Sensitivities of Mexican Isolates of Phytophthora infestans to Chlorothalonil, Cymoxanil, and Metalaxyl. Ludwik S. Sujkowski, Visiting Scientist, Department of Plant Pathology, Cornell University, Ithaca, N.Y. 14853. Barbara A. Fry, Technician, Department of Plant Pathology, Cornell University, Ithaca, N.Y. 14853; Richard J. Power, Scientist, E. I. DuPont de Nemours and Co., Ag Products Crop Research Lab, Newark, Del. 19714; Stephen B. Goodwin, Research Associate, and Tobin L. Peever, Research Associate, Department of Plant Pathology, Cornell University, Ithaca, N.Y. 14853; Ronald A. Hamlen, Scientist, E. I. DuPont de Nemours and Co., Ag Products Crop Research Lab, Newark, Del. 19714; and William E. Fry, Professor, Department of Plant Pathology, Cornell University, Ithaca, N.Y. 14853. PLANT DIS. 79:1117. Accepted for publication 31 July 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-1117.

Seventy-five genetically diverse isolates of Phytophthora infestans from central and northwest Mexico were evaluated in vitro for their sensitivities to chlorothalonil, cymoxanil, and metalaxyl. Sensitivities were determined by measuring radial growth on agar medium amended with fungicide. Isolates were classified into five sensitivity categories based on their estimated EC50 (50% effective concentration) for each fungicide. For chlorothalonil and cymoxanil, the majority (83 and 70%, respectively) of isolates had EC50 that fell between 0.1 and 1.0 μg/ml (ppm) and the mean EC50 values were 3.1 and 0.8 ppm, respectively. Three isolates had mean EC50 values for chlorothalonil in excess of 10 ppm. For metalaxyl, sensitivity phenotypes were more evenly distributed and the mean EC50 value was 52.5 ppm. Forty-four percent of the isolates had EC50 values in excess of 100 ppm. Variation in sensitivity phenotype was lowest for cymoxanil, also low for chlorothalonil, and higher for metalaxyl. For no fungicide were EC50, values correlated to mating type or allozyme genotype.