IDENTIFICATION OF LOCI CONTRIBUTING TO QUANTITATIVE FIELD-RESISTANCE TO BLACKLEG DISEASE, CAUSAL AGENT LEPTOSPHAERIA-MACULANS (DESM.) CES. ET DE NOT., IN WINTER RAPESEED (BRASSICA-NAPUS L.)

Blackleg, caused by Leptosphaeria maculans, is one of the most importa nt diseases of Brassica napus. Genomic regions controlling blackleg re sistance at the adult plant stage were detected using 152 doubled-hapl oid (DH) lines derived from the F-1 'Darmor-bzh' x 'Yudal'. The rapese ed genetic map used includes 288 DNA markers on 19 linkage groups. Bla ckleg resistance of each DH line was evaluated in field tests in 1995 and 1996 by measuring the mean disease index (I) and the percentage of lost plants (P). From notations recovered in 1995, ten quantitative t rait loci (QTL) were detected: seven QTL for I and six QTL for P, expl aining 57% and 41% of the genotypic variation, respectively. Three of them were common to I and P. From data recovered in 1996, seven QTL we re identified: five QTL for I and two different QTL for P, accounting for 50% and 23% of the genotypic variation, respectively. One I QTL, l ocated close to a dwarf gene (bzh), was detected with a very strong ef fect, masking more QTL detection. It was not revealed at the same posi tion and with the same effect in 1995. Four major genomic regions were revealed from 1995 and from 1996 with the same parental contribution. One of them, located on the DY2 group, has a resistance allele from t he susceptible parent. Five- and two-year-specific QTL were detected i n 1995 and 1996, respectively.