The disease tar spot, caused by the fungus Phyllachora maydis, was first discovered in the United States in 2015. Since 2015, tar spot has become a major corn disease in the Midwest, causing yield losses up to 60 bushels per acre in fields where it is established. The disease was confirmed in Kentucky in 2021, and has spread throughout the Commonwealth, but has not yet caused yield loss. Kentucky’s climate is not optimal for tar spot development, and it often is diagnosed late in the growing season when fungicide treatment is not needed. However, farmers and others in agriculture are concerned about the disease and can be tempted to spray again, even as corn approaches maturity, out of concern for the disease. 

In 2024, late confirmations of tar spot occurred in several northern counties in Kentucky. In one area, a consultant had fields in dough stage, with confirmed tar spot. After consultation with the corn disease extension program at the University of Kentucky, this consultant opted not to recommend an additional fungicide application. 

Based on existing numbers, the farmers who opted not to spray a second fungicide application saved approximately $32-36 per acre and saw an increased return on investment compared to farmers who sprayed a second fungicide application in response to late-season tar spot.