| Aggregate sheath spot of rice, caused by Rhizoctonia oryzae-sativae , occurs in California, USA and other countries. In this study, six pairs of single-locus microsatellite primers were developed to study the population structure of R. oryzae-sativae among and within three rice growing areas in California over a three year period. The diversity of R. oryzae-sativae was characterized by culture morphology, pathogenicity tests, somatic compatibility groups (SCGs), single-locus microsatellites (SLMs), and multi-locus microsatellites (MLMs). A high level of gene flow was indicated among growing areas by low population subdivision using analysis of molecular variance, moderate to no population differentiation based on the fixation index (FST), and low genetic distance between populations. Evidence of sexual reproduction was apparent in R. oryzae-sativae because of gametic equilibrium of most pairs of microsatellite loci, high numbers of unique multilocus genotypes, and high genotypic diversity based on genetic analyses. There was no correlation of phenotypes to either SCGs or genetic markers and incongruence of parsimonious trees from each of five primers of MLMs based on phylogenetic analyses. This information suggests that both sexual and asexual reproduction influence the population structure of R. oryzae-sativae. Basidiospores were likely the main vehicle of gene flow among populations, including short and long distances. In addition, several plant products, including plant extracts and plant essential oils, were examined for antifungal activity against R. oryzae-sativae . Cinnamon oil was the most efficacious plant product in vitro; it was tested in the greenhouse for the control of the disease on two rice cultivars, M-205 and M-206, inoculated with two R. oryzae-sativae isolates, 3B and 13B. In the greenhouse study, cinnamon oil failed to reduce the disease caused by isolate 13B at any concentration. Cinnamon oil suppressed the disease caused by 3B on one of the cultivars at a concentration of 37.5%, and on both cultivars at a concentration of 62.5% and 87.5%. However, cinnamon oil at 87.5% decreased dry weights of rice compared to the control. Cinnamon oil has potential to play a role in management of aggregate sheath spot but more research is needed on rates, time of application, and efficacy under field conditions. |
