Histopathological study of resistant ( Vitis labrusca L.) and susceptible ( Vitis vinifera L.) cultivars of grapevine to the infection by downy mildew

Plasmopara viticola (Berk. & M. A. Curtis) Berl & de Toni is a biotrophic oomycete responsible for downy mildew on grapevine. This study aimed to compare the development of P. viticola between a resistant interspecific hybrid of Vitis labrusca (‘Ives’, synonym ‘Bordô’), and a susceptible V. vinifera (‘Cabernet Sauvignon’) cultivars at different times after inoculation with the pathogen. The host leaf tissues were analysed at 0, 6, 12, 24, 48, 72, 96 and 120-h post-inoculation (hpi), by means of light optical microscopy and scanning electronic microscope (SEM). Light optical microscopy was used to analyse the pathogen development in the mesophyll intercellular spaces, as well as the histochemical changes occurring at host level during the infection. Structural differences were observed in stomata, and in the shape of mesophyll cells in the resistant cultivar after infection. SEM analysis showed differences in the pathogen development between both cultivars. All the stages of pathogen development were observed on the leaf surface of cv. ‘Cabernet Sauvignon’, but not in the cv. ‘Ives’. The results showed that cv. ‘Ives’ and ‘Cabernet Sauvignon’ display structural and histochemical differences to the downy mildew infection. Implications of these differences to the plant–pathogen interactions are discussed.