Identification of Brassica oleracea germplasm with improved resistance to Verticillium wilt

Two hundred and ninety-nine accessions representing 11 cultivar groups of Brassica oleracea and eight additional accessions of the wild species B. cretica, B. incana , B. insularis and B. villosa were screened for resistance to Verticillium wilt. A disease index (DI) was calculated for each accession, and a correction of the DI was carried out to compensate for a fluctuating infection level between 11 independent trials. A total of 235, or 77% of the accessions tested, had a DIcorr less or equal to the oilseed rape cv. 'Express' (DIcorr = 2.81), the reference cultivar. Only one accession of the wild species, B. incana, showed an enhanced level of resistance (DIcorr = 2.01). Twenty-four accessions, distributed over eight cultivar groups of B. oleracea were selected for subsequent crosses involving B. rapa. Hybrid plants with 14 accessions were resultant and seed was obtained from crosses where the cultivar groups acephala, alboglabra, botrytis, capitata, gemmifera, italica and sabellica were used as female parents. When progeny of the produced resynthesized rapeseed lines were evaluated for Verticillium wilt resistance, three lines showed a significantly lower disease index (P less than or equal to 0.01) compared with the cv. 'Express'. This source of resistance is now being crossed to advanced breeding material of oilseed rape.