A sensitive bioassay for detecting viable Plasmodiophora brassicae in canola field soils

We describe a biodegradable-cup bioassay for detecting viable Plasmodiophora brassicae in soil samples. Soil samples either artificially inoculated with P. brassicae resting spores or collected from canola fields were aliquoted into biodegradable cups containing 20 g of soil per cup. Two cups representing the same soil sample or inoculum concentration were placed in each pot filled with Sunshine Mix. Six seeds of the canola cultivar Westar were sown into each cup and thinned to four seedlings per cup ten days after planting. After four weeks, roots were examined for the presence of clubroot galls. Across three independent inoculated-soil experiments, galls were observed in samples containing as few as 1 resting spore g−1 soil. In contrast, under qPCR assays evaluated in parallel, consistent amplification across three technical replicates was obtained only at 100 resting spores g−1 soil or greater. In field samples, the bioassay produced galls from 11 qPCR-positive samples and seven of ten qPCR-negative samples. Although the bioassay is not intended for rapid diagnosis or direct quantification, it provides a practical tool for annual clubroot surveys and for studies requiring recovery, propagation or characterization of viable P. brassicae from soil samples collected across diverse geographic regions.