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Epidemic Development of Bacterial Blight on Rice Carrying Resistance Genes Xa-4, Xa-7, and Xa-10

Epidemic development of bacterial blight caused by Xanthomonas oryzae pv. oryzae on three near-isogenic lines (IRBB lines) carrying bacterial blight resistance genes Xa-4 (IRBB4), Xa-7 (IRBB7), and Xa-10 (IRBB10) was studied in two farmers' fields located in Calauan and Mabi-tac, Phillipines, during 1993, 1994, and 1995. The plots were planted continuously during dry and wet seasons, with disease assessments done during the wet seasons, when weather was conducive to bacterial blight development. Disease incidence (number of hills infected) and severity (diseased leaf area) were assessed and area under the disease progress curve (AUDPC) values were computed. Mean disease incidence and severity in Calauan (AUDPC = 3,241.5 and 716.7, respectively) were significantly higher than in Mabitac (AUDPC = 2,431.2 and 558.0, respectively). In Calauan, AUDPC values for disease incidence and severity on IRBB4, IRBB10, and IR24 did not differ significantly during the three years of testing. In Mabitac, the AUDPC values for disease incidence and severity were not significantly different among IRBB4, IRBB10, and IR24 in 1993 and in 1995. However, in 1994, disease incidence (AUDPC = 3,037.7) and severity (AUDPC = 891.0) on IRBB4 were significantly lower than on IRBB10 and IR24. Bacterial blight was lowest on IRBB7 at both sites and throughout 1993 to 1995, indicating that this genotype is effective in suppressing the disease even with the presence of an indigenous virulent population of X. oryzae pv. oryzae.