Nerol as a Novel Antifungal Agent: In Vitro Inhibitory Effects on <i>Fusarium oxysporum, Pestalotiopsis neglecta</i>, and <i>Valsa mali</i> and Its Potential Mechanisms against <i>F. oxysporum</i>

This study explores the in vitro antifungal effects of nerol, a linear acyclic monoterpene alcohol of plant origin, on <i>Fusarium oxysporum</i>, <i>Pestalotiopsis neglecta</i>, and <i>Valsa mali</i>. To further investigate the antifungal mechanism of nerol against <i>F. oxysporum</i>, we examined changes in mycelial morphology and cell membrane integrity-related indices, as well as the activities of antioxidant and pathogenicity-related enzymes. The results demonstrated that nerol exhibited significant concentration-dependent inhibition of mycelial growth in all three fungi, with EC₅₀ values of 0.46 μL/mL for <i>F. oxysporum</i>, 1.81 μL/mL for <i>P. neglecta</i>, and 1.26 μL/mL for <i>V. mali</i>, with the strongest antifungal activity observed against <i>F. oxysporum</i>. Scanning electron microscopy revealed that nerol severely disrupted the mycelial structure of <i>F. oxysporum</i>, causing deformation, swelling, and even rupture. Treatment with 0.04 μL/mL nerol led to significant leakage of soluble proteins and intracellular ions in <i>F. oxysporum</i>, and the Na⁺/K⁺-ATPase activity was reduced to 28.02% of the control, indicating enhanced membrane permeability. The elevated levels of hydrogen peroxide and malondialdehyde, along with propidium iodide staining of treated microconidia, further confirmed cell membrane disruption caused by nerol. Additionally, after 12 h of exposure to 0.04 μL/mL nerol, the activity of superoxide dismutase in <i>F. oxysporum</i> decreased to 55.81% of the control, and the activities of catalase and peroxidase were also significantly inhibited. Nerol markedly reduced the activities of pathogenicity-related enzymes, such as endo-1,4-β-D-glucanase, polygalacturonase, and pectin lyase, affecting fungal growth and virulence. In conclusion, nerol disrupts the cell membrane integrity and permeability of <i>F. oxysporum</i>, reduces its virulence, and ultimately inhibits fungal growth, highlighting its potential as an alternative to chemical fungicides for controlling <i>F. oxysporum</i>.