populations of rice (Oryza sativa L.)

     S. Hitt- talmani, K. GIRISHKUMAR, P.G. BAGALI, 
     SRINIVASACHARY and HE. SHASHIDAR

     Department of Genetics and Plant Breeding, University of Agricultural Sciences, 
     GKVK, Bangalore 560 065, India

     Sheath rot of rice caused by the fungus Sarocladium oryzae has become a serious desease of rice crop in various parts of South Asian subcontinent. The characteristic feature of this disease is the inhibition of emergence of young panicles resulting in discoloration and rotting of flag leaf sheath, thus affecting the grain yield adversely (Estrada et a!. 1979). One hundred and eighty-eight F7 recombinant inbred lines (RILs) developed by single seed descent from a cross between a West African japonica variety Moroberekan and an Indian indica variety C039 were used in this study (Wang et a!. 1994). This experiment was carried out at the Main research station, Bangalore, India during the period 1997-98. The sheath rot disease incidence or severity was computed based on the actual number of infected plants to the total number of plants in a row and expressed as percentage of diseased with sheath rot. In a separate study, one hundred and fourteen doubled haploid (DH) lines of the cross IR64 x Azucena were screened for sheath rot disease at Hebbal, Bangalore, India and Agricultural Research Station, Ponnampet, India during the same year. The percentage of diseased with sheath rot was computed for all the genotypes.
     The panicles of the RILs and the DH lines showed considerable variation for sheath rot incidence. The RILs CM59, CM 159, CM283 and CM260 showed minimum incidence of sheath rot disease while CM62, CM81, CM 143 and CM277 were highly susceptible to sheath rot. To identify the QTL associated with sheath rot resistance interval analysis was done by using MAPMAKERJQTL (Lincoln eta!. 1992) based on percentage of sheath rot incidence for all the genotypes. We identified nine QTL on seven different chromosome (Table 1, Fig. 1). Four QTL with LOD score greater than 3.00 were identified. Among them, two were on chromosome 1 and one each on chromosomes 2 and 6. The major QTL was flanked by RGIO9 and RG236 with a maximum LOD score of 6.13 and with a phenotypic variance of 18.80%. Also, this chromosomal region is known to confer leaf blast resistance in this population (Wang et al. 1994). Similarly, regions on chromosome 4 between RG498 and RG864, on chromosome 6 between RG192 and RG1972 and on chromosome 8 between RG2O and RG333 represent regions of major QTLs controlling leaf blast resistance too, thus revealing the multiple effect of the complex loci for resistance to two rice pathogenic fungi. This result agrees with observations of Causse et al. (1994) who reported several bacterial blight resistance QTLs on chromosomes 4 and 11, particularly in the region containing blast resistance QTLs. Hence it can be hypothesized that there are several common chromosomal regions in rice that have DNA sequences conferring resistance to various diseases.

Table 1.QTLs for sheath rot resistance identified in two mapping populations by interval mapping in rice (Threshold LOC > 2.00)