THE RELATIONSHIP BETWEEN LINEAGE AND VIRULENCE IN PYRICULARIA-GRISEA IN THE PHILIPPINES

The relationship between phylogeny and pathotype was examined for a selected group of isolates of the blast fungus Pyricularia grisea from two sites in the Philippines. Based on DNA fingerprinting and cluster analysis with the repetitive DNA probe MGR586, isolates fell into several distinct groups. Probing blots of the MGR-defined haplotypes with avirulence genes A VR2- YAMO and PWL2 produced single or multiple copy profiles that grouped identically to those from the MGR analyses. A set of 234 isolates, representing six of these groups or putative lineages, was selected for analysis of virulence spectrum. These isolates were tested on 15 rice cultivars and six near-isogenic lines. A total of 71 distinct pathotypes (virulence patterns) was detected on the 21 hosts tested. Multiple pathotypes were seen for each of the six lineages analyzed. Although there was no obvious relationship between lineage and pathotype, the distribution of virulence was nonrandom with respect to lineage for a given cultivar. Although reactions were variable for some host-lineage combinations (both compatible and incompatible isolates were found for 43 of the 123 host-lineage combinations analyzed), consistent incompatibility was observed for the majority of host-lineage interactions. In 80 of the 123 host-lineage combinations tested (65%), no compatible reaction was found, although an average of 39 isolates was tested per lineage per host. For each lineage, between nine and 16 cultivars were resistant to all isolates. Individual cultivars showed different spectra of resistance with respect to the pathogen lineages tested. The indica and japonica cultivars tested appeared to have complementary spectra of resistance. The implications of these observations for resistance breeding are discussed.