IDENTIFICATION OF QTL INVOLVED IN-FIELD RESISTANCE TO LIGHT LEAF-SPOT(PYRENOPEZIZA-BRASSICAE) AND BLACKLEG RESISTANCE (LEPTOSPHAERIA-MACULANS) IN WINTER RAPESEED (BRASSICA-NAPUS L.)

Quantitative trait loci (QTL), involved in the polygenic field resista nce of rapeseed (Brassica napus L.) to light leaf spot disease, were m apped using 288 DNA markers on 152 doubled-haploid (DH) lines derived from the cross 'Darmor-bzh' x 'Yudal'. Over two years (1995 and 1996), the DH population was evaluated for light leaf spot resistance on lea ves (L) and stems (S), and for blackleg disease resistance in same fie ld trials. For the L resistance criterion, a total of five and seven Q TL were detected in 1995 and in 1996 respectively, accounting for 53% and 57% of the genotypic variation. For the S criterion, three and fiv e QTL were identified in 1995 and in 1996 respectively, explaining 29% and 43% of the genotypic variation. The locations of the QTL detected were quite consistent over the two years (4- and 2-year common QTL fo r L and S, respectively). Three genomic regions, located on the DY5, D Y10 and DY11 groups, were common to the resistance on leaves and stems . In comparison with the QTL for blackleg resistance described by Pile t et al. (1998), two regions on the DY6 and DY10 groups, were associat ed with the two disease resistances. These 'multiple disease resistanc e' ('MDR') QTL may correspond to genes involved in common resistance m echanisms towards the two pathogens or else to clusters of resistance genes.