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Breakdown of Rlm3 resistance in the Brassica napus-Leptosphaeria maculans pathosystem in western Canada

Abstract : Blackleg disease, caused by the fungal pathogen Leptosphaeria maculans, is a serious disease of Brassica napus. The disease is mainly controlled by genetic resistance and crop rotation. However, L. maculans has displayed a high evolutionary potential to overcome major resistance genes in B. napus. This study aimed to analyze the major-gene and adult-plant resistance (APR) of Canadian B. napus varieties/lines (accessions) and the avirulence allele frequency in L. maculans populations in western Canada. For resistance identification, a set of L. maculans isolates with known avirulence genes were used to characterize major resistance (R) genes in 104 Canadian B. napus accessions and 102 seed samples collected from growers' fields; with 104 B. napus accessions further evaluated for APR under controlled conditions. In addition, avirulence genes of 300 L. maculans isolates collected from infected canola stubbles in growers' fields were determined by cotyledon inoculation and gene-specific PCR assays. The results indicated that R genes were present in the majority of these B. napus accessions, with the Rlm3 gene being predominant while other R genes were rarely detected. APR was identified in more than 50 % of the accessions. Predominance of Rlm3 in 102 seed samples from growers' fields suggested Rlm3-carrying B. napus varieties were currently widely used in western Canada. Avirulence allele frequency identification of field L. maculans isolates revealed the scarcity of the avirulence allele towards Rlm3, AvrLm3. This indicated the breakdown of Rlm3 resistance, which could be due to the over use of this single resistance gene in Canadian B. napus germplasm.
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Submitted on : Wednesday, February 8, 2017 - 8:46:37 PM
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Xuehua Zhang, Gary Peng, H. Randy Kutcher, Marie-Helene Balesdent, Regine Delourme, et al.. Breakdown of Rlm3 resistance in the Brassica napus-Leptosphaeria maculans pathosystem in western Canada. European Journal of Plant Pathology, Springer Verlag, 2016, 145 (3), pp.659 - 674. ⟨10.1007/s10658-015-0819-0⟩. ⟨hal-01462748⟩



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