Tar spot detected in Ontario

Scouting is critical to deal with spread

Tar spot on a corn leaf, including “fish-eyes” (lower right) that can grow and encompass whole parts of a leaf.

Frequent rains and prolonged wetness are likely the contributing factors for Ontario's earliest instance of confirmed tar spot.

"This is the first time we've seen it this early based on overwintering population from Ontario," said Tenuta, field crops pathologist for Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). "Last year, it would have been the spores coming in from the U.S."

Why it matters:

First detected in Indiana and Illinois area in 2015, favourable weather conditions and early infection in 2018 and 2019 saw yield losses of 40 to 50 bushels in some fields as disease pressure reached the Michigan-Ontario border.

Ontario isn't alone in its July 7 confirmation of the airborne pathogen – Michigan, Indiana, and Iowa confirmed cases over the past week, said Tenuta.

Tar Spot appears as tiny, raised, spore-filled black spots scattered across the upper and lower leaf surface but can appear on the husk and leaf sheaths under severe incidents.

Rust and physoderma brown spot can be confused with tar spot, but rust spores leave a smudge on your finger when rubbed. Physoderma brown spot is primarily mid-rib or near the leaf base, whereas tar spot occurs from the middle to the tip of the leaf.

Tenuta indicated the current sighting was low severity, with one to five lesions of a couple of millimetres in size appearing on the bottom three to five leaves, transmitted via soil and corn-on-corn in a late September 2020 confirmed field in West Elgin.

Tar Spot was on Ontario's radar during the annual corn disease survey with Agriculture and Agri-Food Canada, funded by the Grain Farmers of Ontario (GFO) and Canadian Agricultural Partnership (CAP), due to wind patterns and proximity of infected areas in Michigan.

Although the pandemic restricted some surveying, the group focused on targeted fields and found instances of Tar Spot in Ridgetown in late September 2020, followed by Essex, Elgin, Lambton and Middlesex Counties.

In 2021 OMAFRA initiated tar Spot fungicide trials and commercial corn hybrid screenings to assess ratings in Elgin County, which contained the worst-hit fields last year, said Tenuta.

 The pathogen can move short distances in the air. Still, assisted by intense storm fronts, it can travel significant distances, allowing it to move across Iowa through Michigan, the Great Lakes, and Southwestern Ontario.

"I was not surprised that we saw it; I was surprised at how quickly I noticed the tar spot lesions because we were significantly dry in this area," he said. "We had rain, and it took under two weeks for us to see the first small lesions starting to form."

It's not all bad news.

Tenuta is active in several foliar corn disease working groups, including the United States Tar Spot Working Group, which has allowed him to establish trials to evaluate products registered in Canada and the U.S.

"The good news is products that are available to our Ontario farmers are very effective against tar spot," he said.

New fungicides set to hit the Canadian market have proved effective against Tar Spot and Northern Corn leaf blight.

Currently, 65 commercial hybrids are undergoing evaluation in tar spot infected fields to assess their ratings against foliar disease pressure, said Tenuta.

"We've got a great opportunity this year to learn on both the hybrid side as well as the fungicide side," he said. "We are working with our U.S. colleagues to validate forecast models for tar spot, just like we've done with white mould and other disease-forecasting models as well."

Tenuta said scouting corn is critical now as corn moves into its tassel stage, especially in areas with or are at higher risk for disease and insect pressure.

At this growth stage, tar spot would likely appear mid-canopy in new fields impacted by airborne spores and lower in the canopy in areas seeing disease pressure from residue in the soil.

"As plants are infected, as they are older or farther along in the growth stages, then the impact can be less," Tenuta said, optimistic the early planting season will help diminish tar spot impact. "Now, having said that, we've got a lot of growing season left – we've got months left to finish off the crop here."

In 2018, a cool, wet summer growing season resulted in Michigan cornfields seeing yield losses of 40 to 50 bushels in fields where tar spots were detected on July 12. The plants were dying off by mid-to-late August, he said.

"The best thing is if we can get a handle on the hybrid genetic potential so growers can plan ahead for that," he said. "And we do have, as I said, fungicides that are very effective against tar spots."

About the author


Diana Martin

Diana Martin has spent more than two decades in the media sector, first as a photojournalist and then evolving into a multi-media journalist. Five years ago she left mainstream media and brought her skills to the agriculture sector. She owns a small farm in Amaranth, Ont.



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