Genotypic variability and aggressiveness of Bipolaris oryzae in the Philippines

The genotypic diversity of a collection of 352 isolates of Bipolaris oryzae obtained from 11 locations in the Philippines was estimated. The isolates could be divided into 50 haplotypes based on variation in microsatellite DNA with a moderately high genotypic diversity value of 0.88. Thirty nine haplotypes were represented by three or fewer isolates, whereas 80 % of the isolates belonged to only eight haplotypes, each containing 10 to 88 isolates indicating the prevalence of clonality. AMOVA revealed that the greatest variation was associated with the brown spot isolates collected within provinces (50.81 %), among varieties within provinces (48.17 %) and within ecosystems (49.33 %). Intensive sampling from a single field showed that the population was mostly clonal with about 98 % of the isolates belonging to a single VNTR haplotype. However, isolates within this haplotype exhibited a continuous range of aggressiveness when inoculated onto susceptible rice variety IR72. Several types of lesions were observed in the field during sampling, but the isolates obtained from each type of lesion produced a range of different lesion types when inoculated onto leaves of IR72, indicating that the type of lesion observed in the field was not related to the genotype of the pathogen. These results show that rice fields across the Philippines may contain B. oryzae isolates with considerable genotypic diversity, but an individual field may have both clonal and unique genotypes.