Identification of winter and spring Brassica napus genotypes with partial resistance to Canadian isolates of Plasmodiophora brassicae

Abstract

Clubroot is an important root disease of canola/oilseed rape (Brassica napus L.) caused by Plasmodiophora brassicae. To cope with the emergence of new virulent pathotypes of P. brassicae that threaten canola production in western Canada, there is a need to identify novel sources of resistance for breeding programmes. In the present study, a series of nine B. napus genotypes, from different geographic origins, were screened for resistance to Canadian single-spore and field isolates of P. brassicae. The effect of nitrogen fertilization on the resistance level of these genotypes was also evaluated. Under full nitrogen fertilization, several of the tested host genotypes harboured isolate-specific partial resistance to P. brassicae infection, with some of these interactions resulting in disease indices <50. Accessions showing a partial resistance response to the novel pathotype 5X of P. brassicae also were identified. The influence of nitrogen on the level of resistance varied according to the specific plant genotype/pathogen isolate combination. Under low nitrogen fertilization, the genotype ‘Darmor-bzh’ showed a high level of partial resistance to isolates belonging to pathotypes 2 and 3, and ‘Express’ and ‘Grouse’ showed resistance to pathotype 2. The results confirm that nitrogen fertilization is an important factor to consider in combination with the use of resistant varieties for the management of clubroot.