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Genetic Differentiation Among Isolates of Pyricularia Infecting Rice and Weed Hosts. E. S. Borromeo,Division of Plant Pathology, International Rice Research Institute, Los Baņos, Laguna, Philippines, Current address: National Institute of Biotechnology and Applied Microbiology (BIOTECH), University of the Philippines at Los Baņos; R. J. Nelson(2), J. M. Bonman(3), and H. Leung(4). (2)(3)(4)Division of Plant Pathology, International Rice Research Institute, Los Baņos, Laguna, Philippines; (3)Current address: DuPont Agricultural Products, Newark, DE; (4)Current address: Department of Plant Pathology, Washington State University, Pullman. Phytopathology 83:393-399. Accepted for publication 25 September 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-393.

DNA polymorphisms among Philippine isolates of the blast fungus from rice and 16 weed species were examined using restriction analysis of mitochondrial DNA (mtDNA) and DNA blot hybridization using ribosomal, single-copy, and repetitive DNA probes. Four rDNA hybridization patterns and six mtDNA restriction profiles were observed. The most frequent rDNA and mtDNA types were shared by isolates infecting rice and a group of weed species. Cluster analysis of the data obtained using probes for single-copy nuclear loci confirmed the phylogenetic relationships suggested by the mtDNA analysis, but differentiated the rice and non-rice-infecting isolates at the 50% similarity level. High levels of polymorphism between weed- and rice-infecting isolates were detected by six repetitive DNA probes. Probe MGR586 showed a consistently high number of hybridizing bands for rice-infecting isolates and low number of hybridizing bands for non-rice-infecting isolates. Subpopulations of the fungus attacking rice and adjacent weeds in two rice fields were found to be genetically differentiated. The results presented strongly suggest that although Pyricularia populations infecting rice and many rice-field weeds share a common ancestry, populations of the pathogen-infecting weed hosts do not provide inoculum for the rice crop in the Philippines.

Additional keywords: DNA polymorphisms, Magnaporthe grisea, population genetics, Pyricularia oryzae.