Between 1995 and 1997, 278 grape downy mildew (Plasmopara viticola) populations originating from European vineyards were characterised for their sensitivity to cymoxanil in a leaf-disc assay. The sensitivity proรผle revealed a wide distribution, with minimum inhibitory concentrations (MIC) ranging from 10 to more than 800 mg litre-1.
values ranged from 1 to more than 800 mg EC 50 litre-1 with an average of 125 mg litre-1. The sensitivity distribution was stable between 1995 and 1997. Surprisingly, populations from Portugal appeared signiรผcantly more sensitive than those from France or Italy, which could not be linked to diรพ erential cymoxanil usage in these countries. P viticola populations collected outside Europe and never exposed to cymoxanil appeared signiรผcantly more sensitive than exposed European populations, with an average value of 10 mg litre-1. The EC 50 level of sensitivity of European P viticola populations was relatively unaรพ ected by the number of cymoxanil applications made during a season or by the number of years of cymoxanil use. No link was found between the level of sensitivity in the leaf-disc assay and the level of performance of the cymoxanil mixtures used in the รผelds where the populations originated. Speciรผc รผeld trials conducted in Italy and Portugal have shown that the performance of cymoxanil-based mixtures remained good even on populations of the grape downy mildew fungus characterised as less sensitive in the leaf-disc assay. While there are no baseline sensitivity data for pre-commercialisation P viticola populations, the results of our study suggest that a shift in sensitivity (12.5-fold) may have occurred in some areas since introduction of cymoxanil on grapes nearly 20 years ago. Because cymoxanil is never used alone, it is difficult to determine whether or not practical resistance is occurring in European vineyards.