Exserohilum turcicum Race Population Distribution in the North Central United States.

Northern leaf blight (NLB) of corn, caused by Exserohilum turcicum, is a foliar disease common across corn production regions of the world, including those in the north central United States. Previous race population distribution studies identified five physiological races present in the United States, prior to 1995. For this study, 156 E. turcicum isolates were screened on corn differential lines containing Ht1, Ht2, Ht3, Htm1, and Htn1 resistance genes. Isolates were collected from fields in Iowa, Illinois, Indiana, Minnesota, North Carolina, Ohio, and Wisconsin, which included 143 isolates collected between 2007 and 2014 and 13 isolates collected between 1979 and 1985. Twenty different physiological races were observed based on the symptom response of the differential corn lines. E. turcicum race 0, 1, and 1mn were the most prevalent races, comprising 21, 27, and 13% of the 156 isolates, respectively. Race populations were diverse within states and years. Virulence to multiple Ht resistance genes within individual isolates was observed in 47% of those tested, with 3% of the isolates conferring virulence to all Ht resistance genes. Virulence to the Ht1, Ht2, Ht3, Htm1, and Htn1 resistance genes was present in 64, 20, 18, 32, and 27% of the E. turcicum isolates, respectively. Virulence to Ht resistance genes was fairly evenly distributed across states, in isolates collected after 2008. Virulence to Ht2, Ht3, Htm1, and Htn1 decreased after 2010. Variations in race population diversity are difficult to explain without knowing the level of selection pressure present in fields, and information regarding Ht resistance gene deployment in commercial varieties is not publicly available. Although virulence was observed against all Ht resistance genes, qualitative Ht resistance genes could be used in conjunction with quantitative resistance to increase NLB control.