Resistance to metalaxyl in isolates of Plasmopara halstedii
In 1995 and 1996, isolates of Plasmopara halstedii (EU Annex II/A2) showing an atypical reaction to metalaxyl were collected in France and tested in the laboratory for their level of sensitivity to this systemic fungicide. Metalaxyl has been commonly used in France since 1990, and losses caused by P. halstedii have become insignificant despite the spread of new races of sunflower downy mildew. These laboratory studies showed that primary and secondary infections caused by one of these unusual isolates were not controlled by metalaxyl (at the concentration registered for seed treatment). This is the first report of physiological resistance to metalaxyl in P. halstedii. However, loss of efficacy in the field has not yet been observed. The authors stressed that the risk exists and that surveys on the occurrence of resistant isolates should be continued.
Sources
Albourie, J.-M.; Tourvieille, J.; Tourvieille de Labrouhe, D. (1998) Resistance to metalaxyl in isolates of the sunflower pathogen Plasmopara halstedii.
European Journal of Plant Pathology, 104(3), 235-242.