Maturation of resting spores of Plasmodiophora brassicae continues after host cell death

Plasmodiophora brassicae , cause of clubroot on canola ( Brassica napus ) and other Brassicaceae, is an obligate soilborne pathogen that survives in soil as long‐lived resting spores. Factors that affect the maturation of the resting spores are not well understood. To determine if a living host was required for spore maturation, experiments were conducted under controlled conditions and in field trials. Young plants with clubbed roots containing mostly immature spores had the shoots removed, or clubs were harvested at different times. To kill host cells, clubs were frozen or plants were treated with glyphosate. Clubs were then harvested from treated and non‐treated hosts and stored at low temperature (5 or 10 °C) or room temperature (22 °C). Spore concentration and maturation were assessed 2–4 weeks after treatment using propidium monoazide‐assisted quantitative PCR, and results were validated using bioassays. The state of decay of clubs was also observed to determine if it was an indicator of spore maturity. Spores continued to mature after death of host cells or removal of shoots of plants. This demonstrated that a living host was not required for spore maturation. Interestingly, killing host cells prior to storing the clubs at low temperature produced a higher proportion of mature spores and more severe clubroot in bioassays relative to non‐treated clubs held at low temperature. This indicated that death of host cells accelerates spore maturation. Furthermore, mature spores were recovered from both decayed and less decayed clubs, suggesting that club decay is not an indicator of spore maturation.