Developing cultivars resistant to bacterial blight (BB) caused by Xanthomonas oryzae pv. Oryzae (Xoo) is considered the most effective and economical means of controlling this disease. Systematic studies on bacterial blight resistance of rice varieties in the tropics were conducted at IRRI during the last three decades, and several thousands rice germplasm were found to be resistant (Jackson 1997). Many resistant cultivars were genetically analyzed to identify genes for resistance. Considerable research efforts have also been directed to using the resistance genes in rice breeding programs (Khush et al. 1989).

Sidhu et al. (1978) reported that the resistance in DV85, DV86 and DZ78 is conditioned by two genes. At maximum tillering stage, xa5 conveys resistance, and an additional dominant gene, conveys resistance to race 1 at booting stage. This additional gene of DZ78 was designated as Xa7. However, it was not known if the additional genes of DV85 and DV86 were allelic to Xa7.

Chen (1990) also reported that two genes confer resistance to race 1 in cultivars DV85, DV86, AUS295 and ARC5756. One of these genes was xa5.

In order to determine whether the additional gene may be allelic to Xa7, we crossed these varieties with IRBB7, an isogenic line having Xa7 for resistance. The F₁s were resistant to race 1 and the F₂ populations did not segregate for susceptibility (Table 1).

These results show that DV85, DV86, Aus295 and ARC5756 also have Xa7 for resistance to race 1.

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. 103 p.

Jackson, M.T., 1997. Conservation of rice genetic resources: the role of International Rice Genebank at IRRI. Plant Molecular Biology 35: 61-67.

Khush, G.S., D.J. Mackill and G.S. Sidhu, 1989. Breeding rice for resistance to bacterial blight. In: Bacterial blight of rice, p. 207-217.

Sidhu, G.S., G.S. Khush and T.W. Mew, 1978. Genetic analysis of bacterial blight resistance in seventy-four cultivars of rice, Oryza sativa L. Theor. Appl. Genet. 53: 105-111.