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Virulence, Temperature Optima, and Competitive Abilities of Isolines of Races T and O of Bipolaris maydis. K. J. Leonard, Plant Pathogist, Agricultural Research Service, United States Department of Agriculture, North Carolina State University, Raleigh, NC 27607; Phytopathology 67:1273-1279. Accepted for publication 15 April 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1273.

Isolines of Bipolaris maydis races T and O and mating types A and a were compared with respect to (i) growth in vitro at 20, 24, 28, 32, and 36 C, (ii) capacity to induce disease lesions on corn plants with cms-T and normal (N) cytoplasm, and (iii) selection in mixed populations on plants with cms-T or N cytoplasm. All isolines grew fastest at 28 C. At 28 and 32 C race T isolines grew slightly faster than did race O isolines. Growth rates of isolines of mating types A and a were similiar at all temperatures. There were no significant differences in numbers of lesions induced on cms-T and N plants by race T and race O isolines, respectively. Race T lesions were 45% longer than race O lesions on cms-T plants but were 10% shorter than race O lesions on N plants. Isolines of mating type A induced significantly more lesions on both cms-T and N plants than did isolines of mating type a, and the lesions of A isolines were slightly longer than those of a isolines. However, in mixtures of isolines, there was little or no change in the frequencies of the two mating types over five conidial generations. In mixtures of isolines, race T was selected on cms-T plants (fitness of race T was 43% greater than fitness of race O), but on N plants race O was selected (fitness of race O was 12% greater than fitness of race T). The lower fitness of race T on N plants may account for much of the decline of race T in the southern U. S. and northern Queensland. The greater persistence of race T in the northern U. S. and southern Queensland may indicate that the original race T population was better adapted to cool than warm temperatures. Race T conidial isolates collected from the southern USA in 1972 grew faster in vitro at 36 C than did race T conidial isolates collected from the southern or midwestern U. S. in 1970, but at 20, 24, and 28 C the isolates collected in 1970 grew faster than those collected in 1972.

Additional keywords: Cochliobolus heterostrophus, Zea mays, maize, selection.