By Rodrigo Onofre
Tar spot of corn, a disease caused by the fungus Phyllachora maydis, has been confirmed in Nemaha (9/15) and Doniphan (10/05) counties in Kansas. Through a collaborative effort with K-State County Extension Crop Agents, five fields in Nemaha county and one field in Doniphan county have been confirmed positive for tar spot. Tar spot lesions are black, raised, and have a round/elliptical shape. This pathogen can survive in crop residue (Figure 1).

Figure 1. Tar spot of corn. Purple arrows are indicating a few of the tar spot lesions. Photo courtesy of Rodrigo Onofre, K-State Research and Extension.
Tar Spot was detected in US for the first time in 2015 and has quickly spread through the Midwest (Figure 2). To date, it has been reported in Illinois, Indiana, Wisconsin, Michigan, Minnesota, Kentucky, Iowa, Ohio, Florida, Georgia, Pennsylvania, New York, Nebraska, and Missouri. First observations in our neighboring states, Nebraska and Missouri, were made last season. This disease is favored by mild temperature (60F to 73F), high relative humidity (>75%), and a prolonged leaf wetness period (>7h). Severity of tar spot is dependent on the weather. Irrigated corn may be at particularly high risk for yield or silage loss.