Distribution of Phytophthora Root Rot of Alfalfa in Central Mexico and Development of Disease Resistance in Mexican Cultivars of Alfalfa. R. J. Aguirre, Graduate Student, Department of Plant Sciences, University of Arizona, Tucson 85721. R. B. Hine, Professor, Department of Plant Pathology; and M. H. Schonhorst, Professor, Department of Plant Sciences, University of Arizona, Tucson 85721. Plant Dis. 67:91-94. Accepted for publication 30 May 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-91.

Surveys during the summers of 1977 and 1978 in central Mexico (18–22º N latitude) indicated that root rot of alfalfa (Medicago sativa), caused by Phytophthora megasperma f. sp. medicaginis, was widespread in irrigated fields at elevations between 1,600 and 2,000 m in the states of Aguascalientes, Estado de Mexico, Guanajuato, Hidalgo, Michoacan, and Queretaro, Mexico. The pathogen was recovered from eight of 28 soil samples taken from infested fields by use of an alfalfa seedling-bait technique and the antibiotics pimaricin and hymexazol. Observations in the field and pathogenicity studies in the greenhouse demonstrated that all commonly planted alfalfa cultivars in Mexico, including Moapa, Mesa-Sirsa, and Joaquin 11 (all from the United States), Aragon (from Spain), and INIA-76, Bajio-76, Puebla-76, and Mixteca-76 (from Mexico), were susceptible to the pathogen. The level of resistance to the pathogen in the widely planted Mexican cultivar INIA-76 increased from 5.6 to 27.8% survival levels following two cycles of phenotypic recurrent selection. Two isolates of P. megasperma f. sp. medicaginis from central Mexico were similar to each other in pathogenicity but differed from the typical isolate from Arizona.