A modified sand–liquid culture method facilitated easy visualisation of the primary life cycle stages of Plasmodiophora brassicae within clean root hairs of the Arabidopsis host. Pathogen penetration occurred from day 4 onwards and then primary plasmodia developed within the host root. Several Arabidopsis ecotypes tested in varying growth conditions showed differences in disease expression. Defined growth cabinet conditions were found most suitable for studying disease progression in the ecotypes and for achieving uniform infection and disease development. Arabidopsis ecotypes Ta-0 and Tsu-0 known to be partially resistant to a German single-spore isolate of P. brassicae were susceptible to an Australian (Victorian) field population of P. brassicae. The European clubroot differential test was used to confirm virulence and describe the pathotype of the Victorian field population. Knowledge of the interaction of an Australian population of P. brassicae with its host will provide valuable information on a disease which is very difficult to control.