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Sclerotinia sclerotiorumcauses serious yield losses to many crops worldwide.Aspergillussp. Asp-4, previously shown to inhibit germination of sclerotia ofS. sclerotiorum in vitroand in the field, was evaluated in field trials for suppression of this pathogen on oilseed rape. Spray application of Asp-4 to the soil prior to sowing rice in a rice–oilseed rape rotation resulted in a significant reduction in incidence of Sclerotinia stem rot on oilseed rape compared with the non-treated control in two field trials. This application of Asp-4 also resulted in a significant reduction in germination of sclerotia relative to the non-treated control in these field trials, suggesting that this reduction in sclerotial germination led to disease control. Microscopic examination demonstrated that Asp-4 could effectively colonise external and internal portions of sclerotia ofS. sclerotiorum in vitro. Incubation of Asp-4 with sterile sclerotial material induced production of β-glucanase and chitinase activities by this isolate; β-glucanase and chitinase being potentially capable of degrading the glucan and chitin polymeric components of sclerotia. Incubation of Asp-4 with sterile sclerotial material also resulted in a significant reduction in dry weight of this sclerotial material relative to the non-treated control in 96 hin vitroexperiments. Experiments reported here indicate thatAspergillussp. Asp-4 has promise as a biological control agent forS. sclerotiorumon oilseed rape. Experiments reported here suggest that disease control results from inhibition of germination of sclerotial resting structures due to mycoparasitic colonisation by Asp-4. [ABSTRACT FROM AUTHOR]