Downy mildew, caused by Plasmopara viticola, is a highly destructive disease of grapevine. In recent decades, European breeding programs for disease resistance have led to the creation of new cultivars resistant to downy mildew. This resistance limits mycelium growth and the sporulation of P. viticola, thereby decreasing disease epidemics due to the asexual reproduction of the pathogen. However, we still lack biological insight into the effects of partial host resistance on pathogen survival during the sexual stage of its lifecycle. We present here the results of a two-year experiment in which we assessed the production of sexual forms of P. viticola (oospores) and the success of subsequent infections on resistant cultivars with the Rpv1 and Rpv3 loci. We found that disease resistance had little effect on the sexual cycle of the pathogen. Plasmopara viticola produced four times more oospores on susceptible Vitis vinifera cultivars than on disease-resistant cultivars. Macrosporangia resulting from the germination of oospores were found on both resistant and susceptible genotypes, and were produced in surprisingly large numbers on some Rpv1 genotypes. Rates of primary contamination due to macrosporangia were lower on Rpv3 than on conventional cultivars, but higher on Rpv1 genotypes. Thus, P. viticola can complete its lifecycle (both asexual and sexual phases) on partially resistant grapevine cultivars. The maintenance of downy mildew populations from one year to the next presents a significant challenge to the sustainability of genetic resistance in grapevine.