Tar Spot Continues to Spread Across the Midwest

Tar spot continues to move across the Corn Belt. Yellow counties were infected for the first time in 2019.

Tar spot, a fungal disease that can cause severe yield loss on susceptible hybrids, continued to spread quickly across the Corn Belt in 2019. The disease has now moved into nearly all of Iowa’s 99 counties, plus additional counties in Wisconsin, Illinois, Indiana, Ohio and Michigan and has entered Minnesota and Missouri.1

Nathan Kleczewski, University of Illinois Extension pathologist, said that in 2018 alone, the grain loss was around nine billion pounds. Damage was considerably less in 2019, due to fewer cornfields being planted and those that were planted were planted late.2

Tar spot will move at least 250 feet during the growing season and, with the milder winter most are experiencing in 2019-2020, will survive winter conditions, overwintering on infested corn residue on the soil surface. Researchers believe that high relative humidity and prolonged leaf wetness favor disease development.3 Tar spot also moves with equipment and through cornstalks that are baled and then sold or moved to other farms.

Tar spot on corn. Photo credit: K. Wise. Crop Protection Network.

“Tar spot overwinters in the residue, which is probably the primary source of inoculum, and then something causes the spores to pop out at a time it knows the plant is there,” said Kleczewski. “Considering how fast it has moved since 2015, it’s probably spreading further than we think.”

The disease will continue to move in 2020, with new states Minnesota and Missouri beginning to find tar spot in 2019, and as it moves west across Iowa, it is expected to reach Nebraska soon as well.

Tar spot appears as small, raised, black spots scattered across the upper and lower leaf surfaces. Known as ascomatum, these spots can even appear on husks and leaf sheaths in severe cases. Fisheye lesions, tan to brown lesions with dark borders surrounding the ascomatum, can also develop.4

Residue management, rotation and avoiding susceptible hybrids may reduce tar spot development and severity. Talk to your AgVenture Yield Specialist or AgVenture Seed and Technology Manager to learn more about the effects tar spot can have in your fields.

1 Crop Protection Network.
2 AgriNews article, February 1, 2020.
3 Crop Protection Network.
4 Crop Protection Network.
Photo credit: K. Wise. Crop Protection Network.
Map information courtesy of Crop Protection Network.