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The phytohormone, salicylic acid (SA), is required for a number of physiological processes within plants but primarily is a plant defence signalling molecule required for systemic acquired resistance (SAR). <i>Plasmodiophora brassicae</i>, causal agent of clubroot, is a soil borne obligate biotroph and is responsible for losses in Brassicaceae crops, including broccoli. We are investigating the response of a number of <i>Arabidopsis</i> ecotypes to infection by <i>P. brassicae</i> and following treatment with SA. Wild type Tul-0 and Tsu-0 was tolerant to an Australian isolate of <i>P. brassicae</i>, whilst Col-0 and Ler were susceptible. <i>Arabidopsis</i> mutants assessed for their response to the pathogen reveal that over-expressing SA mutants show restriction in gall formation. For these interactions SA is clearly a key molecule in regulating the outcome of the interaction. We are also examining SA-induced SAR in pre-plant and planting age (2 and 6 week old) broccoli seedlings, by pre-treatment of plants with SA. Broccoli plants were root drenched with SA at desired concentrations for 30 minutes before being transferred to pots containing <i>P. brassicae</i> inoculums at either 24 or 72 h post-treatment. A significant reduction in gall formation occurred in broccoli plants when treated with SA at concentrations between 2.5mM and 25mM in each treatment group. SA thus provides protection against clubroot in broccoli at the seedling stage.<p><p>Keywords: Oomycete, Vegetables, Brassicas (Crucifers)