Epsilon Open Archive

Abstract

Plasmodiophora brassicae, causal agent of clubroot disease of crucifers, has tolerant resting spores that permit its survival in the absence of a host plant. The resting spores are expected to germinate when triggered by specific substances excreted from host plant roots, but they also respond to other cues. As the zoospores emerging at germination are sensitive and short-lived, stimulation of resting spore germination is a potential method for management of clubroot disease. Certain non-host plants have been found to increase resting spore germination. The effect of four non-host plants on P. brassicae was studied in laboratory-, greenhouse- and field studies. In the laboratory, root exudate solution of Lolium perenne (Perennial ryegrass) stimulated germination of resting spores more strongly than other non-host plants tested (Allium porrum, leek; Trifolium pratense, red clover; Secale cereale, winter rye) or the host plant Brassica rapa var. pekinensis (Chinese cabbage). When grown in soil, however, no species-specific effect of any of the plants was observed on the persistence of P. brassicae. It has been claimed that resting spore germination increases in response to substances excreted from decaying plant material or to a general increase in soil biological activity. However, no change in pathogen persistence was observed when infested soil was treated with plant material or with a combination of grass material and earthworms. A bioassay made it possible to correlate disease severity with concentration of resting spores in soil. This analysis provides a coarse estimation of resting spore concentration, and smaller changes in spore load may not have been detected. Nevertheless, none of the treatments showed an influence that would render them useful in dealing with heavily P. brassicae-infested soils within short time periods. The studies emphasise the need for methods for direct and precise quantification of P. brassicae in soil. The gaps in our understanding of the life cycle of the pathogen, including the factors that determine its survival, dispersal and germination are important issues that need to be addressed before we can adequately develop management strategies to control P. brassicae.