SENSITIVITIES OF MEXICAN ISOLATES OF PHYTOPHTHORA-INFESTANS TO CHLOROTHALONIL, CYMOXANIL, AND METALAXYL

Seventy-five genetically diverse isolates of Phytophthora infestans fr om central and northwest Mexico were evaluated in vitro for their sens itivities to chlorothalonil, cymoxanil, and metalaxyl. Sensitivities w ere determined by measuring radial growth on agar medium amended with fungicide. Isolates were classified into five sensitivity categories b ased on their estimated EC(50) (50% effective concentration) for each fungicide. For chlorothalonil and cymoxanil, the majority (83 and 70%, respectively) of isolates had EC(50)s that fell between 0.1 and 1.0 m u g/ml (ppm) and the mean EC(50) values were 3.1 and 0.8 ppm, respecti vely. Three isolates had mean EC(50) values for chlorothalonil in exce ss of 10 ppm. For metalaxyl, sensitivity phenotypes were more evenly d istributed and the mean EC(50) value was 52.5 ppm. Forty-four percent of the isolates had EC(50) values in excess of 100 ppm. Variation in s ensitivity phenotype was lowest for cymoxanil, also low for chlorothal onil, and higher for metalaxyl. For no fungicide were EC(50) values co rrelated to mating type or allozyme genotype.