Genetic analysis of an induced mutant of rice for a quantitative resistance to bacterial leaf blight

An induced mutant of rice (designated as M57) showing a quantitative resistance to the Japanese differential races II, III and IV of bacterial leaf blight (BLB) and a qualitative resistance to the races I and V was crossed with the mother variety Harebare which is susceptible to BLB. The results of genetic analysis for the resistance using the F1 and F2 plants derived from the cross and the parents suggested that the quantitative resistance was conditioned by polygenes or some minor gene(s) and the qualitative resistance by single dominant genes. However, it was estimated through an allelism test that the qualitative resistance to the races I and V was associated with pollen contamination from the variety Shizutama carrying the BLB resistance genes Xa-1 and Xa-12. The qualitative resistance genes in the F2 and F3 tests were not found to be linked with the quantitative resistance genes, though the genes were likely to modify, to some extent, the expression of the relevant genes for the quantitative resistance to the races II, III and IV. It was also observed that the induced genes expressing a quantitative resistance to the races II, III and IV were operative for the races I and V, too. A negative trait for practical use, e.g. dwarfism of the M57 mutant was not linked to the quantitative resistance. In the F 3 progeny lines, some lines with a quantitative resistance to all the differential races and with a normal plant type like that of the mother variety Harebare, were selected, as they may become useful materials for the cross breeding for BLB resistance.