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The most important step in managing tar spot is scouting for it.

Kristy Foster Seachrist, Digital editor

July 30, 2019

3 Min Read
Ohio State University

Tar spot was first spotted in Indiana and Illinois fields in 2015. Since then, it has spread.

It’s no secret that in 2018, the corn disease impacted yields heavily in parts of the Midwest. Some agronomists even called it an epidemic that had a significant impact on corn grain and silage yield and quality. Tar spot has since been confirmed in several states including Florida, Iowa, Michigan, Ohio, and Wisconsin.

Move forward to 2019. Researchers have found that it takes specific weather conditions for Tar Spot to grow in the corn fields. It requires a high humidity, seven hours of leaf wetness, a high monthly rainfall, cooler temperatures and high dew points. Unfortunately, exactly what the Midwest has experienced in June and July this year.

Kevin Scholl, an agronomist with Syngenta, said that Tar Spot just needs the right environment and host to become a problem in fields which ends with a reduction in bushel yields.

Scholl and  Nathan Kleczewski, a field crop pathologist in the Department of Crop Sciences at the University of Illinois agree that scouting is critical.

So far, Tar Spot hasn’t been found yet but due to the late planted corn and the resulting longer disease stage, it’s a disease that can’t be ignored this year.

Tar spot is a pathogen that could have been on the leaves from 14 days to as long as 40 days before it is noticed. “The important thing to note is that when you start to see it, know it’s been in the leaf for awhile,” said Scholl.

“Increased severity likely means increased local inoculum for this season,” Kleczewski says. “If you are planting corn in a region that was hit hard by tar spot last season, your risk for disease is elevated compared to areas where disease was sparse or absent.”

Kleczewski notes that planting corn after soybean or into tilled fields could reduce the incidence of the disease, compared with planting into corn residue. He also emphasizes that scouting will be most important in the days and weeks approaching tasseling, when fungicides can still be effective.

“If you notice tar spot showing up prior to tasseling, a fungicide may help. There are several products with a label or 2ee recommendations for tar spot suppression. Like rusts, this is an obligate fungus, and you want to ensure that the ear leaf and leaves above are protected during the critical periods of grain fill. You do not want to chase this disease; revenge sprays will not work,” Kleczewski says.

Scholl, says, if you suspect a problem in your field, then a fungicide like Trivapro from Syngenta with three modes of action which prevents the pathogen might be the key. He said thekey is to keep the upper canopy of the plant free which keeps the photosynthesis factors working which keeps the plant growing longer in the season.

Both Scholl and Kleczewski say scouting for the disese is the most important step in the management process.

“Keep it as bay, if not will get ahead of you and you can’t catch up especially if the environment is right,” says Scholl.

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