Broadening the genetic basis of Verticillium longisporum resistance in Brassica napus by interspecific hybridization

Verticillium wilt caused by the vascular fungal pathogen Verticillium longisporum is one of the most important pathogens of oilseed rape (Brassica napus sp. oleifera) in northern Europe. Because production of this major oilseed crop is expanding rapidly and no approved fungicides are available for V longisporum, long-term control of the disease can only be achieved with cultivars carrying effective quantitative resistance. However, very little resistance to V longisporum is available within the gene pool of oilseed rape, meaning that interspecific gene transfer from related species is the only possibility for broadening levels of resistance in current varieties. The amphidiploid species B. napus can be resyn-thesized by crossing the two progenitor species Brassica oleracea and Brassica rapa, hence resistant accessions of these two diploid species can be used as resistance donors. In this study a total of 43 potential B. rapa and B. oleracea resistance donors were tested with regard to their reaction to a mixture of two aggressive V longisporum isolates, and resistances from diverse lines were combined by embryo rescue-assisted interspecific hybridization in resynthesized rapeseed lines. Progenies from crosses of the two B. rapa gene bank accessions 13444 and 56515 to the B. oleracea gene bank accessions BRA 1008, CGN 14044, 8207, BRA 1398, and 7518 showed a broad spectrum of resistance in pathogenicity tests. Of 45 tested resynthesized lines, 41 lines exhibited a significantly higher level of resistance than the moderately V longisporum-tolerant oilseed rape cultivar Express. These lines represent a promising basis for the combination of different resistance resources in new varieties.