Tar spot of corn has spread to 31 counties in Missouri since 2019, according to Mandy Bish, state plant pathologist with the University of Missouri Extension.
Bish found tar spot in Holt and Marion counties in June of 2023, the earliest it has appeared in Missouri in recent years. Tar spot can cause severe yield loss on susceptible corn 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 cautions growers about late fungicide applications. “I am unaware of any data showing that fungicide applications applied to corn at dent (R5 growth stage) provide an economic return. I am aware of research that demonstrates the opposite.”
She recommends applications at VT-R3 stages. “Treatments at VT through R3 have shown the most consistent results in research on tar spot management,” she says.
Bish recommends the free Tarspotter app (ipcm.wisc.edu/apps/tarspotter), developed at the University of Wisconsin to help guide 2024 fungicide applications in regions where tar spot is known to be present. This mobile app provides spray recommendations based on current agronomic and weather factors in each field.
When scouting, look for individual stroma or black lesions on leaf surfaces. Tar spot appears as small, raised black spots scattered across the upper and lower leaf surfaces. In severe cases, its stomata appear on leaf sheaths, husks and tassels. Corn is the only known host for tar spot.
The Crop Protection Network, an association of land grant universities, offers many resources on tar spot, including a corn fungicide efficacy chart that is updated annually. Go to https://cropprotectionnetwork.org.
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.
Submit samples of suspected tar spot to the MU Plant Diagnostic Clinic for confirmation and to help the clinic track progression of the disease.
Photo Credit: gettyimages-nes
Categories: Missouri, Crops, Corn, Weather