Control effect and mechanism of ginger essential oil and citral on ginger Fusarium wilt

Fusarium wilt in ginger plants is primarily caused by Fusarium oxysporum, and it is extremely difficult to control. Chemical agents are effective for controlling Fusarium wilt. To investigate environmentally friendly and safe plant-derived biopesticides, this study evaluated the inhibitory effects of ginger essential oil (GEO) and citral on F. oxysporum FOX-1 using the mycelium growth rate and spore germination methods. The mycelial growth rate method was used to determine the inhibitory effects of various GEO and citral concentrations on F. oxysporum FOX-1 mycelial growth. The lowest concentrations of GEO and citral that effectively prevented mycelial growth on potato dextrose agar (PDA) plate after 48 h were recorded as the minimum inhibitory concentrations (MICs). The spore germination method was used to assess the effects of GEO and citral at different concentrations (0, 1/2 MIC, MIC) on the spore number and germination of F. oxysporum FOX-1, respectively. Scanning electron microscopy (SEM) was used to observe the mycelial morphology of F. oxysporum FOX-1, and propidium iodide (PI) staining was used to assess cell membrane damage. Furthermore, the effects of GEO and citral on the cell integrity and permeability of F. oxysporum FOX-1 were evaluated by measuring the changes in relative electrical conductivity, proteins, nucleic acids, malondialdehyde, and ergosterol. The effects of MIC GEO and citral on controlling Fusarium wilt in ginger in a pot experiment were determined 15 d after inoculation with F. oxysporum FOX-1. The results indicated that 2 g∙L⁻¹ GEO and 0.5 g∙L⁻¹ citral significantly inhibited the mycelial growth of F. oxysporum FOX-1, with EC₅₀ values of 1.102 g∙L⁻¹ and 0.141 g∙L⁻¹ for GEO and citral, respectively. This indicates that both GEO and citral exhibited dose-dependent effects, with MIC values of 2 g∙L⁻¹ and 0.5 g∙L⁻¹, respectively. In addition, these treatments of 1/2MIC and MIC significantly inhibited the germination of F. oxysporum FOX-1 spores compared to the control (CK). Compared with CK, the 1/2MIC and MIC GEO treatments reduced the number of F. oxysporum FOX-1 spores by 35.6% and 59.3%, respectively. Similarly, the 1/2MIC and MIC citral treatments reduced the number of F. oxysporum FOX-1 spores by 61.0% and 78.0%, respectively. After 12 h of treatment, the germination rates of F. oxysporum FOX-1 spores in the 1/2MIC and MIC GEO treatments reduced by 20.4% and 34.7%, whereas the germination rates in the 1/2MIC and MIC citral treatments decreased by 86.1% and 95.0%, respectively. After 3 d of GEO and citral treatment, the SEM results showed that the cell walls and cell membranes of F. oxysporum FOX-1 were damaged and could not maintain the normal linear morphology of the mycelium. They also showed different degrees of curvature, folds, and depressions. Furthermore, the PI staining revealed that the GEO and citral treatments severely damaged the integrity and permeability of the cell membrane of F. oxysporum FOX-1, resulting in a significant increase in the number of spores. Moreover, this treatment resulted in a sharp increase in cytoplasmic loss, extracellular conductivity, and protein, nucleic acid and malondialdehyde contents in the damaged F. oxysporum FOX-1 cells. After 3 d of treating the mycelium, high concentrations of GEO (2 g∙L⁻¹) and eugenol (2 g∙L⁻¹) reduced the ergosterol content of F. oxysporum FOX-1 by 27.0% and 45.2%, respectively, when compared with CK. GEO and citral treatments also weakened the pathogenicity of F. oxysporum FOX-1. Moreover, after 15 d of inoculation with F. oxysporum FOX-1, the 2 g∙L⁻¹ GEO and 0.5 g∙L⁻¹ citral treatments exhibited efficacy rates of 32.7% and 42.3%, respectively. The 0.5 g∙L⁻¹ citral treatments was not significantly different from that of the positive control, chlorothalonil, which exhibited an efficacy rate of 47.1%. In summary, GEO and citral had significant inhibitory effects on the growth of F. oxysporum FOX-1 and could control Fusamum wilt in gingers. These findings could lay the foundation for the development of botanical antifungal agents for the management of Fusamum wilt.