An effective medium designated phosphate separately autoclaved Reasoner’s 2A supplemented with cycloheximide and tobramycin (PSR2A-C/T) has been developed for the isolation of Flavobacterium and Chryseobacterium strains from the plant rhizosphere. It consists of Reasoner’s 2A agar (R2A) prepared by autoclaving phosphate and agar separately and supplementing with 50 mg L−1 cycloheximide and 1 mg L−1 tobramycin. A comparison was made among the following nine media: PSR2A-C/T, PSR2A-C/T supplemented with NaCl, R2A agar, R2A agar supplemented with cycloheximide and tobramycin, 1/4-strength tryptic soy agar (TSA), 1/10-strength TSA, soil-extract agar, Schaedler anaerobe agar (SAA), and SAA supplemented with gramicidin, for the recovery of Flavobacterium and Chryseobacterium strains from the Welsh onion rhizosphere. Flavobacterium strains were only isolated on PSR2A-C/T, and the recovery rate of Chryseobacterium strains was higher from PSR2A-C/T than from the eight other media. In order to confirm the effectiveness of PSR2A-C/T, bacteria were isolated from onion rhizosphere soil with this medium. Flavobacterium and Chryseobacterium strains were successfully isolated from this sample at a similar rate to that from the Welsh onion rhizosphere.
Crop rotation and intercropping with Allium plants suppresses Fusarium wilt in various crops. However, the mechanisms underlying this phenomenon have not been fully elucidated. This study was designed to assess the role of microorganisms inhabiting Allium rhizospheres and antifungal compounds produced by Allium roots in Fusarium wilt suppression by Allium cultivation. Suppression of cucumber Fusarium wilt and the pathogen multiplication by Allium (Welsh onion and/or onion)-cultivated soils were eliminated by heat treatment at 60 °C, whereas those by Welsh onion-root extract were lost at 40 °C. The addition of antibacterial antibiotics eliminated the suppressive effect of Welsh onion-cultivated soil on pathogen multiplication, suggesting the contribution of antagonistic gram-negative bacteria to the soil suppressiveness. The Illumina MiSeq sequencing of 16S rRNA gene amplicons revealed that genus Flavobacterium was the predominant group that preferentially accumulated in Allium rhizospheres. Flavobacterium species recovered from the rhizosphere soils of these Allium plants suppressed Fusarium wilt on cucumber seedlings. Furthermore, confocal laser scanning microscopy revealed that Flavobacterium isolates inhibited the multiplication of the pathogen in soil. Taken together, we infer that the accumulation of antagonistic Flavobacterium species plays a key role in Fusarium wilt suppression by Allium cultivation.
Dendrobium
endophytic bacteria are of great interest since their functions may contribute to the protection of endangered orchids with ornamental and medicinal values. To understand and reveal the “true roles” of the endophytes, obtaining those axenic cultures is necessary even in the metagenomic era.
Intercropping with Welsh onion and Chinese chive was evaluated as a means to reduce the incidence of Fusarium wilt on springsummer spinach caused by Fusarium oxysporum f. sp. spinaciae (FOS). In preliminary experiments (i.e., plant box experiment and container experiment), intercropping spinach with Welsh onion and Chinese chive suppressed development of spinach Fusarium wilt. Particularly, intercropping with Welsh onion yielded stronger suppression. Subsequently, at two commercial spinach fields in Takayama City, Gifu Prefecture, during the first cultivation season after soil fumigation, Welsh onion and Chinese chive were intercropped on the ridge shoulders along the sides of rain shelters where FOS often survives. As a result, both intercropping systems effectively reduced the incidence of spinach Fusarium wilt (44 to 100% reduction) up to 5 months post fumigation. F. oxysporum was detected from soils in the ridges of plots with spinach-Chinese chive intercropped at a level similar to that in plots with only spinach, and soil populations of F. oxysporum in spinach-Welsh onion intercropped plots were undetectable during cultivation. These results suggest that intercropping with alliums, particularly Welsh onion, on ridge shoulders along the sides of rain shelter after soil fumigation can effectively reduce the incidence of Fusarium wilt on spring-summer spinach.
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