Infection and development of<i>Plasmodiophora brassicae</i>in resistant and susceptible canola cultivars

Abstract Commercial cultivars of canola with resistance to clubroot (Plasmodiophora brassicae) have been registered recently in Canada. However, little is known about how and when resistance is expressed. Time series assessments of root hair infection and cortical infection were made in four cultivars that differed in reaction to two pathotypes, P3 and P6 (Williams' system), of P. brassicae. The cultivars were ‘45H29’ (resistant), ‘InVigor 5030’ (moderately resistant), ‘46A76’ (susceptible) and ‘45H21’ (susceptible to P3, resistant to P6). For assessment of root hair infection (RHI), seedlings were harvested at 4, 8 and 12 days after inoculation (DAI). For assessment of cortical colonization (% root area occupied by the pathogen), plants were harvested at 16, 22 and 28 DAI. In compatible interactions (susceptible cultivar × virulent pathotype), RHI developed quickly and was slower in intermediate and incompatible interactions. The maximum RHI for both compatible and incompatible interactions was 65–70%; maximum RHI was slightly but significantly lower (59%) in the intermediate interaction type. At 28 DAI, cortical colonization was high in ‘46A76’ (P3 = 45%, P6 = 35%), intermediate in ‘InVigor 5030’ (P3 = 23%, P6 = 16%), and no colonization (0%) was observed for either pathotype in ‘45H29’. In ‘45H21’, cortical colonization by P3 was high (35%), but inoculation with P6 resulted in no colonization. In the compatible interactions, cortical colonization by P3 was consistently higher than by P6. There were small differences in the pattern of RHI associated with resistance, but resistance was most clearly expressed during secondary infection and development.