The host ran-e of eighty isolates of Leptosphaeria maculans obtained from various cruciferous hosts in Western Australia was tested by inoculating onto cotyledons of 13 cruciferous hosts, including six Brassira species and one interspecific line of Brassica. These field isolates produced highly variable responses across the hosts tested. One or more isolates overcame the resistance in each of the tested Brassica species with the B-genome. such as B. juncea, B. nigra and B. carinata and of other species tested, including Raphanus raphanistrum. Crambe abyssinica, Sinapis alba, Eruca vesicaria and Raphanus sativus. None of the isolates tested to date were virulent on Camelina sativa. Of all isolates, 48.8% (and 75%, of isolates taken from cultivars containing single dominant gene-based resistance derived from B. rapa ssp. sylvestris) showed high virulence against canola cultivars containing this single dominant gene-based resistance. There was a relationship between host source of isolates with virulence. Results clearly illustrated that extensive levels of variation exist within L maculans populations in Western Australia such that major gene-based resistance in a range of these hosts could be rapidly overcome by one or more field populations of the pathogen. It is noteworthy that this ability to overcome such resistance in various cruciferous hosts was found to be present in certain isolates in the L. maculans population even prior to exposure of the pathogen to such major gene-based resistance sources in commercial crops. Our results indicated that seeking major gene-based resistance from within cruciferae for B. napus canola breeding, including from taxa which to date have shown resistance to L maculans when used in the field as rotation, industrial oil or green manure crops. may lead to both breakdown and wastage of these valuable sources of resistance to blackleg disease. (C) 2004 Elsevier B.V. All rights reserved.