Resistance to metalaxyl in isolates of the sunflower pathogen Plasmopara halstedii.
During 1995-96, P. halstedii isolates showing an atypical reaction to metalaxyl were collected from sunflowers in France, and were tested in the laboratory for metalaxyl sensitivity. Primary and secondary infections in sunflowers caused by one of these isolates were not controlled by the metalaxyl concentration registered for seed treatment. The EC50 of this isolate was 12 800 mg a.i./kg compared with 22 mg a.i./kg for sensitive isolates, suggesting a 582-fold decrease in sensitivity to the compound. There was no reduction in the aggressiveness of the metalaxyl-tolerant isolate. Propamocarb, contact fungicides (fluazinam, folpet and mancozeb), and the mixed formulations dimethomorph + mancozeb, cymoxanil + mancozeb and ofurace + folpet, were effective against primary infections of metalaxyl-tolerant and sensitive isolates, but not against secondary infections. Metalaxyl mixed with fluazinam, folpet or mancozeb was more effective against primary infections of the tolerant isolate than metalaxyl alone. The EC50 of 5 other isolates ranged from 5800 to 32 900 mg a.i/kg, suggesting variability in metalaxyl sensitivity of tolerant isolates. This is thought to be the first report of physiological tolerance to metalaxyl in P. halstedii.