Characterization of a New Race of Exserohilum turcicum Virulent on Corn with Resistance Gene HtN. R. P. Thakur, Plant Pathologist, Cereals Program, International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru P.O., Andhra Pradesh 502323, India. K. J. Leonard, and R. K. Jones. Research Plant Pathologist, U.S. Department of Agriculture, Agricultural Research Service, Department of Plant Pathology, Box 7616, North Carolina State University, Raleigh 27695-7616; and Extension Plant Pathologist, Texas Agricultural Extension Service, Texas A&M University, College Station 77843. Plant Dis. 73:151-155. Accepted for publication 1 September 1988. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1989. DOI: 10.1094/PD-73-0151.

Isolates of a new race of Exserohilum turcicum collected from South Texas in 1986 were compared with isolates of race 1 and race 3 in inoculations of seedlings of corn inbred B37 and B37 backcross lines with Ht2, Ht3, and HtN in controlled environment chambers. The new race resembled race 3 except for its virulence on B37 HtN. Race 3 has the virulence formula Ht1, HtN /Ht2, Ht3. The new race is described as race 4 with the virulence formula Ht1/Ht2, Ht3, HtN. Both race 3 and race 4 were virulent on B37Ht3 at 22 C day/18 C night temperatures but were avirulent at 26 C day/22 C night. Leaf disks were cut from lesions of races 3 and 4 on plants grown under different light and temperature conditions and incubated in moist chambers in the laboratory to compare treatment effects on sporulation. For plants grown under full light (647 ?E·m^-2·s^-1) at 22 C day/18 C night temperatures or 26 C day/22 C night, sporulation was greater on B37 than on B37Ht3 or B37HtN. In lesions from plants grown at reduced light intensity, sporulation on B37 was significantly decreased, but sporulation by races 3 and 4 on B37Ht3 and by race 4 on B37HtN was significantly increased. The resistance of B37Ht2 to race 1 broke down at low light intensity.

Keyword(s): maize, northern leaf blight, Setosphaeria turcica, Zea mays.