Major gene, nonallelic sheath blight resistance from the rice cultivars Jasmine 85 and Teqing

Only partial resistance is available for sheath blight, the second most important disease of rice (Oryza sativa L.) worldwide. This partial resistance has been considered to be polygenic. The partial resistance of the cultivars Jasmine 85 (J-85) and Teqing (TQNG) was studied by evaluating the resistance of F(1), F(2), F(3), F(4), and backcross F(1) generations of crosses between the resistant parents and the susceptible cultivars Maybelle (MBLE) and Cypress (CPRS). F(1) plants from the crosses were resistant. Segregating F(2) populations from both crosses showed ratios of 3:1 resistant/susceptible plants. Segregating backcross F(1) populations showed a ratio of 1:1 resistant/susceptible plants when the F(1) was crossed to a susceptible parent and no segregation occurred when the F(1) was crossed to a resistant parent. These results were consistent with the partial resistance from the two resistant cultivars being controlled by single dominant genes. When the resistant cultivars were crossed, the segregation F(2) population showed a 15:1 resistant/susceptible ratio. The results suggested that the two resistant parents each possessed a nonallelic dominant major resistance gene that segregated independently. Some of the F(4) lines from this cross appeared to have both resistance genes and a higher level of resistance than either resistant parent. This suggests that major genes conferring high levels of partial resistance to sheath blight may be incorporated together into lines to give near complete resistance. This makes the identification of major genes for partial resistance to sheath blight critically important to rice breeding programs.