Magnitude of genotype x environment interaction for bacterial leaf blight resistance in rice growing areas of Uganda

Abstract

Bacterial leaf blight (BLB) of rice (Oryzae sativa L.), caused by Xanthomonas oryzae pv. oryzae, is a major constraint in most lowland rice producing areas of Uganda. The disease is widely distributed in all irrigated and rainfed lowland rice ecosytems in the country. The pathogen (Xoo) is highly variable and its control is rather difficult. Development and deployment of host resistance is the only effective means of BLB management. The objective of this study was to determine the magnitude of genotype by environment (G x E) interaction for resistance to bacterial leaf blight in rice in Uganda. A study comprised of two sets of germplasms, a total of 30 rice genotypes comprising of 13 lines with varying levels of BLB resistance, and 17 F4 lines that had been previous generated through crossing 7 parental lines, and then advanced in bulk from F1, was conducted in Namulonge-Wakiso, Olweny-Lira and Kibimba- Bugiri districts in Uganda. The study also included 7 parental lines and 6 popular varieties used in most farmers’ fields. Variety IR 24 had been used as a universal check against BLB in Asian rice populations. Results revealed differential reactions on a set of near isogenic lines in the background of IR24, and some national and regional cultivars. IRBB1 (Xa1), IRBB2 (Xa2) and IRBB14 (Xa14) showed moderate susceptibility to susceptibility towards field pathogen populations in the three locations. Whereas genotype IRBB4 with gene Xa4 differentiated pathotypes of Kibimba and Lira from that of Namulonge, IRBB10 (Xa10) and IRBB11 (Xa11) differentiated pathotypes of Lira from the rest. Genotypes that had been pyramided with BLB genes of resistance, showed similar reactions to the three field populations. Generally, the near isogenic lines IRBB1, IRBB2, IRBB11 and IRBB14, had the highest leaf area damaged by disease attack. The highest was shown by IRBB11 with the Kibimba pathotypes for which disease attack was 43%. Low attack was observed on pyramided genotypes in all locations and two with single gene, i.e. IRBB8 and IRBB21, respectively. Interestingly, IR24 was as resistant as any of the pyramided combinations. Results also revealed different reactions of the tested genotypes in the three locations. The analysis of variance by AMMI partitioned the main effects of treatments into genotype, environment, and genotype x environment (G x E) interactions. Results also revealed that, the mean sum of squares due to treatments, genotypes, environments and genotype x environment interaction were significant, and contributed 48.2, 15.3, 19.3 and 13.3%, respectively, PCA1 accounted for 73.02% of the total G x E sum of squares.

Key Words: Oryzae sativa, pyramid, Xanthomonas