The biology of Plasmodiophora brassicae Wor. -: A review of recent advances

This review highlights salient research concerned with Plasmodiophora brassicae Wor., the causal agent of clubroot disease in members of the Brassicaceae over the past 20 years. Key features to emerge in this period include: the development of a greater understanding of pathogen virulence and host resistance factors and the biology of host-pathogen relationships. The latter is particularly aimed at improving our understanding of the mechanisms of symptom expression and the ways by which host metabolism is subverted by P. brassicae. In both areas of study the use of molecular based technology has both aided investigations and provided background knowledge with which to interpret findings. Studies of the ecological relationships of the organism and its environment have also yielded much improved understanding of biotic and abiotic interactions. It is accepted that soil suppressiveness derived from the presence of antagonistic microbes can diminish the impact of P. brassicae on its host. Similarly the effects of calcium, boron, nitrogen and alkaline pH values affect resting spore germination, the motility of primary zoospores and reproductive efficiency of primary plasmodia in the root hair. The presence of calcium in the soil may also affect the expression of resistance factors by the host. Movement within the host from primary root hair infection to secondary cortical infection may be aided by the formation of amoeboid stages in the life cycle. The technical level means for isolating and studying single spores have become more common and use continues to be made of the European Differential Series (ECD) with which to classify pathotypes. Finally, the taxonomy of this microbe remains unclear and enigmatic. Progress in understanding its biology is limited by an inability to identify its affinities.