Tar Spot Primer | Pioneer® Seeds
Agronomy •  7/1/2022

Tar Spot Primer from the Experts

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5 tips, insights and actions to help take the offensive against this corn country yield robber.

Tar spot slashed U.S. corn yields by about 231.3 million bushels in 2021 – more than any other disease, according to the Crop Protection Network.

U.S. production losses soared from 64 cents an acre in 2020 to $13.69 per acre in 2021, totaling nearly $1.247 billion in crop losses last year. That’s according to the latest statistics from the Crop Protection Network, a collaborative effort of U.S. and Canada land grant universities.

And as tar spot continues its march into corn-producing states, the extent of the 2022 infestation will hinge on weather and crop management, says Darcy Telenko, assistant professor of field crop plant pathology at Purdue University.

This foliar pathogen favors long periods of wetness and high humidity. The first 2022 cases aren’t expected until July, she says.

Will Tubbs, an Iowa-based market specialist for Corteva Agriscience, urges growers to act now. “Tar spot is probably the biggest unknown issue we have in corn production, just from an agronomic standpoint, and going into 2022.” 

Photo - Tar spot lesions on corn leaf - closeup

HARD TO STOP: Farmers are finding that if tar spot is in the area the best approach for treatment is to apply fungicide even before the disease arrives. Tar spot is challenging growers across the Upper Midwest.

1. Be proactive, check hybrid ratings

Tubbs’ biggest tip is to be proactive, starting with choosing hybrid seeds with potential disease tolerance.

“Once that corn does get out of the ground, go out and scout, walk through your field,” he says. “Have a plan ready with your retailer or chemical supplier if you do find something in your field and need a fungicide application.”

Mike Koenigs, market development specialist for Corteva Agriscience in Illinois, notes that for 2022, the bigger seed companies came out with the first tar spot tolerance ratings on hybrids. You can check whether a hybrid has some tolerance or is more susceptible to tar spot than others, he says.

No hybrids are fully resistant, he warned. 

2. Time fungicide, keep tar spot at bay

You can start seeing symptoms early, at tasseling time, or later.

“What we’re seeing is that it really takes off in the crop canopy just before dent,” Telenko says.

Consider applying fungicide as a preventative before you spot a problem, and in areas prone to tar spot, Koenigs advises two applications.

“With Corteva, we’ve got our Aproach Prima brand and our Aproach brand, both are labeled for control of tar spot,” Koenigs says, adding that they offer options.

“Both those products have a flexible application window and flexibility with tank mix partners,” he adds, whether it’s a nutritional product or an insecticide.

“The critical component of that fungicide is the timing of the application,” Koenigs stresses. “We need to get that applied before the tar spot disease takes off. And that's always challenging – to predict when the disease is going to take off.”

There’s not much latitude with this pathogen.

“Generally, once you see it, it's very difficult, if not impossible, to stop,” Tubbs says. “You may be able to slow it down with some different things. But if there's enough disease in the area, then it's pretty much going to run its course. And it takes over very, very quickly.

“Within a month, you can go from a few symptoms, or a few black small, bumpy lesions on the leaf surface, to all-out plant death in more severe cases.”

Koenigs recommends applying between growth stage VT to R4, “a critical window in which we need to keep tar spot at bay; we cannot let tar spot take off within that window or you will see yield loss,” he says.

“If you can stop the disease in the lower canopy, and keep it from working its way up, you can still maintain significant yield. But if you don't stop it and let the disease take over the plant, then it's significant yield loss.”

3. Extra steps for conducive conditions, hotspots

The nation’s hotspot has historically been in a big region shaped like a horseshoe around Lake Michigan, with some areas of extreme infestation and others, light, Koenigs says.

2021 and 2018 were the worst years in northern Illinois, southern Wisconsin and eastern Iowa, Tubbs says. For 2022, those growers are planning fungicide applications before they even see tar spot symptoms, around VT and as tassels are emerging from the plant.

Some growers want to apply another fungicide application later in the season for more protection because of how quickly and severely the yield losses can occur late season, when tar spot could set in, he says.

Those Midwest corn growers who irrigate will need to take extra steps, Koenig noted.

4. Long in Mexico, tar spot’s U.S. arrival brings questions

The disease was discovered more than a century ago in Mexico but wasn’t found in the US Corn Belt until 2015, in Indiana and Illinois, Telenko says. It was initially thought to be more of a cosmetic problem. Because it had only one of the two pathogens found in Mexico’s tar spot, experts didn’t think it was going to be a big issue in the Midwest.

The inoculum began slowly building up, though, and four years ago, “we just had the perfect weather conditions that caused it to really ramp up and cause significant issues,” Telenko says.

“In 2018, we had a wet season, and that caused it to take right off into our crop canopy in those states. We saw the first documented real loss in corn, which was in Indiana, Illinois, Michigan, Wisconsin and parts of Iowa. And that season, we documented 20 to 60 bushel an acre, on average, loss.”

The raised black speckling on upper and lower leaf surfaces are fungal fruiting structures that, if viewed under a microscope, appear as hundreds of sausage-shaped spore cases. The pathogen overwinters on residue on the soil surface, but extra tillage has had minimal impact, experts say.

5. As infestation grows, losses mount

The Crop Protection Network reports about 1.1% of U.S. corn production destroyed by tar spot in 2018, with losses totaling about $658 million that year.

Then last year, tar spot spread in every direction. The Crop Protection Network reported 231,311,565 bushels lost in 2021, representing 1.44% of the US corn crop.

Check out Tarspotter, a forecast model developed by the University of Wisconsin, and consult with your extension agent and local Corteva sales representative to help you make management decisions.

This content produced by Farm Progress for Corteva Agriscience.