Abstract
The role of the soft rot bacterium, Erwinia carotovora subsp. carotovora (Jones) Bergey et al., as a source of inoculum for soft rot of Chinese cabbage was examined by spraying a rifampicin-resistant strain (RRS) on spring and summer-sown Chinese cabbage (cv. Matsushima-kohai W-1116) leaves. The organism was then suspended in sterilized water at a concentration of 106cfu/ml and 2.0ml per plant was sprayed on the leaves at 20, 33, 47 and 60 days after seeding. In addition, 2.0ml/plant of the suspension at 108, 106, 104, 102 or 100cfu/ml were sprayed on the leaves at 45 days after seeding, respectively. The RRS sprayed on leaves of Chinese cabbage survived until harvest, but most of the plants were diseased, at 50-60 days after seeding both in the sprayed and control plants. Disease severity did not differ between the sprayed and control plants in the spring- and the summer-seeded crops except for the middle growing stage of the spring crop. The RRS from soft, rotten petioles and rhizosphere soil of diseased plants were reisolated by dilution plating onto modified Drigalski's medium containing rifampicin (RMDM). The RRS was predominantly reisolated from rotten tissues and rhizospheres of the diseased plants, irrespective of the spraying time and concentration of the used strain. Naturally occurring soft rot bacteria were detected on leaf surfaces of the plant at a population level of 105-106cfu/g of fresh tissue at 76 days after seeding in the spring crop and at 33 days after seeding in the summer crop. Soft rot bacteria artificially sprayed on leaves of Chinese cabbage survived until harvest and played a role as a source of inoculum for soft rot disease of Chinese cabbage in the field.
