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The fungus Leptosphaeria maculans causes blackleg disease of canola (Brassica napus). This fungus has a high evolutionary potential, and extensive sowing of a cultivar in a region can lead to blackleg resistance bred into the cultivar becoming ineffective within 3 years. Monitoring of disease severity and virulence of fungal populations across canola-growing regions of Australia in 2011 revealed a high risk of breakdown of resistance of cultivar Hyola50 on the lower Eyre Peninsula, South Australia, but not elsewhere. Cultivar Hyola50 has two resistance genes, based on the pattern of segregation of resistance in the plant and avirulence in the fungus. One of these is Rlm1, a gene that was rendered ineffective previously in commercial crops of canola in Australia and France. Acreage sown to Hyola50-related cultivars decreased dramatically in 2012 after release of a warning to farmers in this region not to sow …