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Biocontrol of Plant Diseases by Bacillus subtilis

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... Biocontrol organisms (or their natural by-products) have been shown to have adverse effects on various plant diseases. For example, Bacillus sp. is one of several candidates (Huang et al. 2019;Shoda 2019). There are commercially available biocontrol products derived from Bacillus sp. for soil-borne diseases including the product "Serenade" containing B. amyloliquefaciens (Matzen et al. 2019). ...
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Pythium myriotylum is a destructive soil-borne phytopathogen, causing yield losses in ginger and many other crops. Environmental and regulatory concerns drive the need to find biological alternatives to conventional pesticides used to manage P. myriotylum. Trans-cinnamic acid (TCA) alone, and fermentation broth from strains of symbiotic bacteria of eight species of entomopathogenic nematodes alone, and in combination with TCA, were tested for their effect on zoospore germination and mycelial growth of P. myriotylum. TCA significantly inhibited mycelial growth. Fermentation broths from seven of the eight strains of symbiotic bacteria directly inhibited mycelial growth, especially those isolated from Steinernema feltiae (strain SN) and S. riobrave (strain 7–12). Moreover, adding TCA significantly increased the inhibitory effect on mycelial growth of the fermentation broths of seven of the strains tested. All bacteria fermentation broths showed inhibitory effects on zoospore germination. However, TCA alone was not inhibitory to zoospore germination but was inhibitory to mycelial growth. Antimicrobial effects on mycelial growth and zoospore germination were proportional to the concentration of symbiotic bacteria isolated from S. feltiae (strain SN). These results show that TCA and symbiotic bacteria of entomopathogenic nematodes may have potential to provide biorational control of P. myriotylum.
... Many soil-borne microorganisms that are suppressive to plant pathogens and have the potential to be successful biocontrol agents have been identified. Examples include Bacillus subtilis (Shoda, 2019), Pseudomonas spp. (Weller, 2007), Sporidesmium sclerotivorum (Adams and Ayers, 1981), Gliocladium catenulatum (Mcquilken et al., 2001), Streptomyces spp. ...
Thesis
There is considerable interest in the use of biological control agents as a control strategy against plant pathogens. However, significant variability in their success at pathogen suppression across field trials has resulted in them often not being seen as commercially viable. This thesis uses mathematical modelling to explore the interactions between a soil- borne biocontrol agent, a soil-borne pathogen, and the roots of a host plant. Understanding the dynamics of these organisms in more detail can allow us to optimise the use of biological control agents. We construct a model that focuses on the roots of a plant and whether they are infected by a pathogen or colonised by a biocontrol agent, as well as including any free-living pathogen and biocontrol agent in the surrounding soil. Although this model can be used across multiple systems, this thesis focuses on modelling the infection of winter wheat by Gaeumannomyces graminis var. tritici and suppression by the biocontrol agent 2,4-DAPG fluorescent Pseudomonas spp. Parameter values are obtained for the model through fitting it to data from this system. Application timing and amount of application were both found to affect the ability for a biocontrol agent to suppress a pathogen. Including a break crop into a simulation had a negative impact on both the pathogen and biocontrol agent, suggesting that combining multiple strategies for epidemic suppression may not always be effective. Planting a crop with even spaces between plants, or in rows with 12cm between each row, was found to reduce epidemic severity more than if rows were spaced further apart for the first year of an epidemic. However, large-scale dispersal of free-living material between growing seasons from agricultural machinery reduced any benefit of specific spatial arrangements of a crop in the second year of an epidemic. Aggregation of the pathogen and biocontrol agent were found to affect epidemic severity, with the greatest reduction from a highly aggregated pathogen and a uniformly distributed biocontrol agent. We suggest that a greater focus on optimising application, as well as a detailed understanding of how the spatial dynamics of a biocontrol agent and pathogen can affect this application, may enhance the success of biocontrol agents and allow them to be seen as a viable control strategy.
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Bacillus subtilis KB21 is an isolate with broad spectrum antifungal activity against plant pathogenic fungi. Our aim was to produce and purify antifungal lipopeptides via fermentation using B. subtilis KB21 and verify their antifungal mechanism against pepper anthracnose. When the KB21 strain was cultured in tryptic soy broth medium, the antifungal activity against pepper anthracnose correlated with biosurfactant production. However, there was no antifungal activity when cultured in Luria-Bertani medium. KB21 filtrates showed the highest degree of inhibition of mycelia (91.1%) and spore germination (98.9%) of Colletotrichum acutatum via increases in the biosurfactant levels. Using liquid chromatography-mass spectrometry (LC-MS) and LC-tandem MS (LC-MS/MS) analyses, the component with antifungal activity in the fermentation medium of the KB21 strain was determined to be the cyclic lipopeptide (CLP) antibiotic, iturin A. When the iturin fractions were applied to pepper fruits inoculated with conidia of C. acutatum, the lesion diameter and hyphal growth on the fruit were significantly suppressed. In addition, iturin CLP elevated the gene expression of PAL, LOX, and GLU in the treatments both with and without following fungal pathogens. Overall, the results of this study show that iturin CLPs from B. subtilis KB21 may be potential biological control agents for plant fungal diseases.
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