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Application of molecular quantification of Plasmodiophora brassicae in soil

Bruce Gossen: Agric & Agri-Food Canada

<div><em>Plasmodiophora brassicae</em> Wor. causes clubroot, a disease with serious impact on canola (<em>Brassica napus</em> L.) and other brassica crops. Its long-lived resting spores and ability to erode genetic resistance make it difficult to manage, especially in an extensive field crop like canola. The resting spores are tiny and nondescript, so counts of spores extracted from soil are highly variable and inaccurate. Spore numbers are often so high in heavily contaminated soil (10<sup>6</sup> to 10<sup>9</sup> spores g<sup>-1</sup>) that assessment of the efficacy of IPM strategies using bioassays is impossible. Estimates of spore concentration based on molecular assessments have recently been enhanced using a competitive internal positive control (qPCR CIPC) and assessment of spore viability using pretreatment with propidium monoazide (qPCR PMA). These techniques have demonstrated that spore numbers decline rapidly in the first 2 years after a susceptible crop, and that high levels of viable spores can be present deep (e.g., 0.5‑1 m) in soil. Field trials in southern Ontario Canada demonstrated that solarisation for 2 weeks can result in a substantial reduction in spore concentration. Also, forage grasses (non-hosts) that form a dense sod can stimulate germination of resting spores and reduce populations in soil. Despite these improvements, issues around collecting and subsampling representative samples are a huge constraint to the adoption of these technologies in risk assessment and disease forecasting.</div>