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Tar spot continues to loom on Ontario’s doorstep

August 17, 2020  By OMAFRA Field Crop Team

This week’s field crop report from the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) Field Crop News team discusses a new potential corn threat: tar spot.

Tar spot in corn

There is a lot of discussion and questions surrounding tar spot this year in Ontario. Tar spot is a new corn disease in the U.S. Midwest and, since its initial confirmation in northern Indiana in 2015, has moved into new areas including Michigan. Although the disease has not been confirmed in Canada or Ontario, it was detected in bordering counties in Michigan last year. As a result, the likely counties where tar spot will be first found in Ontario are Essex, Chatham-Kent, Lambton and Huron due to wind patterns and close proximity to infected areas in Michigan. For real-time tar spot tracking, which includes Ontario, please visit the Corn ipmPIPE website at

The concern with tar spot has led to the submission of many samples and pictures for examination. To date, all of these samples have been negative for tar spot, and the vast majority of samples have been confused with insect frass (poop)! It is easy to distinguish insect frass from tar spot lesions – you just need some water (or spit). Wet the leaf spot and rub the area between your finger – hence the scientific procedure name of “spit-test”! If the spot rubs off the leaf, it is not tar spot, which produces raised black lesions that are embedded in the leaf and don’t rub off.


What does tar spot look like?

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. These are known as “fisheye” lesions. In Latin America, where tar spot is more common, fisheye lesions are associated with another fungus, Monographella maydis, which forms a disease complex with Phyllachora maydis known as the tar spot complex. M. maydis has not been detected in the United States.

Scout now!

Corn during the mid to late grain stages (R3-R6) is when tar spot is most commonly found, so now is a good time to scout for the disease. Keep in mind, a few other diseases such as rust and physoderma brown spot can be confused with tar spot. Rust forms orange/red lesions which erupt through the leaf surface (volcano-like) and as they get older can turn black/dark brown. But, when you rub the lesions, the spores do rub off, leaving a smudge on your finger. Physoderma form flat brown lesions primarily on the leaf mid-rib or near leaf base, unlike tar spot, which usually occurs from the middle toward the tip of the leaf.

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

Soybean cyst nematode (SCN) and sudden death syndrome (SDS)

2020 has been a bad year for SCN and, more recently, SDS. For more information on identifying and managing these diseases, visit the Field Crop News website.


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