Morphological and genetic variation of the brown spot pathogen (Bipolaris oryzae) of rice in the Philippines

The majority of the Bipolaris oryzae isolates from Arakan, Cotabato [Philippines] showed similar colony characteristics on potato dextrose agar (PDA) and rabbit food agar (RFA) while a few isolates exhibited variations on both media. The effect of selection, culture media and culture age on type of conidial was also determined for the isolates from Cavinti, San Pablo, and Palawan since some Bipolaris oryzae isolates showed intercalary germination instead of bipolar germination of conidia which is one of the taxonomic characteristics of the Bipolaris species. Rabbit food agar and water agar as well as longer incubation time prior to harvesting of conidia increased the percentage of bipolar germination. The genetic variability of 325 isolates obtained from multiple locations in the Philippines was analyzed with VNTR marker wherein 50 VNTR haplotypes were obtained giving a somewhat high genetic diversity (Hsubt) value of 0.89. Intensive sampling from a single field showed that the population was mostly clonal with almost all the isolates belonging to one VNTR haplotype. Only three distinct haplotypes were defined from 90 isolates analyzed giving a very low haplotypic diversity estimate (Hsubt) of 0.04. Isolates within the haplotype that predominated (haplotype 48) had a continuous range of aggressiveness when inoculated onto susceptible Oryza sativa cv. IR72 ranging from 6.10 to 80.94% diseased leaf area. There was no relationship between original lesion type observed in the field and the lesion types produced on the inoculated IR72 plants which appeared to become more resistant with age. No major relationship between haplotypes and geographical location or host variety was observed. These results show that rice fields may contain B.oryzae populations with considerable genetic diversity, but the majority of infections arise from a small subset that can increase to form large clonal populations, likely through conidia acting as secondary inoculum.