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The sensitivity of 251 isolates of Phytophthora infestans to the phenylamide fungicide metalaxyl was assessed by an in vitro radial growth assay on fungicide-amended agar media. Isolates were collected from many regions of North America from 1987 through 1993, including 15 states in the United States and British Columbia, Canada. A small sample of isolates from Europe and Israel was included for comparison. Isolates that grew less than 40% of the control on media containing 5pg of metalaxyl per ml were considered sensitive; all other isolates were scored as resistant. Field trials and floating leaf-disk assays were used to confirm the accuracy of the amended-agar technique. All isolates collected from 1987 through 1989 were sensitive. Metalaxyl-resistant isolates were detected in 13 of 15 states and in British Columbia during 1992 and 1993.

With one exception, sensitivity to metalaxyl was absolutely …