LAUSR

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 9 Yudal. DH lines inoculated with the P. brassicae isolate eH were grown under lowand 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.