Missouri corn growers should be on the lookout for a fungal pathogen that causes yield loss.

Kaitlyn Bissonnette, University of Missouri Extension plant pathologist, says tar spot is expanding in the state, and farmers should scout fields this growing season.

Bissonnette first identified tar spot in far northeastern Missouri counties — Scotland, Lewis and Clark — in 2019. Last year, it appeared in Marion County. She expects it to spread across the northern half of Missouri and then move into the middle of the state.

Emerging disease

Tar spot, caused by the fungal pathogen Phyllachora maydis, first arrived in the U.S. in 2015. Since then, it has spread to 10 states. In 2018, Bissonnette says it caused significant yield losses in Illinois, Indiana, Wisconsin and Michigan.

Tar spot appears as circular or oval raised black dots (stromata) on both sides of leaves, the sheaths and husks, Bissonnette says. A brown or tan halo often surrounds the raised black spots. It can appear on brown and green tissue, alone or in numbers.

This fungal disease is often confused with other diseases such as southern corn rust. Unlike the pustules of southern corn rust or common rust, the stromata of tar spot do not break through the leaf’s surface.

TOUGH TO SEE: A single stroma of the tar spot pathogen is seen on a brown senescing leaf. At low levels, tar spot can be difficult to detect, as it is only a single spot on a single leaf.

The tar spot pathogen can infect multiple times in a single season. It overwinters on soil surface residues and spreads by wind and heavy rain.

Conditions that favor the pathogen include monthly average temperatures of 63 to 72 degrees F, relative high humidity greater than 75%, seven or more hours of leaf wetness at night, monthly rainfall of more than 5.9 inches, and 10 to 20 foggy days a month.

Tar spot is most likely to appear during mid-to-late grain fill in Missouri cornfields. Bissonnette recommends scouting in mid- to late August through harvest.

Control options

A Crop Protection Network report by Darcy Telenko of Purdue University estimates tar spot yield losses at 0.34 to 1.45 bushels per acre for every 1% increase in severity. Affected plants have reduced ear weight, poor kernel fill and loose kernels.

Few fungicides are labeled for tar spot management, but researchers are testing the effectiveness of many commercially available products. The Crop Protection Network has an efficacy comparison table based on trials conducted by land-grant university researchers.

Use practices such as crop rotation to reduce tar spot development, Bissonnette says. Corn-on-corn fields face the highest risk of tar spot.

Farmers can report tar spot to Bissonnette at [email protected] or 573-882-9106.

Source: The University of Missouri Extension, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.