APPLICATION OF DOUBLED HAPLOID TECHNOLOGY AND DNA MARKERS IN BREEDING FOR CLUBROOT RESISTANCE IN BRASSlCA OLERACEA
DAC PINK and GJ KING
Horticulture Research International, Wellesboume, Warwick, CV35 9EF, UKBackground and objectives
Clubroot (Plasmodiophora brassicae) is an important disease of brassicas. It is characterised by an undifferentiated proliferation of root tissue to form galls (clubs). These reduce the efficiency of the root system so that infected plants will more lodge more readily. Severely affected plants may be stunted resulting in loss of yield and marketability. Soft-rot bacteria and other organisms may infect galls, resulting in a further reduction of root efficiency . The disease is difficult to control because of the longevity of the resting spores in the soil. Soil sterilisation can eradicate these but is prohibitively expensive on a field scale. Traditionally the disease has been controlled on a field scale by the application of lime to raise soil pH but high application rates are often necessary. Clubroot resistant cultivars are therefore an attractive means of control.
Host resistance to P. brassicaeis available in B. oleracea. With the exception of resistance in the cabbage cv. Badger Shipper controlled by a single dominant gene, resistance appears to be a recessive quantitative character . Sources of resistance to clubroot are found amongst agronomically poor cabbages and kales. Attempts to utilise these in backcross breeding programmes have been largely unsuccessful and relatively few "clubroot resistant' cultivars exist. The use of novel breeding techniques enables more effective and efficient breeding for clubroot resistance. Some of the difficulties of assessing a quantitative recessive character can be overcome by the production of homozygous doubled haploid (DH) lines. The use of molecular markers linked to resistance can also improve the effectiveness of a selection programme since they are not subject to environmental influences.
Materials and methods