Graduate Studies

 

First Advisor

Amitava Mitra

Department

Plant Pathology

Date of this Version

Spring 2024

Document Type

Dissertation

Comments

Copyright 2024, Serkan Tokgöz. Used by permission

Abstract

Essential oils (EOs) are natural hydrophobic compounds, which are highly complex, typically comprising 20 to 60 constituents, with two or three main components making up 20% to 95% of their composition, alongside other minor constituents. Research has firmly established the potent antibacterial and antifungal properties of EOs and their primary components. To assess their efficacy, one EO, nutmeg oil, and seven main components, including safrole, toscanol, safraleine, cinnamaldehyde, carvacrol, carvone, and thymol, were tested against four economically significant fungal pathogens (Alternaria solani, Fusarium graminearum, Rhizoctonia solani, and Sclerotiorum sclerotiorum), one oomycete pathogen (Phytium ultimum), and four bacterial pathogens (Clavibacter michiganensis subs. nebraskensis, Xanthomonas campestris pv. campestris, Pseudomonas syringae pv. tomato DC3000, and GFP-tagged P. syringae pv. tomato). In vitro studies were conducted to evaluate the inhibitory effects of these selected EOs and major components on fungal and bacterial growth, determine the Minimum Inhibition Concentration (MIC), Minimum Fungicidal Concentration (MFC), and Minimum Bactericidal Concentration (MBC) of the compounds, and investigate their impact on spore production and germination of F. graminearum. Among the tested components, carvacrol, carvone, cinnamaldehyde, safraleine, and thymol exhibited significant growth inhibition of the pathogens. These five major components were further tested in wheat heads against F. graminearum to assess their efficacy in reducing infection, mycotoxin levels, and wheat quality parameters in greenhouse conditions. Similar studies were conducted on tomato seedlings infected with P. syringae pv. tomato to evaluate the impact on disease severity, agronomic features, and the relative expression levels of genes associated with resistance pathways, including SAR, ISR, and R-genes. The in vitro screening revealed varying efficacy of the main EO components against fungal and bacterial pathogens, depending on the tested components, their concentrations, and the pathogens. Cinnamaldehyde, carvacrol, and thymol demonstrated the strongest growth inhibition against both fungal and bacterial pathogens, with MIC, MFC, and MBC concentrations ranging from 0.25 mg/mL to 5 mg/mL. These components also inhibited spore production and germination at relatively low concentrations. Furthermore, safraleine and carvacrol significantly reduced Fusarium Head Blight (FHB) infection and deoxynivalenol (DON) levels while improving yield and maintaining nutritional properties. Application of cinnamaldehyde and thymol resulted in a significant decrease in chlorotic areas on leaves and the incidence of bacterial speck lesions in tomato seedlings. Additionally, EO application upregulated the expression of defense-related genes PR1, PR2, and PR3 in tomato plants infected with P. syringae pv. tomato DC3000. The testing of major EO components establishes a foundation for discovering novel alternative biocides for controlling fungal and bacterial diseases.

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