Use of partial host resistance in the management of bacterial blight of rice

The progress of bacterial blight epidemics, caused by Xanthomonas oryzae pv , oryzae, varies with environment. The irrigated lowland rice production en vironment in central Terai (plain) is less conducive to the disease than th e irrigated lowland rice production environment in eastern Terai in Nepal. The effect of partial resistance on bacterial blight was studied in central Terai during the wet seasons of 1994, 1995, and 1996. Three partially resi stant rice cultivars, Sabitri, Laxmi, and IR54 (possessing the Xa4 gene), a nd susceptible check IR24 were included in this study. Analysis of pooled d ata from the 3 years of experiments indicated that rice cultivars differed in resistance based on three epidemiological parameters: disease severity ( DS), area under the disease progress curve (AUDPC), and rate of disease inc rease (r). Estimates of DS, AUDPC, and r were reduced and yield loss was ne gligible in the partially resistant cultivar Laxmi compared with the suscep tible check IR24. IR54, which has partial resistance to X. oryzae pv. oryza e, also showed low DS, AUDPC, and r, which prevented yield loss due to bact erial blight. The locally adapted cultivar Sabitri showed intermediate esti mates of DS, AUDPC, r, and yield loss. Earlier and more severe disease deve loped in the susceptible check IR24, resulting in a yield loss of 22%. Yiel d losses were mainly due to reductions in number of tillers, number of grai ns per panicle, and 1,000-grain weight. These data indicate that the use of rice cultivars with high levels of partial resistance will be an effective tool for the management of bacterial blight in central Terai and similar e nvironments in Nepal.