Estimation of Metalaxyl Resistance inPhytophthora infestans

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Abstract
A method for detecting low frequencies of resistance to metalaxyl was developed for Phytophthora infestans. Potato [Solanum tuberosum] 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 .times. 3mm) 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 phylamide fungicides in the field.