the grapevine downy mildew,Plasmopara viticola,is one of the most devastating pathogens in viticulture. effective control is mainly based on fungicide treatments, although resistance development in this pathogen is reported for a number of resistance. in this study we describe for the first time the molecular mechanism of resistance to a carboxylic acid amide (CAA) fungicide.we identified a family of four cellulose synthase (CesA) geans containing conserved domains that are found in all processive glycosyltransferases. phylogenetic analysis revealed their close relationship to the cellulose synthase of phytophthora sp.