Adaptation to Brassica host genotypes by a single-spore isolate and population of Plasmodiophora brassicae (clubroot)

Plasmodiophora brassicae, the cause of clubroot of crucifers, is an increasingly important pathogen of canola (Brassica napus) in Alberta, Canada. In response, clubroot-resistant canola genotypes are being deployed to help reduce yield losses. Two experiments were conducted to examine the effect on P. brassicae virulence of repeated exposure of a population and single-spore isolate of the pathogen to the same host. The first experiment examined changes in the index of disease over five cycles of infection on seven Brassica hosts (European Clubroot Differential [ECD] 02, ECD 04, ECD 05, ECD 15, '45H26', '45H29', and 08N823R). The second experiment tested the virulence of five cycled populations ('45H29', 08N823R, ECD 05, and ECD 15) and three cycled single-spore isolates ('45H29', 08N823R, and ECD05) on four resistant canola genotypes ('73-77', '73-67', VT-SD-09, and '9558C'). The results from these experiments clearly demonstrate the ability of both single-spore isolates and populations of P. brassicae to rapidly erode the resistance present in the two canola genotypes, '45H29' and 08N823R. Although the index of disease increased on these two genotypes, the four resistant canola genotypes remained resistant to all the cycled populations and single-spore isolates in the second experiment. These results underscore the importance of crop rotation in the management of clubroot in Alberta. © 2012 The American Phytopathological Society.