Effect of clubroot (Plasmodiophora brassicae) on yield of canola (Brassica napus)

Abstract

Clubroot, caused by Plasmodiophora brassicae, is a major disease of canola (Brassica napus). This study evaluated the effect of clubroot development on the yield of this crop under field and greenhouse conditions. Three canola cultivars, ‘45H31’ (susceptible), ‘45H29’ (1st-generation resistance) and ‘CS2000’ (2nd-generation resistance), were inoculated with different quantities of P. brassicae inoculum and monitored for yield, pods per plant and 1000-grain weight. The field experiment was conducted over 2 years in biosecure clubroot nurseries inoculated with pathotype 5X or a mix of pathotypes 5X and 3H at 5 × 104, 5 × 106 and 5 × 108 resting spores per plant. In the greenhouse experiment, plants were inoculated with pathotypes 5X, 3H or a mixture of both at 1 × 103, 1 × 104 and 1 × 106 resting spores per plant. In both the field and greenhouse, clubroot incidence and disease severity index (DSI) increased along with inoculum concentration; the highest levels of disease were observed in the susceptible cultivar, while the lowest levels were found in ‘45H29.’ Yield, pods per plant and 1000-grain weight decreased as the DSI increased in all cultivars. Yield was affected by DSI and cultivar but not pathotype. Regression analysis indicated that under greenhouse conditions, an increment of 1% in DSI resulted in a decrease of 0.49% in yield; under field conditions, this percentage was reduced to 0.26%. While the rate of yield reduction was similar among cultivars, overall yield loss was lower in the resistant hosts, as they developed less disease.