Detoxification of canola chemical defenses (phytoalexins and others) is an important mechanism used by the blackleg fungus Leptosphaeria maculans/Phoma lingam to overcome the plant’s natural defenses. Phytoalexins are anti-microbial defense metabolites produced de novo by plants in response to pathogen attack and other forms of stress. L. maculans is successful in detoxifying several cruciferous phytoalexins into different products. For example, brassinin, a key phytoalexin from crucifers, is transformed into indole-3-carboxaldehyde. This thesis includes investigation of phytoalexin metabolism by L. maculans and related work: (i) transformation pathways of cruciferous phytoalexins and analogues; (ii) design and synthesis of potential inhibitors of brassinin detoxification. In continuation of previous work, homologues, analogues and structural relatives of brassinin were analysed for metabolism by L. maculans. Products of metabolism of these compounds were identified and the overall metabolic pathways were established. It was concluded that structural relatives of brassinin metabolized differently from brassinin. Antifungal bioassays of the products suggested that all these transformations were detoxification reactions. Among the phytoalexins, rapalexin A was not metabolized whereas, erucalexin was metabolized. Results of these metabolism studies using L. maculans along with the syntheses and antifungal activities of the metabolites will be presented. In the second part of thesis, inhibition of the detoxification of brassinin by L. maculans using quinolines and isoquinolines was investigated. These compounds resulted from replacement of indolyl containing structures with quinoline and isoquinoline moieties, and various substitutions such as phenyl, thiazolyl, bromo, chloro, hydroxy and methoxy groups. All these compounds were tested for their effect on brassinin detoxification and antifungal activity. Overall, a significant effect on the rate of brassinin detoxification in cultures of L. maculans was detected in the presence of compounds 6-bromo-2-phenylquinoline, 2-phenylquinoline, 3-phenylquinoline, 1-thiazolylisoquinoline. 6-Bromo-2-phenylquinoline was the most effective compound in slowing down the metabolism of brassinin and also was a weak inhibitor of the growth of L. maculans (virulent on canola). Results of the syntheses and evaluation of the compounds are discussed.