Sensitivity of Phytophthora infestans Populations to Metalaxyl in Mexico: Distribution and Dynamics. J. M. MATUSZAK, Department of Plant Pathology, Cornell University, Ithaca. NY 14853. J. FERNANDEZ-ELQUEZABAL, National Potato Program, INIFAP, Toluca, Mex. Mexico; W. K. GU, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; M. VILLARREAL-GONZALEZ, National Potato Program, INIFAP, Toluca, Mex., Mexico; and W. E. FRY, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Plant Dis. 78:911-916. Accepted for publication 2 July 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0911.

Sensitivity to metalaxyl was determined for 966 isolates of Phytophthora infestans from seven locations in the states of Chiapas, Coahuila, Mexico, Michoacan, Puebla/Veracruz, and Sinaloa in Mexico. Isolates were collected from 1983 through 1989 in Mexico State and in 1988 or 1989 elsewhere. Sensitivity to metalaxyl was determined in vitro as radial growth on metalaxyl-amended medium, after comparison with the floating-leaf-disk method indicated the techniques provided equivalent results. Metalaxyl-sensitive isolates predominated in collections from Chiapas and Michoacan in 1989 and from Mexico State in 1983, 1984, and 1986. Sensitivity varied within some collections, and the frequency of sensitive isolates varied among collections. The overall frequencies of sensitive isolates from wild Solanum species in the large 1988 collection or from tomatoes were not significantly different from the frequency of sensitive isolates from nearby potatoes. In Mexico State, a major potato production area, the frequency of metalaxyl-sensilive isolates was higher in 1983, 1984, and 1986 than in 1987, 1988, and 1989. The frequency of sensitive isolates remained stable during epidemics on untreated potatoes repeatedly sampled in 1988 in the Toluca Valley of western Mexico State. No change occurred in the frequency of sensitive isolates collected from the same untreated site from 1988 to 1989.

Keyword(s): fitness costs, fungicide resistance