A >2-year crop rotation reduces resting spores of Plasmodiophora brassicae in soil and the impact of clubroot on canola

• We show, for the first time, that most P. brassica resting spores disintegrate within 2 years.
• The low number of spores change little between 2 and 4 years, and may survive longer.
• Reduction of P. brassica inoculum in the soil via crop rotation alleviates clubroot impact.
• A >2-year crop rotation also reduces canola yield losses under heavy clubroot infestation.

Resting spores of Plasmodiophora brassicae (clubroot) can survive for many years in soil, so crop rotation has not been considered as a major tool for clubroot management. However, it was not known if 2- to 5-year crop rotations would reduce the pathogen inoculum in soil and alleviate disease impact. Two field trials were conducted at a heavily infested site. In 2012, susceptible, moderately susceptible and resistant canola cultivars were seeded to plots with a 0- to 4-year break from susceptible canola. Resting spores in soil were estimated using a quantitative PCR assay. In the second trial on adjacent plots in 2013, three resistant cultivars were evaluated. A break from susceptible canola ≥2 years resulted in a >90% reduction in resting spores relative to continuous canola or a 1-year break, which, however, failed to reduce the clubroot severity substantially on susceptible or moderately susceptible cultivars. Plants of these cultivars died before maturity in plots of continuous canola or a 1-year break, and were stunted in plots of 2- to 4-year breaks. A ≥2-year break increased the yield of all cultivars relative to continuous canola, but the yield was low (<400 kg ha−1) for susceptible or moderately susceptible cultivars with all rotation intervals. The yield for resistant cultivar was >1230 kg ha−1 in continuous canola, and a 2- to 4-year break increased the yield by 32–76%. It was showed here that, for the first time, most P. brassicae resting spores disintegrate within first two years in soil. We also demonstrate that a 2- to 4-year break under heavy infestation alleviate the impact of clubroot on susceptible or moderately susceptible cultivars only marginally, but increase the yield of resistant cultivars substantially. Therefore, a >2-year break from canola is recommended with use of resistant cultivars for clubroot management on canola.