Rice Genetics IV - IRRI books - International Rice Research Institute

ecent years, Katy has been used as a source of blast resistance in the southern U.S. Inblast-prone areas, major-gene resistance has been ineffective due to the rapid developmentof new races of the pathogen. In these areas, partial resistance (along withchemical control) has been used (Ezuka 1979). With a better understanding of therelationship between resistance genes and pathogen lineages, gene pyramiding usingmolecular markers is now a viable approach.Bacterial blight. The exploitation of major genes for bacterial blight (caused byXanthomonas campestris) resistance has been more successful than that for blast (Mew1987, Khush et al 1989). A large number of genes for resistance have been identifiedand several have been transferred to high-yielding varieties (Ogawa and Khush 1989).Xa4 has been a particularly successful gene, showing some residual resistance evenafter compatible races of the pathogen became common. In the presence of virulentpathogen races, genes may differ in their level of resistance (Ona et al 1998). Again,gene pyramiding is currently being pursued in an effort to develop more durable andbroader resistance (Huang et al 1997), although varietal mixtures have also been advocated(Ahmed et al 1997).InsectsBrown planthopper (BPH). This insect devastated high-yielding rice crops in the 1970suntil resistant varieties were developed. Variety IR26, possessing the resistance geneBph1, was developed at IRRI (Khush 1989). When this resistance broke down in1976, the recessive bph2 was used. In general, BPH resistance was believed to besuccessful; however, some workers contend that this pest is caused by mismanagementof pesticides and continuous rice planting, and that the biotypes are not fixed buthighly fluid. Most likely, resistance has had some success in reducing losses from thispest (Gallagher et al 1994).Green leafhopper (GLH). This insect rarely causes direct damage but is more aproblem as the vector of rice tungro viruses. A large number of resistance genes havebeen identified (Ghani and Khush 1988). Resistance to the vector also confers resistanceto the viruses; however, new GLH biotypes can develop and require the incorporationof new resistance genes.Gall midge. Several genes conferring resistance to gall midge have been identifiedand transferred into high-yielding cultivars (Tomar and Prasad 1992, Pani andSahu 2000). Resistance has been very effective in reducing damage from the disease.Although several biotypes exist, resistance has been relatively durable.Resistance to abiotic stressesIn general, tolerance of or resistance to abiotic stresses is a polygenic trait. One exceptionis low-temperature-induced chlorosis or wilting at the seedling stage. Severalgenes controlling this trait were identified and single-gene segregation ratios are usuallyobserved in indica × japonica crosses (Kwak et al 1984, Nagamine 1991). However,it appears that this gene has not been exploited in breeding for cold tolerance.Application of Mendelian genetics in rice breeding 33