Interaction of pH and temperature affect infection and symptom development of Plasmodiophora brassicae in canola

The occurrence of clubroot on canola (Brassica napus), caused by the soilborne protozoan Plasmodiophora brassicae, in western Canada is currently centred in a region of slightly acidic soils near Edmonton, AB. Warm temperatures and slightly acidic conditions are known to favour the development of clubroot. The current study was conducted as part of a larger project to assess the risk that P.brassicae will spread to other areas in the prairie region, e.g., where soil pH is neutral or alkaline. The interaction of temperature (10, 15, 20, 25, 30 degrees C) and pH (5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0) on root hair infection (RHI) and clubroot symptom development in canola roots was studied under optimum moisture conditions and moderate (1-5x10(6) resting spores per seedling) inoculum levels under controlled conditions. The highest levels of RHI (max = 61%) and clubroot incidence and severity (max. = 100%) developed at pH 5.0-6.5x20-25 degrees C. Clubroot levels were intermediate at pH 7.0-8.0x20-25 degrees C, and very low at 10 and 15 degrees C, regardless of pH. Surveys of clubroot-infested canola fields in Alberta demonstrated that there was only a weak correlation between soil pH and clubroot level (r =-0.30 for incidence and r =-0.33 for severity, based on 267 fields). The absence of a strong correlation supports the results of the controlled environment study. We conclude that moderate levels of clubroot can develop at pH levels well above its pH optimum when temperature and moisture are suitable. This may be the underlying cause of failures in clubroot control that occasionally occur in infested vegetable fields treated with lime. This study indicates that there is a substantial risk that moderate levels of clubroot will develop in regions where soils are neutral or slightly alkaline if other conditions (temperature, moisture, inoculum load) are favourable for disease development.