14 Ate Today? Thank a Farmer. Better Farming | June/July 2024 ‘THIS IS A GOOD YEAR TO BE OUT SCOUTING & KNOW WHAT’S GOING ON IN YOUR REGION.’ EMERGING DISEASE: TAR SPOT By EMILY CROFT Lesions (stroma): Small, raised, round, irregular shape. Tar Spot Albert Tenuta photo Tar spot is an emerging foliar leaf disease in corn which first appeared in the U.S. in 2015 and was later identified in Ontario in 2020. As the disease continues to spread and impact Ontario’s corn crop, it’s important that producers are informed about how to identify tar spot and which resources are available to monitor it in their area and beyond. Tar spot is caused by a fungus called Phyllachora maydis, which produces black, tar-like spots on the leaves of the corn plant. It is somewhat climate dependent, thriving in cool temperatures with high humidity. The disease is transmitted through spores, which are airborne and are now known to also overwinter in residue. Albert Tenuta, field crop pathologist at OMAFRA, says that the mild 2023/2024 winter leads to questions about how that will affect the spread of tar spot this growing season. “The question we have this year is, since we’ve had a mild winter, what does that do for tar spot? It could be an issue here in areas where it’s overwintering and in the U.S.,” says Tenuta. “This is a good year to be out scouting and know what’s going on in your region as well as what’s happening in surrounding areas, including south of the border.” How it got here Ontario’s corn crop has been affected by tar spot since 2020. “Tar spot was first reported in a field near Ridgetown in September 2020, and was later confirmed in fields in Essex, Chatham-Kent and Lambton,” says David Hooker, associate professor in the Department of Plant Agriculture at University of Guelph – Ridgetown Campus. “The regions with the greatest risk are the counties in the southwestern part of the province, although tar spot has been found in fields as far east as Toronto. The disease has been moving eastward for years, so corn producers across all of Ontario and Quebec need to familiarize themselves with the disease and how to manage it.” Tar spot was first identified in the U.S. in 2015, in Illinois and Indiana. Bob Thirlwall, agronomic solutions advisor at Bayer Crop Science, says that weather played a large role in the spread of tar spot to Ontario. “It was spread to Ontario on storm fronts or weather patterns from the western U.S.,” says Thirlwall. “The spores were transferred by the wind and landed on the Ontario corn crop and spread from there.” Regions previously affected by tar spot have an increased risk of continuing to be infected year-after-year, depending on environmental conditions. Fisheye lesion (tan halo).
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