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 u sing an F-1-derived doubled haploid (DH) population of 105 lines and 1 38 restriction fragment length polymorphism (RFLP) markers. The host-p athogen interaction phenotype was assessed qualitatively and by severa l quantitative measurements using different environments and plant dev elopmental stages. A single major locus controlling blackleg resistanc e (LEM1) was mapped to linkage group 6 based on qualitative scores of the interaction phenotype on inoculated cotyledons. This resistance lo cus was also identified by interval mapping using quantitative measure ments of the interaction phenotype on cotyledon- and stem-inoculated p lants. Four other genomic regions were significantly associated with q uantitative measurements of resistance on cotyledon and stem, among th em a marker locus interval in linkage group 17 that included a pathoge nesis related gene (PR2). Two genomic regions associated with resistan ce in field-evaluated plants were different from those identified in c otyledon- and stem-evaluated plants. The use of different environments and plant developmental stages for mapping disease resistance loci is discussed.