3 Scouting Tips to Stay Ahead of Tar Spot

Tar Spot ( Missy Bauer )

Tar Spot has been lurking in Midwest fields since 2015—and possibly even earlier by escaping detection. In five years, it has spread to eight states and decimated yield potential in its path.

“For a lot of us this is a pretty new disease, there are a lot of unknowns,” says Missy Bauer, Farm Journal associate field agronomist and co-owner of B&M Consulting. Tar Spot pressure had increased dramatically across the northeast Corn Belt, with 50 bu. per acre losses recorded in Michigan.

As of 2019, Tar Spot has been confirmed in Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio and Wisconsin. It seems to spread quickly and fields that have had the disease in previous years will likely see it pop up again when corn is planted.

tar spot

Source: https://corn.ipmpipe.org/tarspot/ 

Here are a few scouting notes:

  • Lesions literally look like small tar spots on ears and leaves and the spots cannot be rubbed off with your fingers.
  • Lodging can be concern as the stalk will cannibalize itself from reduced photosynthesis to save the ear.
  • The plant will rapidly turn brown and stop photosynthesizing if the field is not treated with an effective fungicide.

You’ll have to scout yourself or physically get a scout in the field to catch the disease early. It’s best to catch it early, and treat it early.

“One thing that makes it kind of scary is it can develop rapidly,” Bauer says. “If you get behind it, it will be ahead of you [and affect yield] in two weeks.”

tar spot tar spot

^^^August 24 to September 7 disease progression. Photo credit: Marty Chilvers, Michigan State University

Fungicide timing for tar spot is different than typical tassel sprays you might be used to.

“That fungicide timing with tar spot, depending on when it gets started, might be even too early,” Bauer says. “Fungicide helped [save yield] dramatically in 2019, so be ready to spray.”

If the plant is killed too early by the disease, you’re likely to see yield loss.

“Keep an eye on later, full season hybrids,” says Ken Ferrie, Farm Journal field agronomist and owner of Crop Tech Consulting. “Keep them protected the last 30 days of [grain] fill. This is nothing we can’t handle—just stay on top of it.”


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