Abstract: The system of plant defence against fungi is complex. It encompasses several cross-talking pathways that respond to various signals. Methyl jasmonate, a plant growth regulator is one of the various signals that stimulate a defence response. In order to comprehend the involvement of jasmonate in the defence of Arabidopsis against Erysiphe cruciferarum, a mutant that is defective in jasmonate accumulation, was used (fad3-2fad7-2fad8). This mutant is susceptible to E.cruciferarum infection, which results in powdery mildew. The wild type plants were resistant towards powdery mildew. Mutant plants sprayed with methyl jasmonate received substantial protection against E.cruciferarum infections. The incidence of disease in methyl jasmonate treated mutants was reduced to levels observed in the wildtype. The coi1 mutant however was not protected through treatment with methyl jasmonate. This therefore indicates that methyl jasmonate induces the plants resistance through the activation of the plants defence system and not as an antifungal agent. One of the genes activated in the defence response is the AtVSP gene. The transcript in wildtype plants was induced when infected by E.cruciferarum. Transcript levels in mutants were comparable to the constitutive levels observed in the controls. Therefore it is apparent that jasmonate-signalling is essential in protection against E.cruciferarum infections of Arabidopsis.