Selection for metalaxyl resistance in potato crops infected with Phytophthora infestans: effects of fungicides and initial frequency of resistant sporangia

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Abstract
Potato crops (cv. Alpha) were grown in walk‐in plastic tunnels (33 m2) in winter 1987 and spring 1988. Crops were left untreated or treated (four sprays at intervals of 2 weeks) with either metalaxyl, Ridomil‐MZ (metalaxyl+ mancozeb) or Ridocym (metalaxyl +mancozeb+cymoxanil). Crops were inoculated with sporangial suspensions of Phytophthora infestans containing 01, 1 or 10% metalaxyl‐resistant sporangia. Disease progress and percentage resistant sporangia in tunnels were estimated for a period of 50 days. In the treated crops cumulative disease, rate of disease progress and increase in frequency of resistant phenotypes (selection coefficient) were higher in metalaxyl‐treated crops than in Ridomil‐MZ or Ridocym‐treated crops. Cumulative disease, rate of disease progress and increase in frequency of resistant phenotypes were unaffected by the frequency of resistant sporangia in the original inoculum. Metalaxyl, Ridomil‐MZ and Ridocym speeded up the selection for resistance by 92, 29 and 17%, respectively, relative to untreated crops. The time interval required for the resistant subpopulation to predominate in treated crops was inversely related to initial resistance. It was longest in crops inoculated with 0.1 % resistant sporangia and treated with Ridocym. Results indicated that metalaxyl mixtures were advantageous over metalaxyl alone, both in delaying the buildup of resistance and in improving disease control, but did not prevent resistant subpopulations increasing from 01 to 100% in a single season.