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Resistance to Mefenoxam and Metalaxyl Among Field Isolates of Phytophthora capsici Causing Phytophthora Blight of Bell Pepper

Incidence of Phytophthora blight in bell pepper fields that were sprayed for the first time with Ridomil Gold (mefenoxam) according to labeled recommendations was higher in North Carolina in 1997 than in previous years. Mefenoxam is the more active enantiomer contained in the racemic fungicide metalaxyl. A total of 150 isolates were obtained from 17 fields at eight grower locations. Among isolates from all locations, 30% were classified as sensitive, 10% as intermediate, and 59% were resistant to mefenoxam. Mefenoxam-resistant isolates were found in 82% of the fields sampled (14 of 17 fields). The proportion of resistant isolates in individual (fields ranged from 28 to 100%. The mean effective concentration (EC₅₀) values for mefenoxam-sensitive isolates was 0.568 μg ml^-1 (ranging from 0.12 to 1.1 μg ml^-1), whereas the mean EC₅₀ value for mefenoxam-resistant isolates was 366.5 μg ml^-1 (ranging from 3 to 863 μg ml^-1). The mean EC₅₀ value for metalaxyl-sensitive isolates was 0.27 μg ml^-1 (ranging from 0.00002 to 1.3 μg ml^-1) and for metalaxyl-resistant isolates was 470.34 μg ml^-1 (ranging from 10 to 966 μg ml^-1). The greatest proportion of resistant isolates came from fields where mefenoxam was used alone rather than in combination with other fungicides. Both mating types were found among resistant isolates, suggesting that these isolates may persist in soil in subsequent years. Field isolates of Phytophthora capsici resistant to mefenoxam on pepper have not been reported previously and now pose new challenges for management of this important disease.