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Tar Spot Takeaways
By Emily Unglesbee
Tuesday, September 28, 2021 9:05AM CDT

ROCKVILLE, Md. (DTN) -- If you want to know what dominated Jim Donnelly's summer, just take a peek at his phone.

"I've got more pictures of tar spot on my phone than I do my kids," the DeKalb agronomist lamented. "I don't love it more than my kids; that's just the kind of year we had!"

Donnelly's territory stretches across northern Illinois, but his experience is a familiar one to farmers across the Corn Belt, which saw epidemic levels of the disease this summer. Fueled by plentiful inoculum from previous years' outbreaks, lots of early and midsummer rains and plenty of susceptible hybrids, tar spot in corn made it clear this yield-robbing disease is here to stay, said Michigan State University plant pathologist Martin Chilvers.

So what now? Here are five big takeaways from the season:


Growers who saw significant tar spot infestations in their fields should be watching their corn stalks like hawks, Chilvers warned. "Get out and check fields for lodging or stalk integrity because tar spot really shuts the plant down early and significantly affects stalk quality," he said.

Harvest those heavy tar spot fields first, even if they're still wet, he added. "This is a year where you might want to pay a little bit more for drying if it means you don't have to worry about stalks falling over."

Tar spot has proven it is capable of stealing significant yield. Chilvers said he is getting reports of 60 to 100 bushel-per-acre (bpa) losses this year, not unlike the 2018 growing season.


Scientists have learned quite a bit about tar spot of corn since it first surfaced in Illinois and Indiana in 2015. One early finding was the disease is happy to camp out in the Midwest, all winter long, on infected residue.

Now it's becoming clear that residue management strategies, such as crop rotation and tillage, only have limited effectiveness.

"It is moving on the wind, airborne," Chilvers explained. "So even if you did everything right to manage infected residue in your field, you can still get it because it's a regional problem."

Or, as Donnelly puts it, "If you had it this year, it will be there forever."

But don't despair! Like most diseases, tar spot needs certain key environmental conditions to create an epidemic year.


Wet summers seem to be a common denominator in bad tar spot years, Chilvers said. "Early on, it was a quiet start to June, but then we had 10 consecutive days of moisture and my alarm bells went off -- that will prime everything."

Donnelly agreed, noting the worst areas for the disease in his state tend to be ones that got above normal rainfall amounts in June, and even as far back as May. "That seems to be a key underlying factor, and then other things like repeated rainfalls, leaf wetness, higher humidity, cloudier weather -- those are all things that will exacerbate it."

Another early warning sign is the disease showing up prior to tasseling, Donnelly said. "The later it sets in after tasseling, the lower it seems our risk will be."

The disease made big strides this year, surfacing for the first time in states like Kentucky, Pennsylvania and Georgia, as well as the Canadian province of Ontario. See its spread here: https://corn.ipmpipe.org/….


Fungicide applications are proving to be very effective at warding off the worst effects in a bad tar spot year, but researchers and companies are still poring through data to fine-tune what the best timing and frequency is.

Careful scouting for the disease's presence and watching for favorable weather conditions is a must, Chilvers said. An app created by the University of Wisconsin researchers, called Tarspotter, can help growers with those decisions. See it here: https://ipcm.wisc.edu/….

The one-and-done application mindset might not be sufficient with tar spot in bad years, Donnelly said. "With the aggressive nature of tar spot, under high disease pressure a second application could be warranted," he said.

Researchers are in agreement that V5 to V6 fungicide applications are too early to be helpful, Chilvers said. In a heavy pressure year, timing an application close to silking, from VT to R1, followed by another application three weeks later, seems to work well.

But commodity prices might not always support two applications, even in a bad tar spot year, he added. "Right now, it's easy to talk about two fungicide applications," he noted. "But if you're stuck with a single application -- if you have to schedule an aircraft, for example, and can only do one application -- I'd lean toward R1 to R3."

Finally, the quality of a fungicide application is proving extremely important for this fast-moving disease, Donnelly said. "With aggressive diseases like tar spot and southern corn rust, coverage is critical," he said.

See the Crop Protection Network's fungicide efficacy chart for tar spot here: https://crop-protection-network.s3.amazonaws.com/….

See more on timing of fungicide applications for Tar Spot from CPN, as well: https://cropprotectionnetwork.org/….


Part of the reason tar spot can rage so successfully in wet summers is that most hybrids in the country have at least some susceptibility to it.

"I would contend that all germplasm in North America is susceptible, but there are varying degrees of tolerances within it," Donnelly said.

Farmers can't rely solely on those tar spot-tolerant hybrids in a bad year like 2021, both Chilvers and Donnelly noted. "In a light disease year, maybe that varietal resistance will be enough to keep it at bay," Chilvers said. "But in a year like this, you will need fungicide as well."

Big disease years do motivate companies to hunt down more hybrid resistance and they also supply them with more data to do so, Donnelly said. Talk to your seed dealer about what hybrids they can recommend for tar spot tolerance but remember that the disease may not be prominent next year, he added.

"We have to be prepared for it, but don't assume next year will be a train wreck like this year," he cautioned.

"I really hope that in five years' time, tar spot will be more like northern corn leaf blight or gray leaf spot, in that they are always present, but we have a better handle on them," Chilvers concluded. "But it will take time for more tar spot resistant hybrids to come through."

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow her on Twitter @Emily_Unglesbee

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