Investigation of the interaction of Plasmopara viticola with susceptible and resistant grapevine cultivars

The infection process of Plasmopara viticola (Berk. & Curt. ex de Bary) Berl. et de Toni, which causes downy mildew of grape, was studied on four different cultivars of Vitis vinifera. Independent of whether susceptible or resistant grapevine cultivars are colonized by Plasmopara viticola zoospores, the infection cycle is initiated. Three to 4 days after infection, however, the infection process is terminated in resistant cultivars. Infected areas turn necrotic and brownish. In addition to SEM observations, the host response referring to peroxidase activity was also investigated. In the tissues of 'Orion' and 'Phoenix', two newly-bred Plasmopara-resistant cultivars, a high activity of peroxidase and a wide distribution of oxidized phenolic products were observed. This peroxidase induction occurred much earlier as compared to the infection of two traditional susceptible cvs. 'Kerner' and 'Riesling'. The data indicate a correlation between the inducibility of peroxidase in infected plants and the degree of resistance or susceptibility to downy mildew infection.