Infectious crop disease ‘tar spot’ spotted early in Missouri

ST. LOUIS — Four confirmed cases of tar spots have been spotted on corn this year in Missouri. Phyllachora maydis, a fungus, is the cause. The fungal disease appears on crops as tiny, black, and raised circular dots. This is the earliest it has been spotted in the state.

Tar spots are primarily found on corn leaves, stalks, and husks. Cool temperatures and humid conditions are the conditions that spur this disease in corn.

Tar spot was first confirmed in northeast Missouri in 2019, reached northwest Missouri in 2022, and spread to numerous north-central counties in 2023.

Tar spots prevent the crop from receiving proper sunlight, reduce moisture, and reduce silage corn feed quality. Once the disease has reached the corn, it can cause significant yield damage as the fungus creates spores that can spread quickly.

Cases of tar spots have been emerging in the U.S. within the last few years. Mandy Bish, a plant pathologist for the University of Missouri, confirmed the four cases in Howard County, Chariton, Platte, Lafayette, and Saline counties. Cases have already been confirmed in Kansas, Iowa, and Indiana.

Corn is the biggest crop grown in Missouri, alongside soybeans. According to the Missouri Department of Agriculture, Missouri ranks 10th among states in corn production.