Transfer of resistance to clubroot (Plasmodiophora brassicae) to swedes (Brassica napus L var napobrassica Peterm) from B-rapa

In 1975, tests with UK populations of Plasmodiophora brassicae not only revealed a lack of effective clubroot resistance in swedes (Brassica napus), but also the outstanding resistance of the European Clubroot Differential (ECD)04 (B. rapa). It was, therefore, decided to transfer the resistance genes from ECD04 to swedes, using the most pathogenic UK population of clubroot (C56) available for screening purposes. An autotetraploid form of ECD04 was crossed with tetraploid kale (B. oleracea) using the latter as female parent. One of the euploid, 2n = 38, hybrids secured by embryo rescue in 1976 was crossed to the swede cultivars Marian and Ruta Otofte. Three further backcrosses of clubroot resistant plants to lines derived from modem swede cultivars were made over the period 1980 to 1982. Selfing commenced in 1983 to produce F-2 populations. From F-3 to F-5 there was family selection for yield and agronomic characters, as well as single plant selection for clubroot resistance. In 1991, the six most promising F-5 families were multiplied for subsequent evaluation in replicated yield trials in Dundee. The most promising family entered official trials at the beginning of 1993 and, 2 years later, was added to the National List as cv. Invitation and granted Plant Breeders' Rights. The first certified seed was sold in 1996, 20 years after the original synthetic B. napus was produced. The breeding programme provided evidence for only one of the three postulated dominant genes in ECD04 being required for resistance to C56 and also good evidence of differential resistance from tests with other clubroot populations. Hence, whilst the differential resistance in cv. Invitation should prove useful in the UK in the immediate future, it may not be durable in the longer term. It is, therefore, argued that the next and more difficult goal to achieve should be to introduce high levels of non-differential resistance from B. oleracea.