Host-Pathogen Interaction in Rice-Bacterial Blight PathosystemVirulence pattern among 52 isolates of Xanthomonas oryzae pv. oryzae, the causal organism of bacterial blight disease was tested on 16 rice genotypes possessing 11 known Xa genes conferring resistance. Significant differences among the host genotypes as well as the pathogen isolates and in their interaction, suggested that the host genotypes differed in vertical resistance and the pathogen isolates differed in virulence. None of the genotypes exhibited resistant reaction against all the isolates, while one Japanese and two IRRI differentials were knocked down by all the isolates. The set of 16 rice genotypes possessed the Xa genes viz. Xa1, 2, 3, 4, 5, 6, 7, 10, 11, 12 & 13. The isolates carried 4-11 virulence factors, out of a total number of 11 v-factors that could be evaluated from this set of host genotypes to overcome the resistance offered by the corresponding Xa genes. The pattern with virulence to Xa1, 2, 4 & 11 and avirulence to the genes Xa6, 7, 5, 13 & 10 was very common. The wide distribution of the virulence factors over different states of India suggested nonparallelism between virulence pattern and geographical distribution of the isolates. The 52 isolates could be classified into five groups using hierarchical agglomerative method of cluster analysis based on the number of v-factors possessed by each of them viz. 11, 10, 8, 7 & 4, which were equivalent to the pathotype grouping of 1, 4, 7, 14+15 & 16, respectively. The application of the methods of numerical taxonomy emerged as a valuable tool in classification of bacterial isolates into virulence groupings.
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