The infection by Leptosphaeria maculans of Brassica napus cultivars with major gene resistance derived from Brassica rapa subsp. sylvestris was studied in southeastern Australia. Following the commercial release of these cultivars in Australia in 2000, plants with stem cankers were first reported in 2002 at two geographically isolated regions in South Australia and New South Wales. In 2003, this study showed that the major gene resistance had been overcome in an area of approximately 50,000 ha in South Australia and in two fields in New South Wales (0.5 and 30 ha). There was no relationship between disease severity and incidence in 2003 and the proximity to the sites where resistance breakdown occurred in 2002. At some locations, the frequency of isolates able to overcome the B. rapa subsp. sylvestris-derived resistance had increased between 2002 and 2003. Isolates cultured from canola cultivars with either B. rapa subsp. sylvestris-derived resistance or polygenic resistance showed host specificity when inoculated onto cultivars with B. rapa subsp. sylvestris-derived or polygenic resistance, respectively. The most likely cause of the resistance breakdown was the rapid increase in frequency of L. maculans isolates virulent on this particular resistance source. The selection pressure leading to this increased frequency was probably mediated by the planting of cultivars harboring the major resistance gene in the same locations for a 3-year period, and the ability of the pathogen to produce large numbers of asexual and sexual spores.
Keywords: oilseed rape.