Abstract/Summary

ABSTRACT Plasmodiophora brassicae Wor., is viewed here from the stand point of being a highly evolved and successful organism, well fitted for the ecological niche that it occupies. Physical, chemical and biological components of the soil environment are discussed in relation to their effects on the survival, growth and reproduction of this microbe. It is evident that P. brassicae is well equipped by virtue of its robust resting spores for survival through many seasonal cycles. Germination is probably triggered as a result of signals initiated by root exudates. The resultant motile zoospore moves rapidly to the root hair surface and penetration and colonisation follow. The short period between germination and penetration is one of greatest vulnerability for P. brassicae. In this phase survival is affected at the very least by:- soil texture and structure, its moisture, pH, calcium, boron, nitrogen content and the presence of active microbial anatagonists. These factors influence the inoculum potential (sensu Garrett, 1956) and its viability and invasive capacity. There is evidence that these effects may also influence differentially the survival of some physiological races of P. brassicae. Considering the interaction of P. brassicae with the soil environment from the perspective of its biological fitness is an unusual approach, most authors consider only the opportunites to destroy this organism. The approach adopted here is borne of several decades spent studying P. brassicae and the respect that has engendered for it as a biological entity. This review stops at the point of penetration, although some of the implications of the environment for successful colonisation are included since they form a continuum. Interactions with the molecular and biochemical cellular environment are considered in other chapters in this Special Edition.