Plasmodiophora brassicae – host and environment interactions

Sammanfattning: In this thesis, three separate experiments have been performed on different aspects of the interaction between the causal agent of clubroot, Plasmodiophora brassicae, and its hosts. In the first experiment the pathotype of the P. brassicae single spore isolate, which is currently used to construct a reference genome, has been classified using the ECD bioassay. The disease severity of infected plants was scored according to two different scales and the pathotype was determined according to three previously published guidelines. The results were compared to previous published studies describing the e3 isolate. The life cycle of P. brassicae is not well understood. A small-scale cultivation system was tested to study the early interactions between P. brassicae and host plants on a molecular basis using DNA and RNA analyses from plant roots and rhizosphere. Infection rates were determined by quantification of P. brassicae DNA in root samples. DNA and RNA extractions from plant roots were successful but the system did not provide consistent infection. Disease development rates and gene expression analyses did not generate reliable results. The third experiment was a pilot study to investigate the influence of root exudates from inoculated and non-inoculated plants on P. brassicae resting spore germination behaviour. Root exudates of several host and non-host plants are known to stimulate resting spore germination under experimental conditions but the effect of non-host plants is often diminished under field conditions. It was tested if the infection of the plant roots by P. brassicae could influence the germination of resting spores in the environment. The results indicated that infected roots may increase resting spore germination. The effect seemed to vanish as the P. brassicae infection reached later stages in its lifecycle.