Acta Hortic. 1248. ISHS 2019. DOI 10.17660/ActaHortic.2019.1248.82 Proc. XII International Conference on Grapevine Breeding and Genetics Eds.: S. Delrot et al. 607 Evidence for sexual reproduction and fertile oospore production by Plasmopara viticola on the leaves of partially resistant grapevine cultivars L. Delbac 1,a , L. Delière 1,2 , C. Schneider 3 and F. Delmotte 1 1 SAVE, INRA, Bordeaux Sciences Agro, ISVV, Villenave d’Ornon, France; 2 UE Vigne Bordeaux Grande-Ferrade, INRA, ISVV, Villenave d’Ornon, France; 3 SVQV, INRA, Université de Strasbourg, Colmar, France. Abstract 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. Keywords: grapevine downy mildew, disease-resistant variety, Rpv1, Rpv3 INTRODUCTION Plasmopara viticola, the causal agent of grapevine downy mildew, is an obligate biotrophic oomycete that attacks Vitis vinifera (Viennot-Bourgin, 1949; Lafon and Clerjeau, 1988; Burruano, 2000; Luis et al., 2013). All major V. vinifera cultivars are highly susceptible to this pathogen (Yin et al., 2017). It was introduced into Europe from North America in the 1870s (Millardet, 1881) and subsequently spread to all major wine-growing regions of the world (Galet, 1977; Lafon and Clerjeau, 1988; Gessler et al., 2011). European P. viticola populations have a low level of genetic diversity, consistent with the occurrence of a bottleneck at the time of introduction (Fontaine et al., 2013). P. viticola has direct quantitative (Savary et al., 2009; Fermaud et al., 2016) and qualitative (Pons et al., 2018) effects on yield. Fungicide treatment is currently the only available method for controlling this pathogen on V. vinifera (Caffi et al., 2010; Gessler et al., 2011). However, the systematic use of chemicals has led to fungicide resistance in P. viticola populations (Chen et al., 2007; Delmas et al., 2017), decreasing the efficiency of a growing number of products (Gessler et al., 2011). Rational fungicide management approaches based on decision support systems are now being implemented (Delière et al., 2015). a E-mail: lionel.delbac@inra.fr