Anatomic and genetic factors associated with the susceptibility of grapevine cultivars to Plasmopara viticola clade aestivalis and clade riparia

Abstract

The susceptibility of commonly grown grapevine cultivars in the province of Quebec to Plasmopara viticola (Berk. & M.A. Curtis) Berl. & De Toni clades riparia and aestivalis was compared. The relationship between anatomic (density and size of stomata and type of leaf hairs) and genetic composition (percentages of Vitis vinifera L., Vitis riparia Michx. and Vitis aestivalis Michx.) factors of the grapevine cultivars and their susceptibility to the two clades was investigated. The grapevine cultivars were classified according to their susceptibility to each clade. The aggressiveness of the clade riparia was positively correlated with stomata size and negatively correlated with type of leaf hairs. However, the aggressiveness of clade aestivalis was positively correlated with stomata size and density, estimated percentage of V. vinifera ancestry, and the published downy mildew susceptibility of grapevine cultivars; and negatively correlated with estimated percentage of V. riparia ancestry. Furthermore, the grapevine cultivar classification showed that, for P. viticola clade riparia, 44.4%, 44.4%, and 11.1% of the grapevine cultivars were classified as minimally susceptible, moderately susceptible, and highly susceptible, respectively. Alternatively, for P. viticola clade aestivalis, 11.1%, 22.2%, and 66.7% of the grapevine cultivars were classified as minimally susceptible, moderately susceptible, and highly susceptible, respectively. Although some grapevine cultivars fell in the same susceptibility groups for both clades, 78% of grapevine cultivars were classified in different susceptibility groups. The findings of this study provide new information on grapevine and P. viticola interactions, and highlight the importance of knowing which clade of P. viticola is present so that downy mildew control measures can be adapted accordingly.