Biocontrol mechanisms of Bacillus sp., isolated from tomato rhizosphere, against Fusarium oxysporum f. sp. lycopersici

Aims
The effect of Bacillus amyloliquefaciens AK-0 (AK-0) on ginseng root-rot disease caused by Cylindrocarpon destructans was investigated.

Methods and Results
From 190 ginseng rhizoshpere bacteria, AK-0 was selected for further analysis; its morphological characteristics were investigated by microscopy. AK-0 was identified as B. amyloliquefaciens using the Biolog system, 16S rRNA gene sequence analysis, and examination of morphological and biochemical characteristics. Bacterial population and media optimization were estimated by the bacterial growth curve. The no. of AK-0 cells was relatively higher in brain-heart infusion (BHI) medium than other media. The potential antifungal effect of AK-0 culture filtrate on the in vitro conidial germination of C. destructans, and root rot development on root discs and 4-year-old ginseng roots were assessed. PCR analysis of antibiotic biosynthesis gene expression suggested that the release of antibiotic compounds is involved in the antifungal effect of AK-0 and the suppression of ginseng root-rot.

Conclusion
These results indicate that the CF of AK-0 has antifungal effects on fungal pathogens of ginseng, resulting in the suppression of root-rot disease caused by C. destructans.

Significance and Impact of the Study
AK-0 is a potential source of novel bioactive metabolites. AK-0 CF exhibited antifungal effects against C. destructans on ginseng roots. PCR analysis indicated that the AK-0 harbors genes involved in the biosynthesis of antimicrobial compounds.

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Biological characteristics of Bacillus amyloliquefaciens AK-0 and suppression of ginseng root rot caused by Cylindrocarpon destructans

Biocontrol activity and putative mechanism of Bacillus amyloliquefaciens (SF14 and SP10), Alcaligenes faecalis ACBC1, and Pantoea agglomerans ACBP1 against brown rot disease of fruit.

Biocontrol efficacy of Bacillus siamensis LZ88 against brown spot disease of tobacco caused by Alternaria alternata

Citrus trees face threats from several diseases that affect its production, in particular dry root rot (DRR). DRR is a multifactorial disease mainly attributed to Neocosmospora (Fusarium) solani and other several species of Neocosmospora and Fusarium spp. Nowadays, biological control holds a promising control strategy that showed its great potential as a reliable eco-friendly method for managing DRR disease. In the present study, antagonist rhizobacteria isolates were screened based on in vitro dual culture bioassay with N. solani. Out of 210 bacterial isolates collected from citrus rhizosphere, twenty isolates were selected and identified to the species level based on the 16S rRNA gene. Molecular identification based on 16S rRNA gene revealed nine species belonging to Bacillus, Stenotrophomonas, and Sphingobacterium genus. In addition, their possible mechanisms involved in biocontrol and plant growth promoting traits were also investigated. Results showed that pectinase, cellulose, and chitinase were produced by eighteen, sixteen, and eight bacterial isolates, respectively. All twenty isolates were able to produce amylase and protease, only four isolates produced hydrogen cyanide, fourteen isolates have solubilized tricalcium phosphate, and ten had the ability to produce indole-3-acetic acid (IAA). Surprisingly, antagonist bacteria differed substantially in their ability to produce antimicrobial substances such as bacillomycin (five isolates), iturin (ten isolates), fengycin (six isolates), surfactin (fourteen isolates), and bacteriocin (subtilosin A (six isolates)). Regarding the PGPR capabilities, an increase in the growth of the bacterial treated canola plants, used as a model plant, was observed. Interestingly, both bacterial isolates Bacillus subtilis K4-4 and GH3-8 appear to be more promising as biocontrol agents, since they completely suppressed the disease in greenhouse trials. Moreover, these antagonist bacteria could be used as bio-fertilizer for sustainable agriculture.

Potential Role of Rhizobacteria Isolated from Citrus Rhizosphere for Biological Control of Citrus Dry Root Rot

Strain L2 (a Bacillus megaterium species that was isolated from tobacco's rhizosphere) was applied to inhibit the pathogen Alternaria alternata in an attempt to reduce tobacco brown spot disease. This study focused on inhibition experiments, in which culture filtrate inhibited A. alternata’s mycelium growth, its spore germination rate and its spore output. The results indicated that an approximately 1.5%–20% concentration of the bacteria-free filtrate, could effectively inhibit hyphae growth. Additionally, bacterial suppression of germ tubes and mycelium growth was observed, rather than lysis. Furthermore, the filtrate caused some of the germinal tubes to undergo vacuolization and even rupturing. The inhibitory effect was associated with the change of the concentration: the greater the concentration, the greater the impact on A. alternata.

https://www.tandfonline.com/doi/pdf/10.1080/13102818.2015.1068135

Hui Huang

Papers

Effects of bacteria-free filtrate from Bacillus megaterium strain L2 on the mycelium growth and spore germination of Alternaria alternata