Diversity in virulence of Xanthomonas oryzae pv. Oryzae from Northern Vietnam

A survey of the geographical distribution of pathogenic races of Xanthomonas oryzae pv. oryzae (Xoo) causing bacterial blight (BB) of rice in northern Vietnam was conducted. We characterized strains of Xoo collected from nine regions in this part of Vietnam in 2001 and 2002 using polymerase chain reaction (PCR)-based DNA fingerprinting and virulence analysis. The pathogenicity of Vietnamese 84 strains was tested for their virulence to BB near-isogenic lines with a single resistance gene. Based on virulence analysis using 11 differential lines (IR24, IRBB1, IRBB2, IRBB3, IRBB4, IRBB5, IRBB 7, IRBB10, IRBB11, IRBB21 and Taichung Native 1), each harboring a single resistance gene, four pathotypes (tentatively designated as races G1 to G4) were identified. All strains were virulent to the resistance genes Xa1, Xa2, Xa10, Xa11, Xa14 and Xa18, and xa5 and Xa21 were effective against a major portion of the Vietnamese Xoo population; followed by Xa7. Most of the Vietnamese hybrid and improved varieties cultivated in northern Vietnam were susceptible to all races and no Vietnamese variety was resistant to all races. Our study revealed that strains from northern Vietnam comprised two major races (G2 and G3) and two minor races (G1 and G4), which were widely distributed and detected at lower frequencies, respectively, in northern Vietnam. Seventy six strains of Xoo were analyzed by DNA fingerprinting using two PCR-based assays. Two groups, designated as clusters I and II, were respectively identified, in which cluster I contained race G4 strains and cluster II race G1 strains. Combining DNA and pathotype analysis, we hereby present the situation of Xoo recently isolated in northern Vietnam.