Six different Vitis cultivars grown in recirculating nutrient solutions supplied with 0, 10 and 112 mg.l-1 SiO2, respectively, were infected with spores of the powdery mildew fungus Uncinula necator and analyzed after 2, 4 and 6 weeks. Whereas the size of the silica endoskeletons induced by the powdery mildew was influenced by the SiO2 concentration, the cultivar-specific resistance could not be increased. However, after 2 weeks all varieties showed a slightly but significantly enhanced resistance in the 112 mg.l-1 solution which disappeared during the following weeks. The results show silica to be essential for a normal powdery mildew resistance but make evident that the Oidium susceptibility of cultivars cannot be overcome by supplementary silica fertilization in the field, the Si contents of most soil solutions being far above the minimal requirements of the grapevine
