Genetic variability for pathogenicity, isozyme, ribosomal DNA and colony color variants in populations of Rhynchosporium secalis.

Samples of Rhynchosporium secalis were collected from two experimental barley populations known to carry a diverse array of alleles for resistance to this fungal pathogen. Classification of 163 isolates for four putative isozyme systems, a colony color dimorphism and 20 ribosomal DNA restriction fragment length variants revealed 49 different multilocus phenotypes (haplotypes). The six most common haplotypes differed significantly in pathogenicity. Genetic analyses of the data indicated that effective population sizes of the fungus were very large, that the effects of genetic drift were small, and that negligible recombination occurred in the populations studied. Frequency dependent selection was suggested as an explanation for the maintenance of variation in pathogenicity in the fungus.