The canola industry in Australia is concerned about a new strain of Leptosphaeria maculans that breaks down single dominant gene-based resistance in Brassica napus derived from Brassica rapa ssp. sylvestris. This study reports the results of an investigation aimed at reducing the damage caused by a new virulent race. Glasshouse studies were undertaken to investigate the outcome of co-inoculation of an avirulent (UWA P11) and a virulent (UWA 192) strain of L. maculans and the potential for these two strains to interact on the cultivar Surpass 400, a cultivar containing single dominant gene-based resistance derived from B. rapa ssp. sylvestris. On its own, the avirulent strain induced a hypersensitive reaction on cotyledons of cv. Surpass 400 while the virulent strain induced a fully susceptible reaction. However, when applied to cv. Surpass 400 in mixtures, the strains gave only a hypersensitive response, even when the avirulent strain was as low as 10% of the volume. The effect of the avirulent strain was localised and there was no evidence of a systemic effect between cotyledons or between separate point inoculations on one cotyledon. The results from this investigation may explain the occurrence of crops of cultivars containing single dominant gene-based resistance derived from B. rapa ssp. sylvestris that escape major collapse from the disease in the presence of virulent strains. In cropping areas where populations of virulent L. maculans represent a significant disease risk, exposure of cultivars with single dominant gene-based resistance to a source of canola residues colonised by an avirulent L. maculans might prove an effective means of reducing the risk. (c) 2005 Elsevier B.V. All rights reserved.