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Changes in Race Frequency of Xanthomonas oryzae pv. oryzae in Response to Rice Cultivars Planted in the Philippines. T. W. Mew, International Rice Research Institute, P.O. Box 933, Manila, Philippines. C. M. Vera Cruz, and E. S. Medalla. International Rice Research Institute, P.O. Box 933, Manila, Philippines. Plant Dis. 76:1029-1032. Accepted for publication 13 March 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-1029.

In an extensive survey of rice-growing areas in the Philippines from 1972 to 1988, 960 strains of Xanthomonas oryzae pv. oryzae (the bacterial blight pathogen) were collected. These strains were separated into six races based on their interactions with a distinct set of five differential cultivars carrying different resistance genes. Race 1, which was predominant before improved modern rice cultivars highly responsive to nitrogen fertilizer and management become widely cultivated, was virulent to rice with Xa-10, Xa-11, and Xa-14 genes. When cultivars with the Xa-4 gene were introduced in early 1970s, race 1 declined but races 2 and 3 appeared throughout the country. By the late 1970s, race 2 constituted 80% of the pathogen population, which corresponded to the proportion of hectarage planted to rice cultivars possessing the Xa-4 gene. This race composition has remained constant to the present time. Among the six races, four (races 1, 2, 3, and 5) have been detected often during the past 20 years. Strains of race 2, the most predominant race, appear to have a narrow spectrum of virulence among rice cultigens with new R-genes. Four races appear to have distinct geographical distributions. Race 1 continues to be detected in the lowlands, where traditional cultivars are planted, or in areas where cultivars with Xa-4 have not been extensively cultivated. Race 3, once the most predominant in the southern Philippines, was found infrequently, as was race 4, which was found only in Palawan. Race 5 exists in the highlands and has dominated the bacterial population in Banaue, the mountain terraces more than 1,500 m above sea level. A very minute population of race 6 was also detected.