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Tar spot comes to corn

First identified in the U.S. in 2015, there is so much to learn about tar spot and its impact

The alarm bells were ringing. In just one year, Asian soybean rust could move in and devastate huge tracts of beans. Palmer amaranth swept north from the mid-south in 2010 to Michigan in only two years.

It’s not that crop pathologists and weed specialists were trying to alarm growers as much as make them aware of two serious threats to crop yields. Happily, Asian soybean rust never developed in the northern U.S. states or in Canada, and Palmer amaranth has yet to enter Ontario with any significance. But it’s best to prepare for the worst and hope for the best.

That’s the same mindset with tar spot in corn. For now, its impact has been limited to six states in the U.S. Midwest. First discovered in Illinois and Indiana in 2015, the most severe incidence was in 2018 in northern Illinois and southern Wisconsin with lower levels in Indiana, Iowa, Michigan and Ohio.

This map shows the six U.S. states where tar spot was confirmed in 2018.
photo: Courtesy Corteva AgriScience

As of mid-July this year, there were no confirmations of tar spot in Illinois nor in any surrounding states, according to Dr. Nathan Kleczewski of the University of Illinois. Soon after, Kleczewski reported very low levels of tar spot stroma had been detected in northwest Indiana, but given forecasts for hot and dry weather, sporulation and disease development seemed unlikely.

If disease levels are so low, why are plant pathologists, chemical companies and university researchers so concerned about this disease?

For one, there’s so little experience with tar spot across most of the corn-growing states: Iowa and Nebraska have yet to confirm the disease’s presence. As for hybrid susceptibility, fungicide timing and narrowing the impact of the pathogen, there is still so much to learn on both sides of the border. Here in Canada growers will watch and learn from the experiences of their U.S. counterparts.

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Two pathogens

What is known is that there are two pathogens that cause tar spot — Phyllachora maydis and Monographella maydis. To date, only P. maydis has been confirmed in the U.S. Midwest, with infection occurring late in the growing season. Yield losses have been estimated at eight per cent by the United States Department of Agriculture (USDA), although it’s believed that if the disease were to infect before R2 or R3, those losses could run higher.

In Mexico, farmers have seen yield losses as high as 30 per cent, but that has occurred where M. maydis is also present and forms a “tar spot complex” with P. maydis. The other key to its development is its ideal environment, specifically temperatures of 16 C to 20 C with wetter-than-normal conditions.

“Whenever there’s a new pathogen introduced into a crop, it’s obviously a concern and it presents a new management consideration for farmers,” says Dave Har­wood, technical services manager with Cor­teva Agriscience.

“In the case of this pathogen, it’s persisted nicely and has adapted to the same environment we have in Eastern Canada.”

Another consideration, adds Har­wood, is that corn grown in the higher latitudes of North America traces back to a unique set of genetics adapted to sub-tropical environments where this pathogen has its roots. Therefore, the genetics grown by crop producers in North America haven’t evolved concurrently with the pathogen, so it’s unknown how the crop will respond when the pathogen is present.

Only one of two pathogens that cause tar spot — Phyllachora maydis — has been confirmed in the U.S. Midwest.
photo: Courtesy Corteva AgriScience

The good news for growers in Ontario is that tar spot only affects corn. Soybeans and wheat are not host plant species, so farmers with three-crop rotations could see lower infection rates with a break in the disease cycle. According to Colin Elgie, an agronomy solutions specialist with Thompsons Limited, a three-crop rotation is a plus.

“Throughout the province, we have a pretty good mix of crops, and that should limit the spread and reproduction to a point,” says Elgie. “However, we’re not exactly sure how long it can survive on corn residue. It shouldn’t be a full three-year rotation that it would need a host to survive on between that. If we have it surviving in the field, it may be with continuous corn or being able to move with wind or storms.”

Elgie believes southwestern Ontario would be the likely entry point for tar spot, although like Harwood, he says information is limited. Parts of the U.S. Midwest are trying to get ahead of tar spot, though, and Ontario growers would be well advised to learn from what’s happening there.

“We don’t know how it behaves and how it differs from being a Central American disease to one capable of adapting to our climate in Ontario,” says Elgie. “It’s that lack of knowledge and research that’s the real concern.”

Harwood and Elgie stress the need for creating awareness as opposed to raising alarms. The introduction of a new disease will open the door on demands from plant breeders, adjustments in agronomic practices and fungicide timing. Dr. Martin Chilvers from Michigan State University, with a team of researchers there, tested two different fungicide modes of action at the R3 stage — a strobilurin (Group 11) and DMI triazole (Group 3) along with a premix. Results indicated several inconsistencies, including R1 applications, some of which did well while others still saw the disease make inroads a few weeks later.

Where to start

This has Elgie suggesting that growers may have to shift their scouting habits to possibly later in the season, based on studies that have shown that symptoms can be delayed two or three weeks after invection. That would see growers pushing beyond their conventional scouting periods during tasseling.

“It might be something that we have to scout for at a different time than we’re used to,” says Elgie. “For ear rots, for northern corn leaf blight and western bean cutworm, we’ve always been looking right around that tassel timing to R1, and we might have to be out scouting later in the season as well.”

Work has already begun in connecting Canadian and U.S. efforts on modes of action, application timing and getting products approved for use in the U.S. and Canada.

“We’ve engaged with our counterparts in the U.S. and are leveraging opportunities that exist there to screen our fungicide products against this pathogen,” says Harwood. “From there, we can create information that would meet the needs of the PMRA, if and when we need to get a label expanded.”

Hybrid susceptibility

The safe assumption with tar spot is that all hybrids currently available are susceptible to the disease. However, that doesn’t mean that seed and chemical companies aren’t studying its spread and trying to come up with viable solutions.

“In testing in the Midwest they’ve found a few hybrids that are showing some resistance or tolerance, but most seem to be susceptible,” says Elgie. “It’s not one of those diseases that breeders have been looking for so they haven’t been selecting for it. Tolerance might be coming through naturally with certain hybrids but until we know more, it’s better to assume that ours are susceptible.”

Harwood agrees and emphasizes that nothing is completely resistant or bullet-proof, and that there are degrees of tolerance that have appeared with tar spot. It’s a bit of a good-news, bad-news scenario where resistance is preferred but over time tends to fail, while tolerance can be more durable, more multigenic and a function of alleles or genes that collectively contribute to a better crop reaction to the disease. Again, given the lack of familiarity and expertise on tar spot’s management, there’s been little time to screen for that genetic code.

“We’ve had screening opportunities in the Midwest since 2015,” says Har­wood. But, he adds, “One thing that we haven’t had the opportunity to do is screen the earliest corn genetics, so that’s in the 2700 heat units and earlier.”

While growers play the waiting game, the best advice is simply, “Be Prepared.” Don’t give in to panic or alarm and understand the need to have a plan on how to deal with the disease, especially adjusting the timing on scouting for signs of the disease as well as the timing on fungicide applications.

Says Elgie, “If we get a plan in place, it’s a lot easier to put that into action, when and if we need to.”

This article was originally published in the September 2019 issue of the Corn Guide.

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Ralph Pearce

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