Soil bacterial community structure was analyzed using model soil systems (microcosms) contaminated with 4 different levels of Cu^<2+>. We used length polymorphism (LPM) of PCR-amplified DNA fragments of 16S ribosomal RNA gene (16S rDNA) to evaluate the diversity of soil bacteria in the model soil systems. PCR (polymerase chain reaction) primers were designed to classify the soil bacteria into three groups as follows: 1) α and δ subclasses of Proteobacteria, 2) β and γ subclasses of Proteobacteria, and 3) Gram-positive bacteria. Bacterial DNA was extracted directly from the soil samples, and LPM patterns for the PCR-amplified DNA fragments were obtained using a high-resolution gel. Considerable changes in the LPM pattern were observed for the β and γ subclasses of Proteobacteria when the Cu^<2+> concentration increased, while minor changes were observed for the other groups. New bands appeared when the Cu^<2+> concentration increased, suggesting that the number of Cu-resistant bacteria species increased, while that of sensitive bacteria decreased with time after the addition of Cu^<2+> to the soil.

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New biological control agents (BCAs) were screened for Pythium damping-off caused by Pythium aphanidermatum using the inoculation assay of cucumber seedlings grown on soil medium derived from water clarifier sludge (Jousui-cake). The conditions for the seedling inoculation assay were set up before the BCA screening, and applied throughout the three inoculation experiments. Using the dilution plate method, microorganisms of BCA candidates, namely 224 bacteria and 56 fungi, were isolated from the rhizosphere or nonrhizosphere soil collected from 22 ecologically diverse sites. Each isolate was mixed in the soil medium derived from Jousui-cake infected with P. aphanidermatum, and the disease incidence was monitored. Based on the results of three inoculation assays, four bacterial and four fungal strains induced a significant reduction of the disease incidence, and were selected as BCA strains. All the bacterial BCAs were identified as Arthrobacter spp., while fungal BCAs consisted of Paecilomyces sp., Malbranchea sp., and Cunninghamella sp. One fungal strain could not be identified. None of these microbial genera had been considered to act as BCAs of Pythium damping-off previously, suggesting that the BCA selection may have been influenced by the seedling inoculation assay using Jousui-cake. Only one of these BCA strains showed in vitro antibiosis against P. aphanidermatum, indicating that the seven other strains suppressed the disease by mechanisms other than antibiosis against the pathogen.

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Suppressive effect of pumice which had been used for 13 years for continuous cropping of Boston lettuce was evaluated by employing the used pumice as medium in hydroponic culture of tomato. Disease development of tomato plants grown in the used and unused (new) pumices was compared after inoculation of Ralstonia solanacearum, the pathogen of bacterial wilt, to the pumices. Disease incidence was remarkably lower in the used pumice than in the unused pumice. Role of pH, EC and Ca in the disease suppression by the used pumice was unlikely. In contrast, total bacterial number, microbial biomass and respiration rate were significantly higher in the used pumice than in the unused pumice, and the suppressive effect of the used pumice disappeared when the pumice was sterilized by γ-ray irradiation. These observations suggest that biological factors, such as higher microbial activity rather than chemical factors may be involved in the suppression of bacterial wilt in the used pumice.

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Three types of soil, CF-soil (amended with chemical fertilizers for 14 y), CF+FYM-soil (amended with chemical fertilizers and 40 t ha^<-1> y^<-1> of farmyard manure for 14 y) and FYM-soil (amended with 400 t ha^<-1> y^<-1> of farmyard manure for 14 y), were inoculated with Ralstonia solanacearum and tomato plants were grown in order to evaluate the degree of suppressiveness of bacterial wilt of tomato by the soils. More than 70% of the tomato plants showed wilt symptoms in the CF- and CF+FYM-soils after 30 days of cultivation in a climatron, whereas less than 10% of tomato plants wilted in the FYM-soil. The survival of the pathogen was higher in the CF- and CF+FYM-soils than in the FYM-soil. Soil pH appeared to affect the disease incidence in all the soil treatments, whereas it may not have been involved in the major suppression mechanisms in the FYM-soil. The disease incidence was reduced when the CF- and CF+FYM-soils were mixed with the FYM-soil, indicating that the disease suppressive property of the FYM-soil could be transferred. When the FYM-soil was irradiated with γ-rays, the disease suppressiveness was lost, implying that the suppressive nature of the FYM-soil could be attributed to biological factors. The disease incidence decreased when the CF-soil was amended with calcium chloride, suggesting that chemical factors might also be involved in the suppression mechanisms. Results of this study suggested that the suppression of bacterial wilt caused by R. solanacearum in the FYM-soil was due to the combined effects of biotic and abiotic factors associated with repeated application of a larger amount of farmyard manure.

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The distribution of Olpidium brassicae in Japan was examined using a baiting plant method and the abilities of O. brassicae isolates to transmit Tulip mild mottle mosaic virus (TMMMV) were evaluated. Seventy-three soil samples were collected from different areas in Japan. The soil samples, or root debris from selected sampling points, were inoculated onto the surface of soil in pots, to which cowpea (Vigna unguiculata 'Kurodane Sanjaku'), oriental melon (Cucumis melo 'Ginsen'), lettuce (Lactuca sativa 'Cisco'), tobacco (Nicotiana tabacum 'White Barley'), oat (Avena fatua 'Hay Oat') or cabbage (Brassica oleracea 'Shutoku') was planted as bait. O. brassicae was detected in the roots of at least one of these baiting plants grown in the 58 pots. The most successful plants in the recovery and propagation of O. brassicae from the soil were cowpea and oriental melon. However, oat, cabbage and tobacco were not good baiting plants because only a few O. brassicae could be recovered and propagated in these plants. All isolates trapped on cowpea or oriental melon propagated well, but some isolates differed in host specificity to lettuce and cabbage. Forty-nine isolates in total, including 11 single sporangial isolates, obtained from the soil or root debris were tested for their transmissibility of TMMMV. Nine isolates failed to transmit TMMMV, while the rest of the 40 succeeded in transmitting the virus from infected tulip to healthy tulip. The results show that O. brassicae inhabits in many field soils in Japan, and the majority of O. brassicae isolates can transmit TMMMV.

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