Abstract
Single spore isolates of a downy mildew fungus (Peronospora parasitica) on Chinese cabbage (Brassica campestris (pekinensis group)), turnip (B. campestris (rapifera group)) and Japanese radish (Raphanus sativus (daikon group)) plants were obtained from naturally diseased plants in fields. Several single spore isolates from each sample were prepared and the host range of fifty single spore isolates in total was examined. Test plants for inoculations were as follows: twenty-one cultivars of Brassica campestris, one cultivar of B. juncea, five cultivars of B. oleracea, one cultivar of B. napus and two cultivars of Raphanus sativus. In case of inoculations with most of the single spore isolates of P. parasitica from Chinese cabbage and turnip, most of the B. campestris, B. napus cultivars and some B. oleracea cultivars were susceptible, but B. juncea plant was tolerant, although host reaction varied. R. sativus cultivars were resistant to all of the isolates. In case of inoculations with P. parasitica from R. sativus, most of the R. sativus, B. oleracea and B. napus cultivars were susceptible and their reactions varied. Most of B. campestris and B. juncea cultivars were resistant to all the isolates. Therefore, populations of downy mildew fungus occurring on B. campestris and R. sativus plants in fields corresponds to complementary sets of isolates with variable parasitisms, suggesting that physiologic races may exist in downy mildew of B. campestris.
