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Callus tissue of different grapevines (Vitis spp.) was inoculated with Plasmopara viticola. Short, highly-branched hyphae with necrosis, and long hyphae with heavy sporulation were observed on resistant and susceptible callus respectively. Thin-layer chromatography and spectrophotometric analysis showed that resistant callus contained greater quantities of gallocatechin derivatives than susceptible callus. Regression analysis between the field disease rating of each variety and its gallocatechin derivatives content indicated 92.2% correlation. Histochemical studies showed that, after infection with P. viticola, flavonoids appeared in the superficial cell walls of the callus, to a lesser degree on susceptible callus than on resistant callus. At a late stage of infection, the superficial cells of resistant callus were suberized, which did not occur in susceptible callus. This study showed that the preformed gallocatechin derivatives, the induced flavonoids and suberized superficial cells might play a role in the resistance of grapevine callus tissue to this fungus.Abbreviations: Callus I = Callus of V. riparia var. Gloire de Montpellier; Callus V = Callus of V. vinifera var. Grenache; TLC = Thin Layer Chromatography; var. = variety; GAD = Gallic acid derivatives; GD = gallocatechin derivatives; RC= resistant callus; SC --susceptible callus.

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