Exploring host-pathogen combinations for compatible and incompatible reactions in grapevine downy mildew

Viticulture in Europe and other parts of the world is continuously suffering significant economic loss from downy mildew epidemics. Although the infection biology of the pathogen Plasmopara viticola is well understood, little is known on the population diversity of this oomycete and on the mechanisms responsible for compatible or incompatible reactions with different host genotypes. The discrimination of strains with different virulence is a fundamental step for the assessment of pathotypes in local populations and could help to develop measures for preventing economic loss. We here describe an extended and slightly modified bioassay for the assessment of virulence by means of sporulation intensity. Simultaneously the necrotic reaction of host genotypes is considered and allows identification of different resistance strategies. Comparison of infections symptoms of 12 host genotypes after inoculation with five different single sporangium strains of Plasmopara viticola showed that: 1. resistance diversity is high in some Vitis vinifera cultivars; 2. Vitis riparia showed the strongest resistance amongst the four tested North American species; and 3. Asian Vitaceae harbor fully resistant as well as highly susceptible genotypes. In addition, microscopic investigation of inoculated leaf discs from Vitis amurensis and Ampelopsis japonica unraveled, that despite lack of sporulation vital pathogen mycelium was observed in the intercellular system. The necrotic reaction in V. amurensis indicates a different mechanism of resistance when compared to A. japonica, where no necrosis was found.