Quantitative trait loci analysis suggests that partial resistance to rice blast is mostly determined by race-specific interactions

The race specificity of quantitative trait loci (QTL) for partial resistance to blast (Magnaporthe grisea) disease was tested by using isolates for which no major resistance gene segregated in a mapping population based on rice (Oryza sativa) varieties Bala x Azucena. Recombinant inbred lines were repeatedly inoculated with isolates CD100, CM28 and PH19 and scored for lesion type, lesion size and number of lesions. Composite interval mapping was employed to identify QTL. Eighteen main effect QTL were detected. Of these, eight were detected in only one isolate, seven detected in two isolates and only three in all three isolates. Diallelic epistasis was identified as an important genetic component but had little discernible pattern. Fourteen main-effect QTL mapped to previously identified QTL for blast resistance and 10 to previously identified major resistance genes. It is concluded that the majority of QTL detected are race-specific. The results also support the hypothesis that partial resistance genes might be defeated major genes, with residual effectiveness and race specificity.