Answer your Tar Spot Questions with On-Farm Research

By:  Stephanie Karhoff

To growers’ dismay, 2021 is shaping up to be a “good” year for Tar Spot (“good” if you are a plant pathologist at least). Though the combine will have the last say on Tar Spot’s yield impact on corn fields across Williams County, it is already evident that Tar Spot has become more widespread across the county, state, and even the Corn Belt.

Tar Spot was first reported in Ohio in 2018 and is caused by the fungus Phyllachora maydis. The tell-tale symptom of Tar Spot are raised, black spots that cannot be scraped off the plant surface. Disease development is favored by wet, humid conditions and the pathogen can overwinter on infested corn residue. 2021 had the perfect trifecta of favorable weather, susceptible hybrids, and presence of fungal spores. This led to higher Tar Spot levels than in the past two years, but also more questions. What hybrids are more resistant? What fungicides are most effective? When should I spray? Should I spray more than once? Will crop rotation or tillage reduce spore levels?

Since Tar Spot is a relatively new disease to Ohio, often there are more questions than answers. The pathogen (Phyllachora maydis) can only survive on its corn host and cannot be grown as a pure culture in the laboratory like other corn pathogens. This makes inoculation trials difficult, and is why OSU Extension State Specialist Pierce Paul is currently seeking fields with a history of Tar Spot to participate in various on-farm research trials. If you are interested in participating in Tar Spot on-farm research, please call ANR Educator Steph Karhoff at 419-519-6047 or e-mail karhoff.41@osu.edu.

Sources:
https://cropprotectionnetwork.org/resources/articles/diseases/tar-spot-of-corn
https://agcrops.osu.edu/newsletter/corn-newsletter/2021-30/tar-spot-more-widespread-cross-state-ohio-2021