RFLP mapping of resistance to the blackleg disease [causal agent, Leptosphaeria maculans (Desm) Ces, et de Not] in canola (Brassica napus L)

We report the tagging of genes involved in blackleg resistance, present in the French cultivar Cresor of B. napus, with RFLP markers. A total of 218 cDNA probes were tested on the parental cultivars Cresor (resist ant) and Westar (susceptible), and 141 polymorphic markers were used in a segregating population composed of 98 doubled-haploid lines (DH). A genetic map from this cross was constructed with 175 RFLP markers and allowed us to scan for specific chromosomal associations between response to blackleg infection and RFLP markers. Canola residues infested with virulent strains of Leptosphaeria maculans were used as inoculum and a suspension of pycnidiospores from cultures of L. maculans, including the highly virulent isolate Leroy, was sprayed to increase disease pressure. QTL mapping suggested that a single chromosomal region was responsible for resistance in each of the four environments tested. This QTL accounted for a high proportion of the variation of blackleg reaction in each of the assays. A second QTL, responsible for a small proportion of the variation of blackleg reaction, was present in one of four year-site assays. A Mendelian approach, using blackleg disease ratings for classifying DH lines as resistant or susceptible, also allowed us to map resistance in the region of the highly significant LOD scores observed in each environment by interval mapping. Results strongly support the presence of a single major gene, named LmFr(1), controlling adult plant resistance to blackleg in spring oilseed rape cultivar Cresor. Several RFLP markers were found associated with LmFr(1).