The antifungal mechanism of Monarda citriodora essential oil, hexanal and their combined vapours on Aspergillus foetidus

Abstract

Background: The increased reliance on synthetic fungicides for managing postharvest pathogens resulted in the rise of fungicide-resistant pathogen strains. Previous studies have reported essential oils (EOs) as a promising alternative to synthetic fungicides. However, higher concentrations are required for their practical application, which can alter the taste profile of the stored produce. Synergistic combinations of EO volatiles can reduce the active concentrations, thereby mitigating undesirable taste profile changes. However, very few studies have emphasised the changes in the mode of action of EO volatiles when applied as a synergistic combination. This study aimed to decipher the mode of action of Monarda citriodora essential oil (MEO) and hexanal vapour combination (M + H) against the newly identified postharvest pathogen of apple Aspergillus foetidus. Results: The M + H vapour treatment synergistically inhibited the spore germination of A. foetidus at the concentration of 144.23 μL L−1 air (MEO) and 96.15 μL L−1 air (hexanal). Propidium Iodide staining and ergosterol content analysis revealed that vapours compromised the plasma membrane integrity. Hence, there was an extracellular leakage of cytoplasmic components, reduced extracellular pH and increased conductivity. Furthermore, calcofluor white staining showed reduced fluorescence intensity, indicating a compromised cell wall. Reduced chitin content in the cell walls of vapour treated mycelia authenticated the results. 2′,7′-Dichlorofluorescin diacetate (DCFDA) staining revealed the generation and accumulation of Reactive Oxygen Species (ROS) within the vapour treated mycelia. Conclusion: The accumulated ROS interferes with ergosterol and chitin synthesis leading to the collapse of the plasma membrane and cell wall

consequently, extracellular leakage occurs. © 2023 Elsevier Ltd