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Tar Spot Found in Minnesota

So far, discovery is limited to just one field

Tar spot of corn was found for the first time in Minnesota in late September, says Dean Malvick, University of Minnesota Extension plant pathologist. Based on reports from Iowa and Wisconsin, Malvick says it was suspected the disease could move into Minnesota.

Tar spot was found in one field in southeastern Minnesota (southern Fillmore County) where the wet summer weather has likely favored infection and spread, says Malvick.

He says this is a good time to scout for tar spot in fields that still have green leaves to get a better idea of how far this disease has spread into Minnesota. Even though it’s known to have occurred so farm in just one location, tar spot could easily have escaped detection in other fields, says Malvick.

In the field where the disease was found, tar spot appeared to be focused in one area that was not visible from nearest road. Walking within the field was required to see it, says Malvick. Infection levels were moderate to high on some plants.

Malvick expects there to be minimal or no effect on yield because the disease appears to be limited to a small part of the field and infection likely started late in the summer. Future disease risk is higher in infected areas, he says.

Tar spot is caused by the fungus Phyllachora maydis. The fungus infects leaves (and sometimes husks) and produces raised, small (0.1 to 0.3 inches) raised black structures on the leaf surface. The black structures are firm, appear fairly smooth on the surface, and do not rub off or break open as do rust pustules.

Tar spot can also produce fisheye symptoms, which are tan lesions with dark borders surrounding the tar spots. Tar spot can  significantly reduce yields, depending on weather, severity, and timing of disease development, he says.

This finding shows that tar spot continues to spread in the Midwest. Time will tell us how much tar spot can spread and damage corn in Minnesota, says Malvick.

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