Risk for tar spot remains generally high

Colleen Kottke
Wisconsin State Farmer
Tar spot fungus intensified in Wisconsin corn in 2017. Hot spots in Southwest Wisconsin saw as much as a 20 percent drop in yields in 2018.

The present risk for tar spot development on developing corn remains generally high for all locations scouted by Damon Smith, an associate professor of plant pathology at UW–Madison.

Smith said the elevated risk of the disease is due to the relatively wet and humid weather that has blanketed the entire state for the past month.

"Tar spot is favored by persistent temperatures between 60 and 70 F and high relative humidity averaging above 75% for a 30-day period," said Smith on his Badger CropDoc blog. "We have been right in the zone for conducive conditions for this disease."

Smith says his team has spent the last few days scouting corn in Walworth, Grant and Lafayette counties looking for signs of the disease.

"Some of these fields are planted to known tar spot-susceptible hybrids and planted in fields with a history of the disease," he said. "So far we have been unable to find any symptoms of tar spot. Fields generally look disease free with just a bit of Anthracnose, which is common this time of year."

App shows the risk for tar spot locations in southern and south-central Wisconsin.

While weather continues to be conducive for tar spot, producers are encouraged to evaluate the likelihood that tar spot might develop early in their respective fields.

"Remember, if you have no history of the disease, then the likelihood of local inoculum being present is low," he said. "Saving the fungicide application for later in the season might be a better option."

Those with a history of the disease or have a susceptible hybrid coupled with a no-till situation, the risk is higher and Smith urges farmers to evaluate the economics of doing an application of fungicide as early as V6.

"Remember, if you do a V6-V8 application of fungicide, conditions could stay conducive later in the season for tar spot," Smith said. "Those early applications will “burn out” by the time the tasseling period rolls around. So if you do put a fungicide spray on at V6, you might have to come back at VT with another application to protect plants during the reproductive phase, should we stay in favorable conditions for tar spot."

Damon Smith

In the meantime, Smith tells producers to keep an eye on the weather and to continue scouting their fields.