tar spots on corn leaf
ON THE MOVE: Tar spot is showing up in fields in the Corn Belt and in Florida. It has not yet been identified in Minnesota.

Tar spot has not been found in Minnesota — yet

The corn leaf disease prefers damp, cool conditions around 60 to 70 degrees F.

Tar spot, a fungal disease found in corn, is making its way across Midwest fields but has yet to be found in Minnesota.

Tar spot was first identified in Illinois and Indiana in 2015 and by last year, had spread to parts of Michigan, southern Wisconsin, Ohio, eastern Iowa and Florida. Plant pathologists believe a storm system initially blew tar spot north from Mexico and Central America and that is has been successfully overwintering in the Midwest.

The disease is recognized by its small, raised black spots scattered across the leaf surface. It may look like other diseases, such as rust, but there is an easy way to tell the difference. Not only can you feel the raised tar spots, but you cannot scrape them off the leaf tissue. Tar spot prefers cool, wet weather, around 60 to 70 degrees F.

Last year, depending on the location, the disease reportedly caused a 30- to 40-bushel per acre loss of corn in some fields.

Paula Halabicki, BASF technical marketing manager, says corn is one of tar spot’s host plants. She recommends scouting corn at any time for the disease, starting at the lower canopy. Look for small black dots or lesions.

“It is quite distinguishable,” she says. “You can feel it.”

If you suspect it, contact your local Extension specialist or your agronomist and get it tested.

If tar spot has been confirmed and if weather conditions favor spreading the disease, you might consider applying fungicide.

Speaking of weather, according to an environmental study conducted by the International Maize and Wheat Improvement Center in Mexico, southern Minnesota was identified as a region vulnerable to tar spot. Scientists looked at conditions that are considered as optimal for the development of tar spot, including temperatures averaging 63 to 68 degrees, seven hours of leaf wetness per night and humidity about 75%. They reviewed three decades of weather data and suggest that tar spot could go across southern Minnesota and into South Dakota and Nebraska, eastern Montana and northern Missouri.

Meanwhile, scientists at the University of Wisconsin are working with a tar spot prediction model called Tarspotter. Several Midwest university Extension specialists and industry colleagues, including BASF, are working to valid it. Go online to learn more about Tarspotter.

You can learn more about tar spot from the Crop Protection Network or Purdue Extension.

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