The glucosinolate-myrosinase (GSL-M) system in oilseed rape and other members of the family Brassicaceae produces toxic products which can limit fungal pathogen attacks on the host. The role of this system in resistance of oilseed rape to 'Sclerotinia sclerotiorum', causal agent of stem rot, was investigated. Mustard powder was used as a GSL and myrosinase source in bioassays. The effect of toxic volatiles derived from hydrolysis of glucosinolates was observed as inhibition of fungal growth. Oxalic acid, a pathogenicity factor of the pathogen, did not affect production of toxic volatiles and inhibition occurred only at very acidic pH levels, regardless of the presence of oxalic acid. This indicated that oxalic acid at physiological concentrations and pH did not affect the GSL-M defense system. Exposure of 'S. sclerotiorum' colonies to inoculated leaves or leaf discs of host species or cultivars revealed that volatiles derived from infected leaf tissues have a toxic effect. This suggested that the GSL-M system is activated during infection of leaves and disease development. Freeze-dried powders of shoot parts of brassica species and cultivars including leaf, petiole, and stem demonstrated significant differences in producing toxic volatiles through their inhibitory effects on 'S. sclerotiorum' mycelial growth 'in vitro', indicating that GSL contents in brassica species and even cultivars have different potentials for toxic products. Tolerance of 'S. sclerotiorum' to toxic volatiles derived from mustard powder and also synthetic isothiocyanates developed during repeated exposure of mycelium to these biocidal chemicals.