POPULATIN STRUCTURE OF PYRICULARIA-GRISEA AT 2 SCREENING SITES IN THE PHILIPPINES

The population structure of the blast pathogen, Pyricularia grisea, was analyzed at two field sites used for evaluating blast resistance in rice. During 1992, 1,516 monoconidial isolates of the pathogen were collected from 38 rice cultivars and lines from the blast nursery of the international Rice Research Institute (IRRI-BN) and from the upland screening site at Cavinti. Each isolate was subjected to DNA fingerprinting and phenetic analysis using the probe MGR586. Nine lineages were detected at Cavinti during the wet season. Although the same four lineages were collected from the IRRI-BN during the dry and wet seasons, the relative abundance of lineages differed in the two collections. While the lineage diversity was greater at Cavinti than at the IRRI-BN in either season, the haplotypic diversities of the collections were similar. Generic differentiation and chi-square analysis indicated that populations of the fungus were differentiated geographically and temporally. The largest proportion of differentiation was attributable to host selection (G(ST) = 0.39). To assess how well field infection reflected compatibility, a subset of isolates was inoculated on their hosts of origin. Isolates were often unable to reinfect their hosts of origin, especially isolates belonging to the dominant pathogen lineage and when collected from young seedlings and those with low diseased leaf area. Our results provided insight into pathogen population structure, and provided useful information for rice improvement and management.