University of Missouri Extension state plant pathologist Mandy Bish says tar spot of corn has spread into many new counties in 2023.

Bish recently confirmed tar spot in five more counties. This brings to 31 the number of Missouri counties in which tar spot has been confirmed between 2019 and 2023. Tar Spot has been confirmed in Pettis, Cooper, Chariton, Howard, and Lafayette Counties in 2023. It was also found in Carroll County in 2022.

Tar spot can cause severe yield loss on susceptible hybrids. The fungal pathogen that causes tar spot, Phyllachora maydis, prefers temperatures in the mid-60s. While Missouri has experienced extremely high temperatures recently, evenings in the next week are forecast to be milder and may cause the disease to continue spreading. The pathogen can survive Missouri winters.

Bish recommends applications at VT-R3 stages. She cautions growers about late fungicide applications.

“Treatments at VT through R3 have shown the most consistent results in research on tar spot management,” Bish says.

Bish says several factors are known to increase disease risk:
• Tar spot previously found in the field or neighboring fields.
• Field is corn-on-corn rotation.
• Irrigation can hasten the onset of symptoms.

Seed dealers may be able to provide insights on what varieties seem most vulnerable.

Photo courtesy of Mandy Bish