Histological study of the responses of two Vitis vinifera cultivars (resistant and susceptible) to Plasmopara viticola infections

Leaves of Vitis vinifera L. cvs. Chasselas (susceptible) and Solaris (resistant) were inoculated with Plasmopara viticola. Samples were then examined by scanning electron microscopy, light and epifluorescence microscopy. On the resistant cv. Solaris, stomatal deposits, identified as callose, were visible around the germinating zoospores 7 h after inoculation. Twenty-four hours after inoculation, infected stomata exhibited secretions that enveloped the zoospores. At this time, infected stomata were surrounded by necrotic tissues. At 120 h after inoculation, undefined material was deposited on the cuticle in the necrotic areas. Stomata in the vicinity of the infection sites contained callose deposits in and around the stomatal openings, but no necrotic zones were observed. On the sensitive cv. Chasselas neither secretion nor callose deposits were observed. Sporangiophores emerged about 96 h after inoculation and were fully developed 24 h later. Sporulation through small stomata-like apertures present all along the primary vein was also observed on the upper leaf surface. The role of stomatal callose deposits in the defense reactions of the Solaris grapevine cultivar against P. viticola is discussed.