Tar spot in corn crops found in northeast Missouri counties

July 16, 2024 | By University of Missouri Extension

Tar spot, a yield-robbing disease of corn, can be identified by small, raised black spots across the upper and lower leaf surfaces. | Photo courtesy of Mandy Bish

COLUMBIA, Mo. — A yield-robbing corn disease, tar spot, is gaining ground in Missouri.

Tar spot has been confirmed in 19 counties in 2024, says University of Missouri Extension plant pathologist Mandy Bish. Corn growers in nearly half of the state’s counties have reported tar spot since Missouri’s first confirmed case in 2018.

The counties with confirmed cases this year are Scotland, Schuyler, Marion, Lewis, Pike, Lincoln, Monroe, Saline, Carroll, St. Charles, Lafayette, Boone, Nodaway, Gentry, Chariton, Andrew, Platte, Audrain and Howard, according to Corn ipmPIPE, a website that tracks corn diseases. MU Extension is one of the institutions contributing to the website.

Tar spot has been present at low levels and restricted to the lower corn canopy in most of the confirmed cases of 2024, says Bish.

Tar spot, like many other corn diseases, overwinters in infested corn residue. The fungus that causes tar spot prefers temperatures in the mid-60s to low 70s.

Tar spot appears as small, raised black spots across the upper and lower leaf surfaces. In severe cases, it can also be found on husks and leaf sheaths. Corn is the only known host.

Tar spot is sometimes confused with insect frass, says Bish. Unlike frass, which can be wiped away easily, tar spot cannot be scraped off, says Bish.

“We have some knowledge on corn hybrids that are less susceptible to tar spot, but we have yet to identify a highly resistant hybrid,” says Bish.

Bish recommends fungicide applications at VT-R3 stages, which have been most consistent during high outbreak years. Fungicide applications earlier than VT and after R4 have not demonstrated a good return on investment, she says.

A June 2024 article on the Crop Protection Network website discusses decision-making for fungicide application.

The Tar Spotter app at https://ipcm.wisc.edu/apps/tarspotter helps to predict the risks of tar spot using several variables, including weather. Farmers can enter site-specific information into the app, which combines this information with research-based models to predict risks and need for a fungicide application.

Learn more and follow tar spot spread at https://corn.ipmpipe.org/tarspot.