UW-Madison Plant Pathology Prof. Damon Smith says tar spot has put moderate pressure on corn growers this year. The growing season has seen conducive conditions for the disease, but no extended dry cycle for the disease to cause a lot of damage.
“While the risk has been high and folks are finding tar spot, overall the severity level — how much disease is actually there — is quite low right now,” Smith says. “We haven’t reached the upper canopy yet, we aren’t seeing yield-limiting levels of disease yet.”
He says if farmers can get their corn crop through the risk window, they’ll be outside of substantial yield reductions from this disease. Smith explains if tar spot reaches the upper third of the canopy and gets to 5-10 percent leaf coverage, growers can see substantial losses. In 2018 and 2021, he says some fields saw yield reductions between 40-80 bushels per acre.
Smith recommends using the tar spot app, Tarspotter, to stay up to date on the disease: https://ipcm.wisc.edu/apps/tarspotter/
Tarspotter is meant to assist farmers in making management decisions for tar spot in corn. The best time to manage tar spot is during V8 to R4 growth stage.
If you’re starting to see tar spot in the lower canopy and you planted late, you might want to apply fungicide once, Smith recommends.
“That later crop is the one you want to watch, and that may be the once you need to put an application on,” he says.
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