[in Japanese]

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In recent years, growing concerns over unstable food supply and the pursuit of sustainable agriculture have made the control of plant diseases an increasingly important issue. Among these, soil-borne plant pathogens—which can persist in the soil for long periods and cause severe damage to crops—remain particularly difficult to manage. This paper focuses on three representative pathogens: Verticillium spp. (causing diseases such as Verticillium wilt), Sclerotinia sclerotiorum (Sclerotinia rot), and Sclerotium cepivorum (white rot). Each of these pathogens exhibits unique ecological characteristics and infection strategies, enabling them to skillfully exploit the soil, plant surfaces, and even the air to expand their infection area and threaten agricultural productivity. Understanding the survival strategies of these pathogens and developing scientifically grounded control measures are now essential steps toward achieving sustainable plant disease management.

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[in Japanese]

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Verticillium dahliae, a soilborne plant pathogen, causes wilt disease in various dicotyledonous plant species. The host range of V. dahliae differs among strains, but the system which determines the pathogenicity of each strain on a plant species remains unclear. An earlier study using a genetic cross between the strains Chr208 and Vdp4, which are respectively nonpathogenic and pathogenic on sweet pepper, revealed that the genomic region GL3 linked to pathogenicity. A similar result was obtained in this study by genetic crossing between the Chr208 (nonpathogenic) and Cns (pathogenic) strains. By contrast, the genomic region GL3 was not consistently linked with pathogenicity in the genetic cross between the strains TV103 (nonpathogenic) and Cns (pathogenic). Using DNA markers to analyze recombinants obtained in this genetic cross revealed that another genetic region GL5 is linked to pathogenicity. It is particularly interesting, however, that recombinants which inherited the GL3 region from the nonpathogenic strain TV103 were less virulent on sweet pepper. Moreover, in the genetic cross between the strains TV103 (nonpathogenic) and Vdp4 (pathogenic), recombinants that inherited either region of the GL3 or the GL5 from the nonpathogenic strain TV103 were nonpathogenic on sweet pepper. These results suggest that, in addition to the GL3, the GL5 region is involved in V. dahliae pathogenicity on sweet pepper.

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In Japan, early growth injury of sorghum from continuous cropping has been observed, especially in andosols. Large-size plot field experiments demonstrated that early growth injury was more severe in andosols than in other soil types and that there are varietal differences in injury severity. The low phosphorus content of the injured plants was suggested to be related to the anthocyanin development of sorghum early growth injury. Potassium content in the early growth-injured plants in continuously sorghum-cultivated soils tended to decrease compared with those in sorghum-uncultivated soils. Other mineral contents of Ca, Mg, Fe, Mn, and Zn were not likely to be related to the development of sorghum early growth injury. The 121℃ autoclave-sterilization treatment of continuously sorghum-cultivated soil improved sorghum plant length and shoot dry weight compared with fresh soil treatment. Meanwhile, soybean plants were not affected by the fresh soil treatment of continuously sorghum-cultivated soil. The 80℃ steam sterilization of continuously sorghum-cultivated soil made the sorghum growth healthy, with the same level of growth in sorghum-uncultivated soil. This suggests that certain causal microbes can lead to sorghum early growth injury. Sorghum allelopathy may be related to early growth injury, but the findings of pot experiments suggested that the allelopathic influence would not be determinable as a causal agent. Certain microbes were observed on the root surfaces of the injured plants. We anticipate that further studies will clarify the possible causal microbes by fungal DNA sequencing analysis of young reddish roots grown in continuously sorghum-cultivated andosols.

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This study aimed to elucidate antibiotic-resistant bacterial communities in the lake sediments of Lake Kitaura, Japan using culture-based and non-culture methods. Based on bacterial 16S rRNA amplicon sequencing using DNA extracted from the lake sediments, PICRUSt2 predictive metagenomic analysis suggested an increased representation of taxa whose reference genomes contain genes associated with tetracycline (TC) resistance. The rate of TC-resistant bacteria increased in river water downstream of the Hokota River, and TC-resistant bacteria appeared in lake sediments upstream of the lake based on the culture-dependent assay. PCR amplification revealed TC resistance genes (tetA, B, M, O, and W) in the river water downstream of the river. However, only the tetW amplicon was detected in the water above the lake sediment and in the lake sediments in the upper reaches. 18 TC-resistant bacterial strains obtained from river water and lake sediments suggested that TC-resistant bacterial communities diversified in the lower reaches of the river. Additionally, we identified TC-resistant bacterial strains that amplified tetAB and tetM by PCR, respectively. Furthermore, tetW-based meta-amplicon sequence analysis showed that infectious and intestinal bacteria from humans and animals inflowed from the lower reaches of the river into the lake. To the best of our knowledge, this study is the first to report that TC-resistant bacteria reach the upper reaches of the lake sediments of Lake Kitaura via the Hokota River.

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