According to the latest Crop Protection Network map, positive cases of tar spot have been reported in 13 of the top 18 corn growing states. Additional cases have been reported in Georgia, Maryland, and New York.
First reported in the Corn Belt in 2015, 18 U.S. states have a history of the disease.
So far this season, Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Nebraska, Ohio, Pennsylvania, South Dakota, and Wisconsin have confirmed infected crops.
Illinois
As of Aug. 1, 19 Illinois counties have reported cases of tar spot. There are only 11 counties in the southern part of the state that have no history of the disease.
Wyffels agronomist Ryan Gentle ranked "late disease pressure," including tar, spot the second biggest yield thread to corn in his territory behind tight tassel wrap.
USDA rated the state's corn crop 4% very poor, 5% poor, 22% fair, 52% good, and 17% excellent the week ended Aug. 3.
Indiana
In Indiana, 32 counties have reported cases of tar spot, as of Aug. 4.
USDA rated the state's corn crop 3% very poor, 8% poor, 27% fair, 51% good, 11% excellent the week ended Aug. 3.
Iowa
As of Aug. 4, more than one third of Iowa's counties confirmed cases of tar spot. The Crop Protection Network noted cases in 37 counties.
Wyffels agronomist Eric Wilson noted foliar disease and late season nitrogen loss are the biggest threats to corn yield in his Iowa territory.
USDA rated the Iowa corn crop 1% very poor, 2% poor, 12% fair, 57% good, and 28% excellent the week ended Aug. 3.
Kentucky
Four Kentucky counties along the border with Indiana reported cases of tar spot, as of Aug. 4.
In the latest Crop Progress report, USDA rated Kentucky corn 2% very poor, 7% poor, 28% fair, 55% good, 8% excellent.
Michigan
In Michigan, 16 counties have corn crops infected with tar spot.
USDA rated the state's corn crop 1% very poor, 8% poor, 35% fair, 40% good, and 16% excellent the week ended Aug. 3.
