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Comparison of Virulence of Xanthomonas campestris pv. oryzae in Thailand and the Philippines. S. Eamchit, Plant Pathologist, Division of Plant Pathology and Microbiology, Department of Agriculture, Bangkhen, Bangkok 9, Thailand. T. W. Mew, Plant Pathologist, The International Rice Research Institute, Los Baños, Laguna, Philippines. Plant Dis. 66:556-559. Accepted for publication 15 August 1981. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-556.

Isolates of Xanthomonas campestris pv. oryzae collected in Thailand and the Philippines were distinguished into groups of strains for virulence on rice (Oryza sativa) cultivars with different genes for resistance to the pathogen. All groups of this bacterial pathogen were avirulent on cultivar DV85 (recessive gene xa-5 and dominant gene Xa-7). Group 0 was avirulent on all cultivars, including IR8 and RD9 that carry no genes for resistance to the pathogen. The Thai group I was comparable to the Philippine group I and was virulent only on IR8 and RD9. The Philippine group II was similar in virulence to the Thai group III but differed from it in that it was virulent on both RD7 (genes not analyzed) and IR20 (dominant gene Xa-4); the Philippine group II was virulent on RD7 but not on IR20. Only the Philippine group III isolates were virulent to IR 1545-339 that has the recessive gene xa-5 for resistance to the pathogen. Groups II and III were virulent on IR 1695 (dominant gene Xa-6) and PI 231129 (recessive gene xa-8); both cultivars have adult plant resistance to X. campestris pv. oryzae. The data suggest that pathogen specialization has occurred and that the specificity of infection of these differential cultivars can be distinguished in the vegetative stage by lesion length or area.