( N Kleczewski and J Donnelly )

Tar spot was first identified in 2015 in seven counties in northwest Indiana and in 10 counties in north-central Illinois. In 2018, tar spot has been confirmed in Illinois, Wisconsin, Michigan and Indiana, according to University of Illinois extension.

The disease is caused by the fungus Phyllachora maydis and Monographella maydis. The most favorable conditions for infection are cool temperatures with high humidity along with sustained long-periods of leaf wetness. Here’s a link to the Purdue Extension bulleting on Tar Spot.

The yield impact of tar spot is being researched as well as potential future controls. Here is a link to a document compiled by the University of Illinois with known hybrid resistance.

Researchers are making a call for all farmers to send samples of suspected tar spot, and they should include valuable information such as hybrid, if a fungicide was applied and county of origin. Contact your extension or University pathology lab for more details.

Identifying the disease can be tricky as it mimics others, such as corn rust and tropical rust. The first symptoms of infection are brownish lesions on leaves, which then become black circular lesions (that have a similar appearance to a spot of tar). According to this Pioneer bulletin, up to 80% of the leaf can be affected—with up to 4,000 lesions on a single leaf.