Abstract
Genetic resistance is a successful strategy for management of clubroot (Plasmodiophora brassicae) of brassica crops, but resistance can break down quickly. Identification of novel sources of resistance is especially important when new pathotypes arise. In the current study, the reaction of 177 accessions of Brassica napus to four new, virulent pathotypes of P. brassicae was assessed. Each accession was genotyped using genotyping by sequencing to identify and map novel sources of clubroot resistance using mixed linear model (MLM) analysis. The majority of accessions were highly susceptible (70–100 DSI), but a few accessions exhibited strong resistance (0–20 DSI) to pathotypes 5X (21 accessions), 3A (8), 2B (7), and 3D (15), based on the Canadian Clubroot Differential system. In total, 301,753 SNPs were mapped to 19 chromosomes. Population structure analysis indicated that the 177 accessions belong to seven major populations. SNPs were associated with resistance to each pathotype using MLM. In total, 13 important SNP loci were identified, with 9 SNPs mapped to the A-genome and 4 to the C-genome. The SNPs were associated with resistance to pathotypes 5X (2 SNPs), 3A (4), 2B (5) and 3D (6). A Blast search of 1.6 Mb upstream and downstream from each SNP identified 13 disease-resistance genes or domains. The distance between a SNP locus and the nearest resistance gene ranged from 0.04 to 0.74 Mb. The resistant lines and SNP markers identified in this study can be used to breed for resistance to the most prevalent new pathotypes of P. brassicae in Canada.
Highlights
Genetic resistance is a successful strategy for management of clubroot (Plasmodiophora brassicae) of brassica crops, but resistance can break down quickly
The resistance available in the first generation of CR canola cultivars in Canada has broken down rapidly to reveal the presence of many new pathotypes of P. brassicae[6,7], which complicates breeding for resistance
To producers in Canada because of its simplicity and suitability for the initial situation in Canada[8]. These three systems have recently been replaced in Canada by the Canadian Clubroot Differential (CCD) set[6], which was designed to differentiate among the many new pathotypes recently identified in Canada
Summary
Genetic resistance is a successful strategy for management of clubroot (Plasmodiophora brassicae) of brassica crops, but resistance can break down quickly. To producers in Canada because of its simplicity and suitability for the initial situation in Canada[8] These three systems have recently been replaced in Canada by the Canadian Clubroot Differential (CCD) set[6], which was designed to differentiate among the many new pathotypes recently identified in Canada. A couple of major clubroot resistance genes and some quantitative trait loci (QTL)[11,12,13] have been identified in B. napus (AC genome), but strong efforts have been made to identify novel sources of resistance to clubroot in other Brassica spp. and transfer them into canola. Six major clubroot resistance genes and at least 10 QTL have been mapped to B. oleracea (C genome)[29,30,31,32,33]
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