What tar spot’s arrival means for Ontario

Cases identified in Lambton and Chatham-Kent, but impact insignificant overall

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Tar spot, a pathogen affecting corn, has moved into Ontario, after five years of international attention.

Tar spot can cause significant yield losses if left unchecked. The problem does not now appear to be widespread.

Why it matters: Evidence from the United States indicates tar spot can depress corn yields by 20 to 60 bushels per acre.

“With what we’re seeing now there should be no major impact for 2020,” says Albert Tenuta, field crop pathologist with the Ontario Ministry of Agriculture, Food and Rural Affairs.

According to Tenuta, reports of potential tar spot infection have not been unusual this year. All but two discoveries proved to be other issues. But one case in Lambton County and the one at the University of Guelph, Ridgetown, were positive.

“The incident in (the Ridgetown field) was a 10 to 20 per cent infection rate, so it was pretty easy to find,” Tenuta says, adding no other cases have been identified in surrounding fields. And while final testing is still ongoing with the Lambton case, everything indicates tar spot as the culprit.

Profile of a pathogen

A Field Crop News report from August detailed tar spot as a corn disease emerging in Indiana in 2015. It has since spread to neighbouring states, including Michigan. Because of location and local wind patterns, tar spot was expected to appear first in Ontario in the counties of Essex, Chatham-Kent, Lambton, and Huron.

“It’s not unexpected to have found it,” says Tenuta. “Projections would have been based on airborne movement, storm fronts. The southwest portion of the province would likely be the areas. These two fell quite into place. There are probably other fields as well.”

Visually, tar spot appears as small, raised, black spots scattered across the upper and lower leaf surfaces. As described by the Field Crop News report, these spots are fungal fruiting structures which, when viewed under the microscope, show hundreds of sausage-shaped spore cases. These spore case groupings can also appear on husks and leaf sheaths when severe.

Brown fisheye lesions featuring dark borders can develop too, and have been linked to other fungal diseases in Mexico, though not yet at higher latitudes.

Tar spot can be confused with other diseases, namely rusts, as well as insect droppings, but the latter two will rub off, while tar spot will not. Tar spot also prefers to infest from the mid-point to leaf tip.

The emerging pathogen is generally found between the R3 and R6 growth stage, but can develop earlier if conditions are favourable. That makes early August a prime time to begin scouting. However, Tenuta reiterates the presence of moisture and cooler temperatures are the main determining factors.

“Most years the disease develops during the milk growth stages of corn,” he says. “A lot of it is based on the environment. Years such as 2015 and 2018 in the United States with greatest problems also had cooler, wetter conditions earlier in the season. This resulted in significant yield losses.

“For every one per cent infection on the ear leaf you could lose anywhere from one quarter to one bushel.”

Like other diseases, Tenuta says researchers on both sides of the border have been co-operating to study the spread of tar spot, while developing responses.

One of the main challenges is the pathogen’s ability to overwinter in crop residue and infect subsequent corn crops. Good rotation, tillage, and removing or otherwise breaking down crop residue can help suppress reinfection risk.

Tenuta says “reduce” is the key word, since risk cannot be eliminated once the pathogen is established in an area.

“There’s work being done right now on hybrids on their susceptibility and tolerance to reduce infection and severity,” he says.

Effective chemical options are also being explored through the Crop Protection Network.

“A later fungicide application may be an option for tar spot. We’re looking at timing and different fungicides. One of the benefits of being part of these research groups and teams is we have access to that research,” says Tenuta.

He adds they are still trying to identify which fungicides best combat tar spot, though several appear promising.

About the author


Matt McIntosh

Matt is a freelance writer based between Essex County and Chatham-Kent. He is interested in all things scientific, as well as rock n' roll, hunting and history. He also works with his parents on their sixth-generation family farm.



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