Effects of root exudates and pH on Plasmodiophora brassicae resting spore germination and infection of canola (Brassica napus L.) root hairs

Clubroot, caused by Plasmodiophora brassicae, is emerging as a serious threat to canola production on the Canadian Prairies. Germination of soil-borne resting spores (RSs) and their subsequent infection of canola root hairs are important requirements for the establishment of the clubroot pathogen in canola. Stimulation of RS germination by certain non-host plant species and elevation of soil pH by application of calcium-rich compost may provide some level of clubroot control in the field. Experiments were conducted under controlled conditions to determine the effects of (i) host and non-host root exudates (REs) and different pH values on the germination of RSs, and (ii) different pH values either alone or in conjunction with a Ca-containing nutrient solution (NS) on root hair infection of a susceptible canola cultivar. The percentage of RS germination was significantly enhanced when incubated with REs prepared in NS from three plant species compared to a NS alone or sterile dH2O. Non-host REs from perennial ryegrass (Lolium perenne L.) produced higher spore germination than exudate preparations from canola (Brassica napus L.) and Chinese cabbage (Brassica rapa var. pekinensis, cv. Granaat) roots. The percentages of RS germination were higher at pH values ranging from 5.0 to <7.0, and lower at pH values >7.0 in both the presence and absence of REs. The effects of various pH values on the percent of root hair infection varied. Root hair infection was higher at relatively lower pH values (6.0–7.0) compared to either lower or higher pH values when a NS was added. It was however lower at pH values below 6.5 compared to those above 6.5 when the experiment was conducted in the absence of a NS. The possible impacts of REs, different pH values, and calcium levels in the soil on the severity of clubroot on canola are discussed.

► Root exudates improved germination of Plasmodiophora brassicae resting spores.
► Host exudates resulted in lower spore germination than perennial ryegrass.
► Resting spore germination was lower above pH 7 compared to pH 5.0–7.0.
► Root hair infection was lower in soils below pH 6.5 than in more alkaline soils.