In 1999, reports of spring rapeseed plants (Brassica napus L.) exhibiting wilt symptoms were received by agricultural extension personnel from farmers near Fort Vermillion and Andrew, Alberta, Canada. Fungal colonies recovered from affected plants after surface disinfection were identified as Fusarium oxysporum by comparison of morphology on carnation leaf and potato dextrose agars with literature descriptions and reference cultures. Root-dip inoculation of young rapeseed plants with spore suspensions prepared from recovered F. oxysporum colonies resulted in rapid development of symptoms seen in the field. An initial estimate of yield loss in an affected field near Andrew was performed by removing all rapeseed plants from three 1 m2 quadrats. Each plant was evaluated according to a simple three point severity scale, and then the seed from each plant was individually threshed and weighed. Fully- and partially-wilted plants yielded 0.2 and 19.3% of asymptomatic plants, respectively. In 2000, wilt symptoms were observed at a plot research site near Ranfurly, Alberta. Disease symptoms were restricted to one B. napus cultivar, Nexera 705. A similar procedure to that used at Andrew in 1999 was applied at Ranfurly, except quadrat size was 2.5 m2, and replicated comparisons were made between Nexera 705 and an unaffected cultivar, Quantum. The average number of unaffected Quantum plants was 99.4%, while only 66.9% of Nexera 705 plants were asymptomatic. No Quantum plants were severely wilted, while 11.7% of Nexera 705 plants were wilted. Yield of Nexera 705 was 38.6% of Quantum. In 2004, the impact of fusarium wilt on yield of 6 susceptible and 9 resistant B. napus cultivars was determined at nine locations in western Canada. Cultivars were selected on the basis of survey results and agronomist's reports. Across all sites, yield of the most severely affected cultivar, 45A55, was 15.9% lower than the least severely affected cultivar 3455. At the most severely affected site, yield of the most severely affected cultivar, Bianca II, was 75.2% lower than the least severely affected cultivar, Option 501. Yield of wilt-susceptible (Canterra 1604) and resistant (Cougar CL) rapeseed was compared at a replicated large-plot (576 m2 per plot) experiment at Lavoy, Alberta in 2005. The susceptible variety was severely affected and yielded 44% of Cougar CL.