Overwintering of Erwinia carotovora subsp. carotovora in Diseased Tissues in Soil and Its Role as Inoculum for Soft Rot of Chinese Cabbage (Brassica campestris, Pekinensis Group)

To examine whether the pathogenic bacterium, Erwinia carotovora subsp. carotovora, causal agent of soft rot of Chinese cabbage (Brassica campestris L., pekinensis group), can overwinter in plant debris and soil and serve as inoculum the following year, we monitored field populations of rifampicin-resistant, phage-sensitive strains of the bacterium. Chinese cabbage (cv. Matsushima Kohai W1116) were planted in field soil in pots that were sunk into the field on Aug. 2, 1996 and eventually reduced to one plant per pot. Outer petioles of the plants were inoculated with mixture of 13 bacterial strains of E. carotovora subsp. carotovora on Sept.5, 1996. After the soft rot spread throughout the plant, the diseased plant was buried in the potted soil. New seeds were sown in the pots on April 30, 1997, and the disease was observed in June and July. The bacterial strains were re-isolated from the potted soil, diseased tissue and rhizosphere soil by the dilution plating method on modified Drigalski's medium containing 100 ppm rifampicin and by the enrichment technique. In addition to rifampicin resistance, phage sensitivities of some of the re-isolated strains were identical to those of the strains buried in the soil with the diseased plant in the previous year. From these results, some of the 13 strains overwintered in the soil and infested plant tissue and acted as primary inoculum the following year. The frequency of re-isolation varied among the strains, perhaps because of competition among the strains, differences in epidemiological behavior and stabilizing selection among the strains, and the presence of different ecotypes of the organism.