Corn tar spot, a fungal disease first confirmed in Virginia in 2022, continues to spread across the commonwealth. This year, the disease has been confirmed in Culpeper County (July 16), Augusta County (July 20), and Shenandoah County (July 23). While tar spot had previously been documented in Shenandoah, Rockingham, and Augusta counties, the Culpeper detection marks its first confirmed occurrence in that county.

In Midwestern states, tar spot has caused documented yield losses of 25% to 30%, with some severely affected fields losing as much as 60 bushels per acre.

"While tar spot is still relatively new to Virginia, it has the potential to become an economically important disease if environmental conditions favor its development," said Yuan Zeng, assistant director of the Southern Piedmont Agricultural Research and Extension Center and Virginia Cooperative Extension plant pathology specialist.

Disease information

Tar spot appears first as raised circular or oval black spots on the upper and lower surface of leaves. Several corn diseases produce similar symptoms, so laboratory testing is recommended to confirm tar spot. Growers who suspect the disease should submit samples through their local Virginia Cooperative Extension office.

Weather plays a major role in disease development. Moderate temperatures, high humidity, extended overnight leaf wetness, and frequent fog create ideal conditions for infection. Once a plant becomes infected, visible signs typically appear within two to three weeks. For information on managing the disease using fungicides, see the Extension publication Tar Spot of Corn.

Mid- to late summer is the appropriate time for farmers to scout for the disease on their plants. Farmers who observe suspicious spots should contact their local Virginia Cooperative Extension office for diagnostic help and for additional recommendations.

Woman using pipette to fill test tubes in a lab.
Yuan Zeng is using soil testing to build a disease forecasting model, helping growers manage destructive soil-dwelling pathogens.

About Zeng

In her Plant Pathology lab at Southern Piedmont AREC, Yuan Zeng uses low-cost, solar-powered spore traps in corn fields to monitor for pathogens that cause corn diseases. Early detection of these pathogens can help farmers optimize fungicide application timing and potentially reduce yield loss to disease.

Learn more about the lab.

For media inquiries, please contact Devon Johnson, dvj@vt.edu.