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 site s. Resistant isolates comprised 31 to 59% of the population at those s ites. Most resistant isolates grew well in the presence of 100 mu g/ml of metalaxyl, whereas sensitive isolates did not grow at 1.0 mu g/ml. Some isolates from mature citrus groves were intermediate in their se nsitivity. Metalaxyl-drench treatments failed to control resistant iso lates in greenhouse tests, but fosetyl-Al was usually effective. The r eplacement series technique was used to compare the relative competiti ve ability of resistant and sensitive isolates in greenhouse tests. Re sults varied according to the isolate used, but at least some metalaxy l-resistant isolates were able to compete favorably with sensitive iso lates 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 Fl orida citrus groves.