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 environment in central Terai (plain) is less conducive to the disease than the 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 resistant rice cultivars, Sabitri, Laxmi, and IR54 (possessing the Xa4 gene), and susceptible check IR24 were included in this study. Analysis of pooled data 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 increase (r). Estimates of DS, AUDPC, and r were reduced and yield loss was negligible in the partially resistant cultivar Laxmi compared with the susceptible check IR24. IR54, which has partial resistance to X. oryzae pv. oryzae, also showed low DS, AUDPC, and r, which prevented yield loss due to bacterial blight. The locally adapted cultivar Sabitri showed intermediate estimates of DS, AUDPC, r, and yield loss. Earlier and more severe disease developed in the susceptible check IR24, resulting in a yield loss of 22%. Yield losses were mainly due to reductions in number of tillers, number of grains 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 environments in Nepal.