Soil Microorganisms

Cultivation of lilies in containers was conducted on a farm-wide scale in Tokushima prefecture using two types of cedar bark medium to evaluate the effects of continuous and renewed treatment on the fungal community structures and chemical properties of these media. The fungal community monitored using denaturing gel gradient electrophoresis (DGGE) was different in both types throughout continuous cultivation and was more affected by type of medium than by cultivation times or season. The chemical properties of cedar bark medium were changed by cultivation management and contributed to differences between the fungal communities. There were no disease symptoms throughout the cultivation period using either continuous or renewed treatment on a small scale, and continuous cultivation was performed three times with few troubles on a farm-wide scale. Hereafter, it is necessary to confirm whether the continuous cultivation of lily is possible more than four times on a farm-wide scale.

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An inoculation test with Ralstonia solanacearum was conducted to evaluate how adding biocontrol agents (BCAs) and lysine or sucrose affects the microbial community structures of tomato roots as well as bacterial wilt of the tomato. In addition, we examined the relationship between the quantity of R. solanacearum DNA and the disease incidence of bacterial wilt. Bacterial and fungal community structures were determined using denaturing gradient gel electrophoresis (DGGE). The addition of BCAs and lysine or sucrose significantly decreased the disease incidence and changed the bacterial DGGE profiles. Canonical correspondence analysis (CCA) indicated that three unique bands observed in lysine treatments were negatively correlated with disease incidence, suggesting that bacterial species corresponding to these bands suppressed the bacterial wilt pathogen. On the other hand, none of the bands in the fungal DGGE profiles were related to disease incidence in the CCA, suggesting that the suppressive effect on bacterial wilt promoted by the lysine treatment was attributed to the bacterial community of tomato roots, not to the fungal community. The quantity of R. solanacearum DNA in the lysine treatment was as small as that in the non-inoculated control, suggesting that the quantity of R. solanacearum DNA in the tomato roots six weeks after sowing might be a useful indicator for assessing the risk of tomato bacterial wilt.

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We confirmed that Phytophthora rot was involved in frequent growth failure after replanting in asparagus production areas in Ehime Prefecture, Japan. According to microscopic observation, under moist conditions, zoosporangia and a septate mycelium showing the typical characteristics of Phytophthora rot formed on the lesions of stems. The isolates from the replanting soil were identified as Phytophthora nicotianae van Breda de Haan (1896) on the basis of the morphological characteristics of the zoosporangia, oogonia, oospores, and antheridia on culture media, compared with the previous reports. Also, asparagus seedlings inoculated with the isolates showed symptom in the field. In a bioassay with asparagus seedlings, the soil in which asparagus death occurred was tested, and the frequency of dead plants in non-sterilized soil was observed. In contrast, the occurrence of dead plants was completely suppressed in sterilized soil (autoclaved for 2 hours at 121℃) or fungicide-treated soil (2.0% of metalaxyl granules). Furthermore, PCR-DGGE confirmed that the density of P. nicotianae mycelia in the rhizosphere of asparagus seedlings increased from three days after transplanting.

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