Twenty-four major genes for resistance to bacterial blight have been identified using Japanese and Philippine races of Xanthomonas oryzae pv. Oryzae. A recessive gene, xa24, for resistance to race 6 of Philippine was identified in varieties DV85, DV86, and Aus295 from Bangladesh (Khush and Angeles 1999). Four varieties (Bhadoia 648, ARC5756, Bazail 414, and Bazail 980) were first placed in DZ192 group. Varieties belonging to this group, have xa5 and are resistant to races 1, 2, and 3 and are moderately resistant to race 4 at seedling stage (Ogawa et al. 1988). Chen (1990) reported that these four varieties are also resistant to race 6 and on the basis of genetic analysis concluded that two recessive genes convey resistance to race 1 and race 6, respectively. Resistance to race 1 is conferred by xa5. However, the genes for resistance to race 6 were not identified.

For allele tests, we crossed these four varieties with IRBB13 which has xa13 and with DV85 which has xa24. The F₁'s of four varieties with IRBB13 were susceptible and the F₂ populations segregated in a ratio of 7:9 indicating that the recessive genes for resistance to race 6 in these varieties segregate independently of xa13 (Table 1). The F₁'s of ARC5756, Bazail 414, and Bazail 980 with DV85 were susceptible and the F₂ populations segregated in a ratio of 7:9 indicating that the recessive genes for resistance to race 6 in

these varieties segregate independently of xa24. The F₁'s of Bhadoia 648 and DV85 was resistant and the F₂ population did not segregate for susceptibility. These results indicate that Bhadoia 648 has xa24 for resistance to race 6 (Table 2).

References

Chen, J.C., 1990. Genetic analysis of resistance to Philippine races of bacterial blight, Xanthomonas campestris pv. Oryzae in some rice cultivars. Ph.D. thesis. University of the Philippines Los Banos (UPLB) p.1- 103.

Khush, G.S. and E.R. Angeles, 1999. A new gene for resistance to race 6 of bacterial blight in rice, Oryza sativa L. RGN 16: 92-93.

Ogawa, T., T. Yamamoto, H. Kaku, S. Taura, G.S. Khush and T.W. Mew, 1988. Resistance to rice bacterial leaf blight. IRRI-Japan Collaborative Research Project. P.1-299.