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Tar spot confirmed in southern Ontario

December 11, 2020  By OMAFRA Crop Talk

In late September, tar spot was confirmed in Chatham-Kent, Ont. Since then, the fungal corn disease has been found in Essex, Lambton, Elgin and Middlesex counties.

Tar spot was discovered relatively recently in the U.S. Midwest, first confirmed in Indiana in 2015. While it was expected to make its way to Ontario due to its spread pattern and confirmed findings in Michigan last year, it’s still a blow to corn growers.

Tar spot can cause severe yield loss; the pathogen causing tar spot (Phyllachora maydis) overwinters in infected corn residue. When conditions, such as high relative humidity and prolonged leaf wetness, are present, the likelihood of tar spot is greater.


Tar spot appears as small, raised, black spots scattered across the upper and lower leaf surfaces. These spots are stroma (fungal fruiting structures). If viewed under a microscope, hundreds of sausage-shaped asci (spore cases) filled with spores are visible. When severe, stroma can even appear on husks and leaf sheaths. Tan to brown lesions with dark borders surrounding stroma can also develop.

Regular scouting, practicing residue management, rotation, and staying clear of susceptible hybrids are strategies to avoid tar spot development and severity.

If you suspect tar spot, please contact OMAFRA field crop plant pathologist Albert Tenuta at


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