Breeding for Clubroot Resistance of Crucifer Crops in Japan

Clubroot disease caused by Plasmodiophora brassicae \iVoronin, is considered one of the most damaging diseases of crucifer crops. In Japan the occurrence of clubroot was first recorded in 1892 on cabbage and turnip, and in 1978 on Chinese cabbage. 71 For a Jong time the infested area was small and severity of clubroot was not high except few fields where traditional turnip cultivars were grown successively. Since about 1965, however, clubroot incidence has been reported in many different regions gro\ving crucifer crops, and the severity and percentage of diseased plants have increased very much. At present, clubroot disease can be found almost all over Japan, especially in the central and northern parts, where clubroot is a serious problem. Intensive crucifer cropping was undoubtedly conducive to a population build-up of the organism in the soil. High concentration of pathogen in soil is apt to make the effect of chemical control insufficient,101 so that crucifer crops a re forced to be replaced by other crops. Clubroot is a constant threat to crucifer crops, especially to cabbage, turnip and Chinese cabbage. Except chemical control, cultivation of resistant cultivars is one of the safest and cheapest methods to combat with clubroot. A breeding work for clubroot resistance was started first on cabbage under a practicable plan at Vegetable and Ornamental Crops Research Station in 197,.1. At present, breeding works for clubroot resistance