MAPPING LOCI CONTROLLING BRASSICA-NAPUS RESISTANCE TO LEPTOSPHAERIA-MACULANS UNDER DIFFERENT SCREENING CONDITIONS

Leptosphaeria maculans, the causal agent of blackleg of crucifers, is a major threat to rapeseed (Brassica napus) production throughout the world. Genes controlling blackleg resistance in B. napus were mapped using an F-1-derived doubled haploid (DH) population of 105 lines and 138 restriction fragment length polymorphism (RFLP) markers. The host-pathogen interaction phenotype was assessed qualitatively and by several quantitative measurements using different environments and plant developmental stages. A single major locus controlling blackleg resistance (LEM1) was mapped to linkage group 6 based on qualitative scores of the interaction phenotype on inoculated cotyledons. This resistance locus was also identified by interval mapping using quantitative measurements of the interaction phenotype on cotyledon- and stem-inoculated plants. Four other genomic regions were significantly associated with quantitative measurements of resistance on cotyledon and stem, among them a marker locus interval in linkage group 17 that included a pathogenesis related gene (PR2). Two genomic regions associated with resistance in field-evaluated plants were different from those identified in cotyledon- and stem-evaluated plants. The use of different environments and plant developmental stages for mapping disease resistance loci is discussed.