COMBINED GENETIC-ANALYSIS OF PARTIAL BLAST RESISTANCE IN AN UPLAND RICE POPULATION AND RECURRENT SELECTION FOR LINE AND HYBRID VALUES

The CNA-IRAT 5 upland rice population has been improved for 4 years by recurrent selection for blast resistance in Brazil. In order to predi ct the efficiency of recurrent selection in different test systems and to compare the relative advantage of hybrids versus pure line breedin g, a combined genetic analysis of partial blast resistance in the CNA- IRAT 5 population was undertaken. A three-level hierarchical design in inbreeding and a factorial design were derived from the base populati on. Partial blast resistance of lines and hybrids was evaluated in the greenhouse and in the field by inoculation with one virulent blast is olate. The means and genetic variances of the hybrids and lines were e stimated. Genetic advance by recurrent selection was predicted from es timates of variance components. The inheritance of partial blast resis tance was mainly additive but non-additive effects were detected at bo th levels of means and variances. Mean heterosis ranged from 4%-8% for lesion size and lesion density to 10-12% for leaf and panicle resista nce. High dominance or homozygous dominance variances relative to addi tive variance and negative covariance between additive and homozygous dominance effects were estimated. A low frequency of favourable allele s for partial resistance would explain the observed organisation of ge netic variability in the base population. Recurrent selection will eff iciently improve partial blast resistance of the CNA-IRAT 5 population . Genetic advance for line or hybrid values was expected to be higher testing doubled haploid lines than S1 lines, or than general combining ability. Two components of partial resistance assessed in the greenho use, lesion size and lesion density, could be used as indirect selecti on criteria to improve field resistance. On the whole, hybrid breeding for partial blast resistance appeared to be slightly more advantageou s than pure line breeding.