This work was designed to ascertain the extent of genetic diversity in the pathogen population of Xanthomonas oryzae pv. oryzae (Xoo), the causal organism of bacterial blight of rice. The virulence of 52 strains of Xoo, collected from 12 rice growing states of India, were clip-inoculated on 16 rice genotypes possessing known genes for resistance. Based on the genetic distance, estimated by multivariate analysis isolates of Xoo could be classified into 13 clusters and five broad groups. The genetic variability of virulence in populations of Xoo was also measured by estimation of diversity indices viz. Shannons information function (H), Levins diversity index (B), Hills generalized entropy (H2), trophic diversity index (D), dominance of virulence factors (λ), the index of evenness or equitability (JH,), genetic richness index (d) and normalized pathotypic diversity index (Hn). Wide genetic diversity in the pathogen populations from the country as well as from the eastern and southern regions was indicated by high estimates of H', B, H2, D, λ, d and low JH,. High estimates of Hn were attained for almost all populations of Xoo. With regard to the states, genotypic and pathotypic diversities were high in Andhra Pradesh and Orissa, while pathotypic diversity was high in the pathogen population from Gujarat, Punjab, Madhya Pradesh, Uttar Pradesh and Tamil Nadu. The existence of high diversities and the grouping of the pathogen isolates into clusters of similar in virulence facilitated a better understanding of the population structure of Xoo, to guide regional rice breeding programs and the deployment of resistance genes in disease control strategy.
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