Abstract:
Information on the inheritance and gene action of Bacterial Leaf Blight (BLB) caused by Xanthomonas oryzae pv oryzae is known for many Asian rice populations, but very little is known about Ugandan rice populations. This study was conducted to (1) determine the inheritance and gene action conditioning the transmission of resistance to bacterial leaf blight into selected rice genotypes; and (2) determine the magnitude of genotype by environment (G x E) interaction for resistance to bacterial leaf blight. The study on the mode of inheritance and gene action involved crossing four BLB resistant genotypes, namely NERICA1, NERICA4, CT12 and IR09A to five susceptible and locally preferred cultivars: K85, K5, GSR-I-0057, CT 145 and IR24. The F2 populations were then inoculated with one BLB isolate-UG 20-2, most virulent in laboratory testing. For G x E interaction, 30 genotypes (17 crosses, 7 parents and 6 checks) and 18 near isogenic- lines based on IR24, having 1-4 known genes for resistance to BLB were evaluated at three locations ; Namulonge, Kibimba and Lira in Uganda.
Results revealed variations in the number and nature of genes involved in the inheritance of resistance to BLB. The resistance could be controlled by single dominant gene (3:1), two independent genes, (one recessive and the other dominant) (13:3), or including two independent dominant genes (15:1) suggesting epistatic gene action. When analyzed quantitatively, highly significant differences among genotypes were recorded (P<0.001). The investigation of GCA effects revealed that parents: NERICA1, NERICA 4, CT 12 and IR 09A all were good general combiners, showing that they can be used in future rice breeding cultivars with resistance to BLB. Reciprocal effects were also significant (P<0.01), indicating that maternal gene effects had a contribution to the modification of genotypic resistance to BLB. It was also revealed that IR 24, which is used as a universal susceptible check in Asian rice populations showed partial resistance in this study.
Predominance of GCA effects (Baker’s ratio = 75%) over SCA suggests that additive genetic effects are more important than non-additive in the inheritance of resistance in the genotypes. The values for narrow sense coefficient of genetic determination (NSCGD) and broad sense coefficient of genetic determination (BSCGD) were high (57% and 77%, respectively). The high significance shown for reciprocal effects indicates the importance of maternal contribution of the parents.
The G x E evaluation in three locations revealed highly significant variations in reaction patterns for the test genotypes and for the near isogenic lines, suggesting the existence of different BLB pathotypes in Uganda.
