RESISTANCE TO METALAXYL IN ISOLATES OF THE SUNFLOWER PATHOGEN PLASMOPARA-HALSTEDII

Plasmopara halstedii isolates showing an atypical reaction to metalaxy l were collected in France, in 1995 and 1996, and tested in the labora tory for their level of sensitivity to this fungicide. Primary and sec ondary infections 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(-1) compared with 22 mg a.i. kg (-1) for sensitive isolate, indicating a 582-fold decrease in sensitiv ity to the compound. There was no reduction in the agressiveness of th e resistant isolate. Using other anti-oomycete fungicides, it appeared that propamocarb, contact fungicides (fluazinam, folpet, mancozeb) an d the mixed formulations dimethomorph + mancozeb, cymoxanil + mancozeb and ofurace + folpet were effective against primary infections made w ith metalaxyl resistant and sensitive isolates, but not against second ary infections. Metalaxyl mixed with fluazinam, folpet or mancozeb was more effective against primary infections with the resistant isolate than metalaxyl alone. The EC50 of five other isolates ranged from 5 80 0 to 32 900 mg a.i. kg(-1), indicating a variability in metalaxyl sens itivity of resistant sunflower downy mildew isolates. This is the firs t report of physiological resistance to metalaxyl in Plasmopara halste dii.