A Cline and Other Patterns of Genetic Variation in Chochliobolus carbonum Isolates Pathogenic to Corn in North Carolina

Lodge, Deborah; Leonard, Kurt

ABSTRACT: Patterns of genetic variation were studied in natural populations of Cochliobolus carbonum Nelson (anamorph Helminthosporium carbonum Ullstrup), a haploid asexually reproducing fungus. Two virulence races (2 and 3) are common on corn in North Carolina. Race 3 has occurred in the Appalachian mountains for at least 25 years, but has recently expanded its range eastward. The expanded range of race 3 cannot be explained by adaptation through parasexual recombination between races or mutation alone. Five polymorphic traits in addition to virulence were compared in races 2 and 3 to evaluate possible recombination between races. If sexual recombination occurred between races, it was rare and was not detected in this study. The simplest explanation for the expansion of race 3 involves historical factors such as increased corn production and changes in weather which aided gene flow. A steep cline was found on the Appalachian escarpment, where the proportion of race 3 isolates dropped from 100% at high elevations to 30% at low elevations over a distance of 7–20 km. No barriers to gene flow were found on the escarpment, suggesting that the environment and possibly cultural practices may restrict race 3 at low elevations and race 2 at high elevations. Race 3 may adapt to conditions in eastern North Carolina only slowly via mutation unless recombination occurs and has gone undetected.