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Association of Mating Type and Virulence in Helminthosporium maydis, and Observations on the Origin of the Race T Population in the United States. K. J. Leonard, Plant Pathologist, Plant Science Research Division, ARS, USDA, North Carolina State University, Raleigh 27607; Phytopathology 63:112-115. Accepted for publication 27 July 1972. DOI: 10.1094/Phyto-63-112.

Cultures of Helminthosporium maydis isolated from diseased corn leaves collected in Iowa, New York, and four Southern states, and from corn seed from Texas and Guatemala in 1971 were tested for mating type and virulence. Race O was found in seed from Guatemala and in 12% of the leaves collected in the South, but in none of the leaves from Iowa or New York. All 50 race T isolates from Iowa and 9 of 10 from New York were mating type A. Mating types A and a occurred in nearly equal frequency among both the race O and race T isolates from the South. With respect to mating type, the midwestern and northeastern populations of race T remained as they were in 1970. The southern population has changed markedly since 1970, when 80% of the race T isolates were mating type A. The implications of these and other observations which suggest a recent origin of the race T population and counter evidence suggesting a widespread and long term existence of race T in the USA and many other parts of the world are discussed.

Additional keywords: southern corn leaf blight.