Effect of environmental parameters on clubroot development and the risk of pathogen spread

Clubroot (Plasmodiophora brassicae) was initially discovered on canola (Brassica napus) in western Canada near Edmonton, Alberta in 2003. Since then, the disease has spread rapidly but is still most common and most destructive on the heavy, slightly acidic soils in the region where the first infested fields were identified. However, there was (and is) concern that the pathogen might continue to spread and become a constraint to canola production across the Canadian prairies. To assess the risk of continued spread, the effect of factors such as temperature and soil type, pH and micronutrients on clubroot development were examined. Temperatures below 17 °C were shown to slow or inhibit the development of P. brassicae at all stages of its life cycle. Alkaline pH also reduced infection and symptom development in both controlled environment and field situations, but alkaline pH did not eliminate clubroot when other conditions were conducive for infection. Differences in the concentration of boron and other micronutrients in soil were shown to affect clubroot development on canola, but the effect of these differences was unlikely to limit the development of clubroot in situations where inoculum concentration was high. Soil moisture, especially in the rhizosphere during primary and secondary infection, had an important impact on clubroot development, but is a difficult factor for most growers to manipulate. Soil type had a small effect on clubroot severity in controlled environment studies, but a strong interaction of soil type with soil moisture is extremely likely under field conditions. Assessment of the interaction of environmental factors affecting infection success with the mechanisms of pathogen dispersal indicated that clubroot on canola has the potential to spread across large portions of the Canadian prairies.