The effect of fosetyl-Al on phytoalexin accumulation was investigated in non-inoculated and Plasmopara viticola-inoculated leaf discs of a susceptible cv. (Riesling) intermediate resistant ( V. rupestris 6544), and resistant cv. (Castor) Vitis spp. In non-inoculated tissues fosetyl-Al induced only a weak phytoalexin response. ε-Viniferin was the predominant compound in both cv. Riesling and cv. Castor after application of low concentrations of fungicide; at high concentrations more resveratrol than ε-viniferin accumulated in cv. Castor. In V. rupestris 6544, levels of both compounds were approximately the same. Following pre-infectional applications of fosetyl-Al, sporulation of Plasmopara viticola was completely suppressed by 40 μg ml −1 in V. rupestris 6544, 120 μg ml −1 in cv. Castor, and 280 μg ml −1fungicide in cv. Riesling. Degradation of ε-viniferin was rapid in inoculated fungicide-treated tissue but the compound was not detected after post-infectional fungicide applications. Resveratrol was the predominant phytoalexin; it accumulated to the highest levels in cv. Castor, high levels in V. rupestris 6544, and low levels in cv. Riesling. Post-infectional fungicide treatments elicited a several fold increase in resveratrol compared with pre-infectional applications, and 400 μg ml −1 was more effective than 200 μg ml −1. Pre-infectional fungicide treatments with 200 μg ml −1and 400 μg ml −1 fosetyl-Al gave total control of Plasmopara viticola in cv. Castor and V. rupestris 6544; in cv. Riesling only the high concentration was effective. Post-infectional applications did not suppress sporulation in cv. Riesling; in V. rupestris 6544 400 μg ml −1, but not 200 μg ml −1 fosetyl-Al inhibited reproduction of the fungus, whereas, in cv. Castor, sporulation was completely controlled by both treatments. The degree of curative control was highly correlated with level of resveratrol accumulation. The results did not provide evidence for a phytoalexin response being the primary mode of action of fosetyl-Al. It is suggested that a combination of direct (= primary) and indirect (= secondary) modes of action exert a double selection pressure on the fungus which may explain the rarity of fosetyl-Al-resistant strains in the field.