Distribution of Phytophthora Root Rot of Alfalfa in Central Mexico and Development of Disease Resistance in Mexican Cultivars of Alfalfa

Abstract

Surveys during the summers of 1977 and 1978 in central Mexico (18-22.degree. 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 1600 and 2000 m in the states of Aguascalientes, Estado de Mexico, Guanajuato, Hidalgo, Michoacan, and Queretaro, Mexico. The pathogen was recovered from 8 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 USA), 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 2 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 [USA].