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Metalaxyl-Resistant Isolates of Phytophthora nicotianae: Occurrence, Sensitivity, and Competitive Parasitic Ability on Citrus

Phytophthora nicotianae was recovered from 8 of 11 nursery sites, and metalaxyl-resistant isolates were detected at 6 of the 8 positive sites. Resistant isolates comprised 31 to 59% of the population at those sites. Most resistant isolates grew well in the presence of 100 μg/ml of metalaxyl, whereas sensitive isolates did not grow at 1.0 μg/ml. Some isolates from mature citrus groves were intermediate in their sensitivity. Metalaxyl-drench treatments failed to control resistant isolates in greenhouse tests, but fosetyl-Al was usually effective. The replacement series technique was used to compare the relative competitive ability of resistant and sensitive isolates in greenhouse tests. Results varied according to the isolate used, but at least some metalaxyl-resistant isolates were able to compete favorably with sensitive isolates in the absence of metalaxyl. In a field test, a high percentage of the population remained resistant to metalaxyl even after 2.5 years without treatment with this fungicide. Excessive use of metalaxyl in nursery situations has reduced the future utility of the product in Florida citrus groves.