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Using induced mutantagenesis, near-isogenic lines of Brassica napus cv Linetta were screened for partial resistance to the pathogen Sclerotinia sclerotiorum. The line HH1 expressed significantly higher levels of resistance than the present commercial oilseed germplasm. To study the mechanisms of resistance a polyclonal antibody based ELISA was developed to detect fungal mycelium and a sensitive oxalic acid assay to quantify the pathotoxin in infected tissue. Partial resistance in HH1 was typified by a distinct reduction in lesion girdling and this was confirmed by the amount of mycelium in inoculated stems of HH1 being much less (and localised closer to the site of inoculation) than in ‘Linetta'. In studies on Brassica species, oilseed rape varieties and near-isogenic lines, a negative correlation was observed between tissue response to the fungus and to the pathogen derived toxin, oxalic acid. HH1 tissue proved highly sensitive to oxalic acid resulting in the rapid production of oxidised polyphenols around the infection site.