PHENOTYPIC VARIATION AND PARASITIC FITNESS OF RACES OF COCHLIOBOLUS-CARBONUM ON CORN IN NORTH-CAROLINA

Two field populations of Cochliobolus carbonum on corn (Zea mays) in t he piedmont of North Carolina, where races 2 and 3 coexist, were sampl ed repeatedly (four times during 51 days in the Wilkes County field an d three times during 32 days in the Yadkin County field) during 1987. Isolates were tested for lesion type and several polymorphic traits, i ncluding mating type, fertility, and fungicide tolerance. The relative fitness of race 3, as determined from frequency changes, was signific antly lower than the fitness of race 2 in each field. The fitness valu es of race 3, with an estimated generation time of 7 days, were 0.82 a nd 0.84 versus 1.0 for race 2. A third race, race 0, was also common i n the Wilkes County field and had a relative fitness of 0.42. The mean frequency of race 3 was 29% in the Wilkes County field, 30% in the Ya dkin County field, and 92% in a third field in the Appalachian Mountai ns in Tennessee that was sampled only once in 1987. These data show th at the frequency of race 3 in the piedmont and mountains changed littl e from 1977 to 1987, and parasitic fitness alone cannot account for th e distribution of race 3. The frequencies of seven polymorphic traits and the genetic diversity within races 2 and 3 remained stable over th e 1987 sampling period, and race frequencies changed, suggesting the p henotypic traits are selectively neutral. As in previous years, trait frequencies differed significantly between races, indicating their gen etic isolation. Within races, however, there was evidence that sexual reproduction may have occurred. In races 0 and 3, there were no indica tions of gametic phase disequilibrium typical of asexual populations. In race 2, there were no significant associations of pairs of traits, but some phenotypes occurred significantly more frequently than was ex pected based on the frequencies of their component traits. Also, there were significant differences in phenotype frequencies between mating types MAT-1 and MAT-2 in race 2. The two mating types differed more fr om a 1:1 ratio in race 2 than in races 0 and 3, suggesting that sex ma y be less important in race 2 than in races 0 and 3.