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Pathotype and Avirulence Gene Diversity of Pyricularia grisea in Thailand as Determined by Rice Lines Near-Isogenic for Major Resistance Genes

Five hundred twenty-seven isolates of Pyricularia grisea were collected from trap rice cultivars of indigenous and exotic origin across three seasons at five sites in Thailand. Single conidium isolates were inoculated onto 15 rice lines near-isogenic (NILs) for resistance genes, one recurrent parent, and two local cultivars. One hundred seventy-five pathotypes were identified, of which 160 were represented by fewer than eight isolates. Predicted pathotype number was estimated at greater than 450 for the study region. Significant differences in pathotype diversity were detected across sites, seasons, and among isolates collected from exotic versus indigenous hosts. Isolates and pathotypes with greater numbers of virulence genes (as inferred from compatibility with NILs) were less common than those with fewer virulence genes. Analysis of virulence distributions among isolates grouped according to their MGR586 DNA-fingerprint similarities (i.e., “lineages”) also showed that, for the most commonly represented lineages, isolates with fewer virulence genes predominated. Lineages represented by one or a few isolates had greater numbers of virulence genes. Lower frequency of recovery of isolates with accumulated virulence genes is consistent with an associated fitness penalty. Resistance genes Pi 1, Pi z-5, and Pi ta² were broadly effective across this population, compatibility with Pi 1 and Pi z-5 was very rare, and no isolate combined compatibility with both genes. Well-represented (more than 20 isolates) MGR586 lineages showed specific incompatibilities with some NILs, but these were restricted to Pi 1 and Pi z-5. No combination of resistance genes would confer resistance across all lineages.