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Blackleg, caused by Leptosphaeria maculans (Desm.) Ces. et de Not., is a major disease of oilseed rape (Brassica napus L.) worldwide. Molecular markers would be useful tools to assist breeding for blackleg resistance. The objective of this study was (i) to map and characterize quantitative trait loci (QTL) for field blackleg resistance in doubled haploid (DH) and F2:3 populations from the cross ‘Darmor’ (resistant) × ‘Samourai’ (susceptible) and (ii) to compare QTL with those previously identified in the cross ‘Darmor‐bzh’ × ‘Yudal’. A total of 134 DH lines and 185 F2:3 families were genotyped with random amplified polymorphic DNA (RAPD) and restriction fragment length polymorphism (RFLP) markers and assessed for a disease index of resistance in 1998 and/or 1999 in one location. Genetic maps derived from the two populations included a total of 257 and 81 markers, respectively. Up to 30% of these markers were common to the Darmor‐bzh × Yudal map previously used. A total of six and four genomic regions were associated with resistance in the DH and F2:3 populations, respectively. They collectively explained 36 to 42% of the variation within each year and population. Three of them were consistent across the two populations derived from Darmor × Samourai cross and expressed dominant or overdominant effects. Four favorable alleles were derived from the susceptible parent. A total of 16 genomic regions were revealed for blackleg resistance in the two crosses Darmor‐bzh × Yudal and Darmor × Samourai studied. Four of them were consistent over the two crosses. The inconsistencies observed between populations and crosses can be explained by different genetic backgrounds and disease infestation levels. For marker‐assisted selection, these results suggest that QTL mapping must be carried out separately for each population.

No abstract

Specificity of interaction at the cotyledon stage was recently demonstrated between the blackleg pathogen, Leptosphaeria maculans, and Brassica napus. Three pathogenicity groups were distinguished, PG2 avirulent towards 'Quinta' and 'Glacier', PG3 avirulent towards 'Quinta', and PG4 virulent on the two cultivars. The genetic control of the interactions was investigated on both the pathogen and the plant. Tetrad analysis was performed following PG3 x PG4 and PG2 x PG4 crosses. 'Quinta' and 'Glacier' were crossed with the susceptible winter oilseed rape cultivar 'Score'. The analysis of F,, Fj and testcross populations suggested that the incompatible interaction between 'Quinta' and PG3 isolates is conditioned by the presence of the dominant single resistance allele Rlml in 'Quinta' and the matching avirulence gene AvrLml in L. maculans. Race-specific resistance of 'Glacier' to PG2 isolates was conditioned by the matching gene pair Rlm2/AvrLm2. Finally, the data suggest that two avirulence genes matching two dominant loci control the 'Quinta'-PG2 interaction. The consequences of the occurrence of race-specific resistance in B. napus are discussed with respect to future breeding for blackleg resistance.