Though not yet found in NJ, keep tar spot to your radar when scouting corn.
Tar spot is a fungal disease of corn that causes small, raised, black spots to appear on both the upper and lower leaf surfaces (Photos 1 & 2). With severe disease, spots may also be observed on corn husks and leaf sheaths. The spots are the fruiting structures of the causal fungus, Phyllachora maydis. In addition to the spots, lesions can develop which appear as tan to brown with dark borders surrounding each spot. Late in the season, tar spots may appear similar to rust. A simple test is to try scraping the spot off with a fingernail; tar spots cannot be scraped off.
Tar spot was first observed in the United States in 2015 when it was found in northern Indiana and Illinois. Since then, tar spot has also been found in Iowa, Michigan, Minnesota, Missouri, Ohio, Pennsylvania, Wisconsin, Florida, and southwestern Ontario, Canada. Annual incidence of tar spot is documented in maps created by the Corn ipmPIPE (Photo 3).
Tar spot is a disease of concern in corn because severe disease can reduce grain yields by 20-60 bushels an acre. As with many fungal diseases, disease risk increases under high humidity and other conditions that prolong leaf wetness. Little is known about fungicide efficacy on tar spot, but we do know tar spot can overwinter on infested corn residue. Corn growers are encouraged to implement best management practices like crop rotation and residue management while also avoiding susceptible hybrids. More information on tar spot can be found in this resource from the Crop Protection Network: “CPN-2012 Corn Disease Management: Tar Spot”.
For information on current projects addressing management of tar spot, check out these two recent publications in Morning Ag Clips:
- [Article] PSU plant pathologists head investigation into tar spot disease in Pa. (10/5/21)
- [Article] Tackling “tar spot” threat to U.S. corn (10/26/21)
Tar spot was confirmed in the nearby Pennsylvania counties of Lancaster, York, Cumberland, Lebanon, and Dauphin in 2021. Although tar spot has not been identified in New Jersey, it is important to be on the lookout for symptoms when you are scouting. If you suspect tar spot on your corn, please don’t hesitate to reach out to us at (609)265-5050 or by email at firstname.lastname@example.org or email@example.com.