POPULATIN STRUCTURE OF PYRICULARIA-GRISEA AT 2 SCREENING SITES IN THEPHILIPPINES

The population structure of the blast pathogen, Pyricularia grisea, wa s analyzed at two field sites used for evaluating blast resistance in rice. During 1992, 1,516 monoconidial isolates of the pathogen were co llected from 38 rice cultivars and lines from the blast nursery of the international Rice Research Institute (IRRI-BN) and from the upland s creening site at Cavinti. Each isolate was subjected to DNA fingerprin ting and phenetic analysis using the probe MGR586. Nine lineages were detected at Cavinti during the wet season. Although the same four line ages were collected from the IRRI-BN during the dry and wet seasons, t he relative abundance of lineages differed in the two collections. Whi le the lineage diversity was greater at Cavinti than at the IRRI-BN in either season, the haplotypic diversities of the collections were sim ilar. Generic differentiation and chi-square analysis indicated that p opulations of the fungus were differentiated geographically and tempor ally. The largest proportion of differentiation was attributable to ho st selection (G(ST) = 0.39). To assess how well field infection reflec ted compatibility, a subset of isolates was inoculated on their hosts of origin. Isolates were often unable to reinfect their hosts of origi n, especially isolates belonging to the dominant pathogen lineage and when collected from young seedlings and those with low diseased leaf a rea. Our results provided insight into pathogen population structure, and provided useful information for rice improvement and management.