CHARACTERIZATION OF QUANTITATIVE TRAIT LOCI (QTLS) IN CULTIVATED RICECONTRIBUTING TO FIELD-RESISTANCE TO SHEATH BLIGHT (RHIZOCTONIA-SOLANI)

Sheath blight, caused by Rhizoctonia solani, is one of the most import ant diseases of rice. Despite extensive searches of the rice germ plas m, the major gene(s) which give complete resistance to the fungus have not been identified. However, there is much variation in quantitative ly inherited resistance to R. solani, and this type of resistance can offer adequate protection against the pathogen under field conditions. Using 255 F-4 bulked populations from a cross between the susceptible variety 'Lemont' and the resistant variety 'Teqing', 2 years of field disease evaluation and 113 well-distributed RFLP markers, we identifi ed six quantitative trait loci (QTLs) contributing to resistance to R. solani. These QTLs are located on 6 of the 12 rice chromosomes and co llectively explain approxi mately 60% of the genotypic variation or 47 % of the phenotypic Variation in the 'Lemont' x 'Teqing' cross. One of these resistance QTLs (QSbr4a), which accounted for 6% of the genotyp ic Variation in resistance to R. solani, appeared to be independent of associated morphological traits. The remaining five putative resistan ce loci (QSbr2a, QSbr3a, QSbr8a, QSbr9a and QSbr12a) all mapped to chr omosomal regions also associated with increased plant height, three of which were also associated with QTLs causing later heading. This was consistent with the observation that heading date and plant height acc ounted for 47% of the genotypic variation in resistance to R. solani i n this population. There were also weak associations between resistanc e to R. solani and leaf width, which were likely due to linkage with a QTL for this trait rather than to a physiological relationship.