Metabolic cost of resistance in clubroot-resistant canola and napa cabbage

Abstract

Genetic resistance is widely used to manage clubroot (Plasmodiophora brassicae) in canola (Brassica napus) and other brassica crops. Replicated field trials were conducted with three clubroot-resistant cultivars and one susceptible cultivar of either canola or napa cabbage (B. rapa) in southern Ontario in 2014 and 2015. These studies indicated that plant growth and development in resistant cultivars of both crops were often reduced at high concentrations of inoculum (resting spores) of the pathogen. In napa cabbage, leaf length was reduced by 31% in 2014 and 22% in 2015 at a site with high spore concentration relative to a site with lower concentration. In canola, height of resistant plants was reduced by 30%, biomass by 43%, and maturity was delayed at a site with high spore concentration relative to an adjacent site with lower spore concentration in 2014. However, there were no differences among sites in 2015. A growth room study demonstrated that plant height of a clubroot-resistant canola cultivar declined by 12–14% at high spore concentrations. Biomass of canola was also lower and maturity was delayed with increasing concentration of spores in some, but not all, field and laboratory studies. We suggest that the expression of resistance to clubroot in resistant cultivars can result in reduced plant growth and development in both B. rapa and B. napus, but that substantial reductions in plant growth and development occur only at high concentrations (> 106 g−1) of resting spores in soil.