Pathotyping and virulence analysis of Xanthomonas oryzae pv. oryzae causing bacterial blight of rice in Tamil Nadu

Bacterial leaf blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the most destructive diseases affecting rice production worldwide. Resistance breeding is considered a durable, effective and eco-friendly approach to control BB. Several BB resistance (R) genes have been identified that confer resistance against various strains of Xoo. The dynamic nature of this pathogen needs proper knowledge of pathotype composition and its virulence among the diverse Xoo strains which is imperative for designing a resistance breeding program.

In this study, we conducted an extensive survey to collect BB infected samples from diverse rice-growing regions of Tamil Nadu. Further, the samples were identified as Xoo through morphological and molecular identification methods. The pathotyping and virulence profiling of Xoo isolates was carried out on a set of rice differentials consisting of 22 near-isogenic lines (NILs) of IR24 possessing different BB resistance genes under glasshouse conditions during Kharif 2021-2022.

Based on the disease reaction in 22 rice BB differentials (IRBB lines), the Xoo isolates were grouped into six pathotypes (I II, III, IV, V and VI). Further, the virulence analysis revealed that the near-isogenic lines possessing single R gene and gene pyramids were susceptible to about 20-88% and 4-20% respectively to Xoo isolates. R genes and their pyramids such as (Xa7, Xa21, Xa4+xa5+Xa21, Xa4+xa5+xa13, Xa4+xa13+Xa21, xa5+xa13+Xa21 and Xa4+Xa21+xa5+Xa21) were found to be effective against all the pathotypes in Tamil Nadu. The pathotype III was predominantly distributed (40%) over the Tamil Nadu. The isolates found in the pathotypes I and II are more virulent and in the pathotypes V, VI are less virulent. The current study identified the virulence nature of diverse Xoo isolates and the effectiveness of resistance (R) genes against the Xoo isolates. Therefore, the effective R genes identified in this study could be an effective source for resistance breeding program against BB in Tamil Nadu.