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Estimation of Metalaxyl Resistance in Phytophthora infestans. D. Kadish, Graduate student, Department of Life Sciences, Bar-Ilan University, Ramat-Gan 52100, Israel; Y. Cohen, Professor, Department of Life Sciences, Bar-Ilan University, Ramat-Gan 52100, Israel. Phytopathology 78:915-919. Accepted for publication 4 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-915.

A method for detecting low frequencies of resistance to metalaxyl was developed for Phytophthora infestans. Potato tuber disks were used because they were more susceptible to the blight fungus than leaf disks or intact plants. When resistance of 5–95% was anticipated in a mixed sporangial population, tuber disks (10 × 3 mm) were placed in petri dishes on either water or 100 mg/L of metalaxyl and inoculated with 3 to 6 sporangia per disk. When 0.01–1% resistance was anticipated, disks were placed on 100 mg/L of metalaxyl and inoculated with 50–800 sporangia per disk. The number of disks on metalaxyl supporting fungal sporulation at 1 wk after inoculation was used to compute resistant frequency. The bioassay is simple, accurate, and may be used as a tool to assess aspects of selection pressure imposed on the pathogen by phenylamide fungicides in the field.