Nitrogen levels can modulate the effectiveness of clubroot resistance in an isolate- and host-specific manner. While the same QTL were detected under high and low nitrogen, their effects were altered. Clubroot, caused by Plasmodiophora brassicae, is one of the most damaging diseases of oilseed rape and is known to be affected by nitrogen fertilization. However, the genetic factors involved in clubroot resistance have not been characterized under nitrogen-limiting conditions. This study aimed to assess the variability of clubroot resistance under different nitrogen levels and to characterize the impact of nitrogen supply on genetic resistance factors. Linkage analyses and a genome-wide association study were conducted to detect QTL for clubroot resistance and evaluate their sensitivity to nitrogen. The clubroot response of a set of 92 diverse oilseed rape accessions and 108 lines derived from a cross between 'Darmor-bzh' (resistant) and 'Yudal' (susceptible) was studied in the greenhouse under high- and low-nitrogen conditions, following inoculation with the P. brassicae isolates eH and K92-16. Resistance to each isolate was controlled by a major QTL and a few small-effects QTL. While the same QTL were detected under both high and low nitrogen, their effects were altered. Clubroot resistance to isolate eH, but not K92-16, was greater under a low-N supply versus a high-N supply. New sources of resistance were found among the oilseed rape accessions under both low and high-N conditions. The results are discussed relative to the literature and from a crop improvement perspective.

Clubroot, caused by Plasmodiophora brassicae, is a worldwide disease affecting Brassica. Until now, the detection of genetic factors (QTLs) implicated in clubroot resistance has been based on estimates of disease index. However, as the amount of resting spores released in soil after club disintegration influences clubroot epidemics and resistance‐breaking dynamics, its genetic control may deserve specific attention. In a previous report, it was shown that nitrogen fertilization modulated quantitative partial resistance toward clubroot symptom development in rapeseed. The present work aimed to identify genetic factors involved in the control of resting spore production and to assess their regulation by nitrogen supply. A flow cytometer method was adapted for rapidly estimating resting spore content in a large series of samples. Linkage analysis was conducted to detect QTLs implicated in resting spore production in a Brassica napus doubled haploid progeny from the cross Darmor‐bzh × Yudal. DH lines inoculated with the P. brassicae isolate eH were grown under low‐ and high‐nitrogen supply. Under low‐nitrogen conditions, resting spore production was reduced compared to high‐nitrogen conditions, regardless of genotypes. Genetic architecture controlling resting spore production and clubroot symptom development was similar. Under high‐nitrogen conditions, resting spore production was controlled by one major QTL (C09a) and a few small‐effect QTLs. By contrast, two major QTLs (C02 and C09a) controlled resting spore production under low‐nitrogen conditions. This work highlighted a large see‐saw effect between the relative contribution of the C09a QTL (high effect under high‐nitrogen conditions) and the C02 QTL (high effect under low‐nitrogen conditions), with possible implications in resistance breeding.