Clubroot development and quantification of Plasmodiophora brassicae DNA in canola root tissue following treatment with lime products

Non-Peer Reviewed

Clubroot (Plasmodiophora brassicae Wor.,) is a soil-borne disease that has become a constraint to canola (Brassica napus L.) production in Alberta, Canada. The disease is managed primarily by the planting of clubroot resistant cultivars, but new P. brassicae pathotypes have emerged that can overcome this resistance. Therefore, the need for other effective clubroot management strategies is imperative. The application of lime creates a less favorable environment for clubroot development by increasing soil pH and calcium levels. A greenhouse experiment was conducted to evaluate the effects of hydrated lime, limestone and different rates of each product on P. brassicae infection and clubroot severity (index of disease) in susceptible and moderately resistant canola cultivars grown in a combination of potting mix and peat moss with varying inoculum levels. Eight weeks after inoculation, indices of disease of 92-100% and 9-13%, respectively, were observed in the susceptible and resistant controls (no lime) treatments. The index of disease decreased to 0% in both the susceptible and resistant cultivars following treatment with any of the four tested rates of hydrated lime. In contrast, the application of limestone resulted in a modest decrease in clubroot severity and only at the two lowest inoculum levels evaluated. Ten-day old canola root tissue was subjected to conventional and quantitative PCR analysis to detect and measure the amount of P. brassicae DNA under the various treatment regimes. When seedlings were exposed to a low level of inoculum, no P. brassicae DNA could be detected in the roots at any of the four rates of hydrated lime evaluated. When seedlings were exposed to a high level of inoculum, P. brassicae DNA could be detected at all but the highest rate of hydrated lime. There were no clear trends in pathogen DNA content in the roots following the application of limestone. The results highlight the need for careful selection of lime products and rates for clubroot management in canola.