National Research Institute of Vegetables, Ornamental Plants & Tea Ano, Mie 514-2392, Japan
Clubroot caused by Plasmodiophora brassicae is one of the severe diseases in Brassica crops in many areas in the world. A number of clubroot-resistant (CR) cultivars of Chinese cabbage(Brassica rapa) had been bred using European fodder turnips as sources of the resistant genes, and are now widely cultivated throughout Japan. However, these CR cultivars are already infected in some of the cultivation areas. While the European turnips are still resistant in the areas, where the CR cultivars are damaged. To elucidate the genetic nature of the resistant genes, Homei 09, a Clubroot-susceptible Chinese cabbage and Siloga S2, a CR turnip were crossed. Doubled haploid lines and an F2 population were generated from the hybrids. The doubled haploids were used to score the resistance, while the F2 were used to construct a linkage map. As the results, three RAPD markers, which are closely linked to each other in linkage group 2, were found to be linked to the major CR gene in ,Brassica rapa. The markers are cloned and sequenced. Specific primers for amplification of the markers were synthesized. Using the primers, Chinese cabbage and turnip germplasms are surveyed. At least one of the markers are found to be useful in breeding CR cultivars of Chinese cabbage.