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 imp ortant disease of rice (Oryza sativa L.) worldwide. This partial resistance has been considered to be pola genic. The partial resistance of the cultiv ars Jasmine 85 (J-85) and Teqing (TQNG) was studied by evaluating the resis tance of F-1, F-2, F-3, F-4, and backcross F-1 generations of crosses betwe en 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 p lants. Segregating backcross F-1 populations showed a ratio of 1:1 resistan t/susceptible plants when the F-1 was crossed to a susceptible parent and n o segregation occurred when the F-1 was crossed to a resistant parent. Thes e results were consistent with the partial resistance from the two resistan t cultivars being controlled by single dominant genes. When the resistant c ultivars were crossed, the segregating F-2 population showed a 15:1 resista nt/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 par ent. This suggests that major genes conferring high levels of partial resis tance to sheath blight may be incorporated together into lines to give near complete resistance. This makes the identification of major genes for part ial resistance to sheath blight critically important to rice breeding progr ams.