$B!!(BThe soft rot of Chinese cabbage (Brassica campestris L.,pekinensis group)has been known as soil-borne disease caused by pathogenic bacterium (Erwinia carotovora subsp.carotovora) inhabiting in soil. The incipient symptoms of the disease appear on the pe-tioles at first then it develops systemically from the wrapping stage following destru-ctive damage. It is listed up as one of the destructive diseases in Japan. Though there re many reports on the disease cycle consisting of saprophytic phase in soil and para-sitic phase on host plants up to date,it has not come to a satisfactory solution.
$B!!!!(BThe present study was carried out to clarify the ecological aspects of the soft rot disease with reference to the establishment of source of inoculum for the disease in Yamagata University Farm. The results obtained are as follows.
1. Growth of the organisms in sterilized and non-sterilized soil
$B!!!!(BThe soil was collected from Yamagata University Farm and sterilized by autoclaving at 121$B!n(B for 2 hrs. The soft rot bacteria inoculated to the sterilized soil at 101cfu/gof dry soil grow rapidly and population level of 106-107 cfu/g of dry soil was obtainedat 3 days after incubation at 25$B!n(B. On the contrary, even the organisms of 106 cfu/g ofdry soil did not grow in non-sterilized soil and could not even be detected by the phage technique at 7 days after incubation showing to be the level below 101cfu/g of dry soil. The results may indicate that most of the organisms inoculated externally to non-sterilized soil may die out within a short period.
2. Growth of the organisms in field and occurrence of the soft rot
$B!!!!(BThe organisms have been reported to inhabit widely in soil. In ordinary circum stan-ces, the population of the organism is below 103cfu/g of dry soil which is undetectablelevel by routine procedures. However, when Chinese cabbage (cv. Matsushima Kohai W-1116,unless otherwise stated) is grown, the organism can exclusively grow in the rhizospheresoils in a wrapping stage at 40-50 days after seeding in both the spring and summer-seeded crops. The population level was 105-107cfu/g of dry soil and was continued untilharvest. The organism was also isolated from leaf surface of the plant from a wrapping stage. The soft rot occurred at about 50 days after seeding corresponding with the latewrapping stage. Susceptibility index of the disease in the spring-seeded crops was hi-gher than that of the summer-seeded crops.
3. The factors concerning with the growth of the organisms in the rhizosphere soils at a wrapping stage
$B!!!!(BThe high population level of the organisms in the rhizosphere soils at a wrapping stage is decreased by cutting the leaves of the plants and can not be detected at 7days after cutting. The results may show that the exclusive growth of the organisms in the rhizosphere soils depends upon the nutrient substances secreted by roots of the plant in which the leaves conduct photosynthesis. Root exudates were sampled at early, middle and wrapping stages, respectively and 10ml of them were inoculated with level of 101cfu/ml of the organisms. The number of the organisms was 107-108cfu/ml after incuba-tion at 25$B!n(B for 24 hrs regardless of sampling time. There was an evident that the rootexudates collected at a wrapping stage had a significant effect on the growth of the organisms. Application of kinetin to the leaves (0.3 ppm) had some effect on the exclu-sive growth of the organisms in the rhizosphere soils.
$B!!!!(BAll the 540 bacterial isolates were obtained from the rhizosphere soils at early,middle and wrapping stages by the dilution plating method with PDA using 10-4 diluent of the soil in the spring and the summer-seeded crops. The 249 isolates on NA plates and the 271 isolates on PDA plates out of the 540 isolates were found to be antagonis-tic to five indicator strains of the soft rot bacterium. Though the antagonistic bacte-ria were isolated from the rhizosphere soils in spite of different growth stages, the number of the bacteria showed a trend to decrease in a wrapping stage. The decrease seems to be one of the factors inducing an exclusive growth of the organisms in rhizo-sphere soils.
4. Role of the organisms growing in rhizosphere soils and inhabiting on leaf surface of the plants as a source of inoculum for the disease
$B!!!!(BAs the time of occurrence of the soft rot coincides with that of growth of the orga-nisms in rhizosphere soils, the organisms growing in the rhizosphere soils and inhabit-ing on leaf surface of the plants were supposed to be an effective source of inoculum for the soft rot. The organism marked with rifampicin-resistant and phage sensitivity was inoculated to sterilized soils, cut straws of rice and rice husks in 300ml conical flask, respectively, then the soil and plant materials were inoculated with the organi-sm. After incubation at 25$B!n(B for 7 days, they were buried at a depth of about 10 cm in the field, respectively. The slices of carrot root were dipped for 4 hrs in the water suspension (108cfu/ml) of the organism and were similarly buried. Chinese cabbage were preliminary grown for 14 days in paper pots filled up with sterilized soils. The plantswere immediately replanted in the buried sites. When the soft rot occurred and develop-ed about 50-80 of susceptibility index, the rifampicin-resistant organism buried in thefield with sterilized soil or plant materials were frequently reisolated from the soft rotten lesions and rhizosphere soils of the diseased plants. The sprayed organisms on leaves of the plants remained there until harvest time and were frequently reisolated from the soft rotten lesions and rhizosphere soils in spite of different spraying time and concentrations of the organism. It was found that the organism in soil and on leav-es played a role as an effective inoculum source for the disease. In other word, the source of inoculum for the disease can not only originate from soil but can also origi-nate from leaves of the plants.
5. An attempt to control of the disease biologically by the antagonistic bacteria
$B!!!!(BThe efficacy of biological control by using the two strains out of the antagonistic bacteria as mentioned above was examined by three different methods such as spraying ofthe organisms on the leaves of the plants with water suspension (106 cfu/ml), dipping of seeds in the water suspension before seeded in the field (bacterization of seeds by the organisms) and transplantation of the plants grown in the organisms-grown sterilized soil (107 cfu/g of soil). It was found that there is a high possibility of controlling the soft rot disease biologically by antagonistic bacteria.
$B!!%O%/%5%$(B$B!J(BBrassica campestris L., pekinensis group) $BFpIeIB$OEZ>m@8B)@-$NIB86:Y6](B(Erwinia carotobovora subsp.carotovora) $B$K$h$C$F$R$-$*$3$5$l$kEZ>mEA@wIB$G$"$k!#K\IB$OCO0h$dG/l$K$*$$$FKhG/#47n(B($B=UGE(B)$B$H#87n!J2FGE!K$KGEl$K$*$1$k46@w8;$N@.N)$rCf?4$K8!F$$7$?$b$N$G7k2L$O