Population structure of Rhizoctonia oryzae-sativae in California rice fields.

Six pairs of single-locus microsatellite primers were developed to study the population structure of Rhizoctonia oryzae-sativae, the cause of aggregate sheath spot disease of rice, among and within three rice-growing areas in California over a 3-year period. A high level of gene flow among growing areas was indicated by low population subdivision according to analysis of molecular variance and moderate to no population differentiation between pairs of populations based on the fixation index (F(ST)). Gametic equilibrium of most pairs of microsatellite loci, high numbers of unique multilocus genotypes, and high genotypic diversity indicated extensive sexual recombination within growing areas. Because there was little differentiation among populations in all hierarchical levels, including among growing areas within sampling years, fields within growing areas, and corners within individual fields, a high level of gene flow was revealed in all levels. Basidiospores were likely the main vehicle of gene flow among populations, including short and long distances. Asexual inocula (sclerotia and mycelia) probably overwinter because a few clones were detected over a 2-year period within the same field. A few clones were shared among fields but were not commonly shared among growing areas.