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Managing small patches of field infestation of Plasmodiophora brassicae (clubroot)

Mary Ruth McDonald: University of Guelph

<div>Clubroot of canola (<em>Brassica napus</em>), caused by <em>Plasmodiophora brassicae</em>, continues to cause crop losses in Canada. Effective methods are needed to reduce the spread of the pathogen within and between fields and to reduce the population of resting spores in soil. A proposed recommendation for treating small areas of infestation is to apply lime, preferably a mixture of quick lime and a standard lime, to increase the soil pH to 7.4 or above, then seed a perennial cover crop. Other methods to reduce resting spore populations are solarisation with or without fumigation. Resting spores were extracted and treated with propidium monoazide prior to qPCR to quantify the number of viable resting spores in the soil profile. Solarization, achieved by covering soil with totally impermeable film for 16 days, increased mean soil temperatures by about 8 <sup>0</sup>C and reduced clubroot severity from 81% to 35% on bioassay crop pak choi <em>(Brassica rapa</em>). Adding the fumigant chloropicrin or metam sodium did not further reduce clubroot severity, but yield increased with decreasing clubroot severity. Trials in controlled environment showed that the grass cover crops perennial ryegrass (<em>Lolium perenne</em>) and smooth brome grass (<em>Bromus inermis</em>) reduced the number of viable resting spores in soil after 8 weeks of crop growth. Soil-core samples should be taken to determine when the population of viable resting spores has dropped below an economic threshold.</div>