Tobacco like many other plants has developed a spectrum of defense mechanisms against attacking pathogens. One of these mechanisms called SAR (systemic activated resistance) has been analyzed in great detail. SAR is triggered by several necrogenic pathogens, but also by CGA 245704 (Bion;). In tobacco and other plants, the biological induction of SAR requires salicylic acid (SA). Bion; mimics the action of SA thereby inducing the same defense mechanisms as the biological induction. A battery of defense genes is activated in either case leading to a broad spectrum resistance against several fungal, bacterial and viral diseases. The influence of the CGA 245704-activated defense mechanisms in the life cycle of Peronospora hyoscyami f. sp. tabacina was examined after foliar spray application, by artificial inoculation and subsequent light microscopic analysis. The CGA 245704-activated tobacco plants (cultivar Xanthi) protected themselves by affecting multiple sites in the life cycle of the blue mold fungus. Germination and appressorium formation were not affected but both of the next steps, penetration and the formation of intra-epidermal vesicles were inhibited. In addition, deformed non-penetrating surface structures were found, a phenomenon which is known to occur in P. hyoscyami f. sp. tabacina on the resistant tobacco species Nicotiana debneyi . These observed effects together resulted in a reduced development of disease symptoms. Based on the cytological observations as well as on the multitude of activated defense genes, it seems likely that Bion; protects tobacco plants by activating multiple defense mechanisms.