Toshio Kikumoto

Ecology and Biocontrol of Soft Rot of Chinese Cabbage

1. Ecology of soft rot bacterium Bacterial soft rot, caused by Erwinia carotovora subsp. carotovora (Ecc), is one of the most destructive diseases of Chinese cabbage [ Brassica campestris L. (pekinensis group)] in Japan. The pathogen is widely distributed in agricultural areas as well as in uncultivated land. In 1949 the soft rot pathogen caused catastrophic losses of Chinese cabbage in Miyagi Prefecture. The next year in our laboratory, Tsuyama and Sakamoto initiated studies on the behavior of soft rot bacteria in the rhizosphere and non-rhizosphere soils, as well as on or in Chinese cabbage plants in order to control the disease. Since 1961, we have continued and developed their work. (1) Growth in rhizosphere of crop plants and

Biological control of soft rot of Chinese cabbage using single and mixed treatments of bacteriocin-producing avirulent mutants of Erwinia carotovora subsp. carotovora.

Two pathogenic strains of E. carotovora subsp. carotovora 2T-2 and TT-4 with high bacteriocin activity but low sensitivity to the bacteriocins of other strains were treated with ethyl methane sulphonate (EMS). Two avirulent mutants A-f-39 and B-e-19 developed from 2T-2 and TT-4, respectively, by this treatment had the same bacteriocin activity as their respective parents and inhibited the in vitro growth of pathogenic strains of this species. The disease control of these two mutant strains were compared in the field in 1995 and 1997 to the control by CGE234M403 (M403) (a commercialized biocontrol agent), a mixture of A-f-39 and M403, and an agrochemical (basic dithianon-copper chloride). The protection obtained with A-f-39 was comparable to M403 and was better than that with the chemical. The mixture of A-f-39 and M403 consistently gave the best results in all the field trials.