Field trials were carried out to test the effect of phosphonate fungicide (AG3) on the severity of clubroot of bok choy (Brassica rapa var. chinensis) and cabbage (B. rapa var. perkinensis and B. oleracea var. capitata) in commercial Ontario muck fields with a clubroot history. Disease severity also was examined in the same infested soil under greenhouse and microplot conditions. In microplot trials with bok choy, AG3 phosphonate concentrations of 0.07 and 0.14% a.i. applied before or after planting consistently reduced clubroot severity (1-to-4 rating) by 0.8 to 1.6 when planted in May or June. However, only the 0.14% a.i. preplanting treatment was effective in trials in July and August. Postplanting drenches of 0.14% a.i. were consistently effective throughout the season. Fresh weight of bok choy was increased or not affected by phosphonate treatments. Under field conditions, one (0.07, 0.14, and 0.21% a.i.) or two (0.07% a.i.) postplant-ing drench applications of phosphonate significantly reduced the incidence of clubroot by 52 to 87% and severity by 1.7 to 2.5 on bok choy in 2004 but not in 2005. In the 2004 trial, two applications of 0.07% a.i. AG3 phosphonate reduced the severity of clubroot comparably to single applications at 0.14 and 0.21% a.i. rates. Fresh weight of bok choy was increased by 34 to 86% with all phosphonate drench treatments in both years. With cabbage, AG3 postplanting drench treatments consistently reduced the severity of clubroot (1-to-5 rating) by a range of 0.7 to 3.3 under greenhouse, microplot, and field conditions. In the greenhouse, a single drench application of 0.07 and 0.14% a.i. AG3 phosphonate 1 day after transplanting cabbage seedlings to the infested muck soil reduced clubroot severity by 2.6 to 3.3 and increased fresh weight of cabbage tops by 66 to 69%. Similar results were seen with cabbage trials under both microplot and field conditions at all AG3 postplanting drench concentrations.