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Tracking tar spot in Ontario

Scouting is the foundation for managing this challenging new disease

In case you haven’t heard, tar spot in corn is finally part of the landscape in Ontario. Tar spot been plastered across the U.S. farm media for the past three years. Advisors, researchers and extension personnel in Ontario have been saying it’s inevitable that, given the diseases’s ease of transmission and rapid spread in the U.S. Midwest, it would arrive here too.

Universities in Indiana and Illinois have circulated reports about tar spot ever since its discovery there in 2015, and they have tracked its advance throughout much of the Corn Belt. During the 2020 growing season, it was confirmed in nine states, covering almost all of Indiana and Iowa, and more than half of Illinois, Wisconsin and lower Michigan (see map below). There were also infections in one corner of Minnesota, a small area of Ohio, one county in Pennsylvania and four counties in southern Florida.

A map from the IPM Research Centers in the U.S. shows regions of the U.S. Midwest and southern Ontario where tar spot in corn has been confirmed (yellow areas indicate positive cases of tar spot, grey areas indicate where tar spot was found in previous years).

Tar spot in corn represents a relatively new concern for growers, not just because of any leaf damage or yield losses, but in its identification and subsequent management. Its appearance on corn leaves is similar to other conditions and diseases in corn. For one, its small black spots can be mistaken for saprophytic organisms like alternaria or insect frass (droppings). The colouration it can cause on leaves can also be mistaken for northern corn leaf blight.

Tar spot, however, is readily identifiable by its so-called “fish-eye” pattern, where there’s the same black spot on the leaf that’s surrounded by a yellow halo. The halo can grow and amalgamate with other fish eyes, occupying a greater area on the leaf surface.

Yet it’s the speed with which the disease spores can spread — and potential yield losses — that garners headlines. The spores are easily dispersed by the winds that come with thunderstorms that strike the U.S. Midwest during the summer months. According to a report issued by Purdue University in August 2019, tar spot’s greatest impact comes under relatively cool conditions — 16 C to 24 C — and periods of high relative humidity (85 per cent) longer than seven hours.

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

Researchers reported yield losses of 20 to 60 bu./ac. But those numbers also came from “the most severely affected regions (with) 100 per cent disease incidence and over 50 per cent severity on the ear leaf before the dent growth stage (R5/R6).”

Closer to home

Subsequently, tar spot was confirmed in isolated locations in Essex, Kent, Lambton, Middlesex and Elgin counties in Ontario late in 2020. However, the infection levels did not affect yield or plant health to any extent.

Still, tar spot’s arrival has served as a call-out to growers that they need to be aware of the disease, and to work with disease extension personnel, researchers, advisors and agronomists.

Far from ringing alarm bells or generating panic, Rob Miller wants to create awareness and an information base that growers can use to make better management decisions. Miller has been working with Albert Tenuta from the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), who, in turn, has been work- ing with others in tracking the disease through Michigan for the past two years. Fortunately, the symptoms of tar spot identified in Ontario occurred later in the sea- son towards the end of September.

“There was low incidence, and low severity on some of the fields and with some of the hybrids,” notes Miller, field biologist/technical development specialist with BASF Canada. “It would have had very minimal impact on yield this year because it did come in so late in the season — the corn was already at black layer and some of it was frosted-off at that time.”

Damage to a corn leaf can be rapid, turning green leaf tissue brown in a relatively short period of time.
photo: Courtesy Trevor Kraus, BASF

The key to remember is that, with tar spot, every year is different, and even if it was confirmed in Ontario in 2020 that does not necessarily mean it will be a problem in 2021. There remains considerable uncertainty with any disease, courtesy of the disease triangle where the pathogen, host plant and environmental conditions must be favourable all at the same time. Even if the pathogen overwinters or blows in with storms from Michigan, the right environmental conditions must develop.

“If growers do identify it or suspect it in their fields, the main thing is getting it tested to confirm it,” Miller says. “Then there’s the opportunity to use the tools to manage that in the future.”

If it continues to appear late in the season, it’s less likely to have an impact. But if it comes in earlier — around tassel or during grain-fill — and it’s on a more susceptible hybrid, that’s where it has the potential to have a negative impact on yield.

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“What little we found in Ontario is manageable, but we still have to look to the U.S. to see where those spore loads are coming from because that’s where we’re going to have the greatest impact,” says Miller. “If you look back at 2018 when we saw it take off in Michigan, there were long periods of leaf wetness during grain fill and it had a greater impact on yield.”

