Tracking the Distribution and Risk of Tar Spot of Corn in Indiana from 2015 to 2022

Tar spot of corn ( Zea mays L.), caused by Phyllachora maydis, was first confirmed in the United States in 2015 in Illinois and Indiana but has since spread to a total of 20 states and Ontario and Quebec, Canada. Severe tar spot epidemics have caused unexpected and significant yield losses in corn in the Midwest. It is critical to document the movement and risk factors of this disease to develop effective management strategies. Tar spot distribution and disease intensity data were collected in Indiana from 2015 to 2022. Tar spot severity data were assessed from images of leaves submitted to the Purdue Pest and Diagnostic Laboratory from 2015 to 2018, and a statewide survey was conducted annually from 2019 to 2022. In each county, two or more corn fields were scouted for tar spot, percent field incidence and average leaf severity were documented, and county-level weather data were collected. In Indiana, tar spot severity was negatively correlated with temperature and positively correlated with precipitation during June and August but negatively correlated with July precipitation. High relative humidity (>90%) was also positively correlated with tar spot severity during June, July, and August. Fields in northern Indiana had the highest severity throughout the survey and have the highest risk for the disease in the future. Pockets of tar spot outbreaks indicate that once it is found locally, favorable environmental conditions of moderate temperatures and fluctuating periods of seasonal moisture may increase the risk of a severe epidemic in a field in future years.