The other factor that makes tar spot harder to manage is its ability to overwinter, which is expected to occur here as it has in parts of Indiana, Illinois and Michigan. Looking at the map of its incidence in Wisconsin and Michigan, it’s easy to assume the disease will progress farther north into Ontario, possibly to the same latitude as Georgian Bay or even the Ottawa Valley.

Looking to the future

With its confirmation in Ontario and the ease of its spread, Dave Curry’s advice to growers echoes much of Miller’s approach: be informed, not alarmed. Curry never saw any examples of tar spot in corn in his field work in 2020, although he concedes it’s likely to spread to neighbouring counties and beyond in 2021.

“This was expected and it’s likely that it was here last year (in 2019) but no one found it,” says Curry, a certified crop advisor (CCA) and crop inputs representative with Lakeside Grain and Feed in Forest, Ont. “It’s definitely something we’re going to be battling and it’s going to change a lot of corn growers’ practices going forward.”

A severe infection of tar spot on a corn leaf.
photo: Courtesy Trevor Kraus, BASF

In terms of the disease’s impact on the plant, Curry likens tar spot to northern corn leaf blight, which has been an issue in Ontario. He recalls a time roughly 10 years ago when a particularly susceptible hybrid saw a 40-bushel yield loss due to northern corn leaf blight.

“It’s a very similar weather pattern and it came in at tassel or early reproductive timing, when that plant still needs that photosynthetic engine to keep chugging along and packing that ear full of starch,” says Curry. “Tar spot’s going to be a later disease but if it’s here even up to the R3 stage, you can still see a fairly significant yield impact. It does sound like it’s extremely aggressive.”

Currently, there are no hybrids with resistance to tar spot, and resistant hybrids — which Curry believes could come be based on Mexican or Mesoamerican germplasm — are still five to 10 years away from commercial availability. In the short-term, at least, genetic resistance won’t be a solution.

What has to happen will come from management of residue and the timing on fungicide use.

“From everything I’ve read it can overwinter, it stays on residue and in general I’m noticing that with the customers I work with, there’s a lot of push towards minimum tillage or vertical tillage or strip till,” says Curry. “The residue is staying on the soil surface and we’re not getting that deep incorporation with conventional tillage to break it down and speed that decomposition process.”

Since its spores have the ability to overwinter, the inoculum load in the soil isn’t going to dissipate; Curry notes it doesn’t break down during the course of a growing season or over the winter. What may make a difference, he adds, is that the density of corn in southern Ontario is less than in most of the U.S. Midwest, thanks in part to a greater diversity of crops (with horticulture) and longer rotations. As a result, the U.S. situation might have been easier for the disease to blow through counties due to a higher potential rate of infection.

“Having a colder winter will hopefully slow the spread so it’s not going to jump to life as early in the spring and it will shut down earlier in the fall,” suggests Curry. “Maybe it won’t move as quickly through Ontario, but it’s definitely going to move. And we do have some management options to help slow the spread locally. But we’re going to have to go in and use a fungicide product. The timing will be critical — at that tassel or early silking stage.”

Compare that to parts of the U.S. Mid- west where it’s appeared earlier and some farmers have had to be out two, three — or sometimes four weeks — earlier with a fungicide application, then following up with a sequential application. That adds to costs as well as stresses personnel and equipment numbers. Curry wonders if there’s enough equipment to cover every acre of corn twice in a growing season.

“Some of these sprayers are already maxed out as it is, so that’s going to be challenged,” he states, adding the other concerns are the type of fungicide products available, and that multiple modes of action will be the preferred option. “If you’re giving two shots on a field, hopefully growers will be using different actives, even though there are only three different types of fungicides to pick from.”

According to Miller, one of the products U.S. growers have access to is a multiple-mode-of-action fungicide called Veltyma, from BASF. It has shown very good activity on tar spot and has been submitted for use with Canada’s Pest Management Regulatory Agency (PMRA). But it will take time to move it through regulatory channels.

On the other hand, both Miller and Curry remind growers that although tar spot has been confirmed in Ontario, it doesn’t mean it’s a foregone conclusion that 2021 will be worse for the disease.

This article was originally published in the January 2021 issue of the Corn Guide.

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Ralph Pearce

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