The mechanism of antifungal effects of Monarda citriodora essential oil and trans-cinnamaldehyde as a fumigant against Aspergillus fijiensis, a postharvest pathogen of lemon

The present study examined the antifungal activity of Monarda citriodora essential oil (MEO) and trans-cinnamaldehyde (trans-c) vapours on Aspergillus fijiensis (C4), a new causative agent of postharvest rot of lemon. The disc fumigation method was used to assess the in vitro Minimum Fungicidal Concentrations (MFCs); the MFCs for MEO was 961.53 μL L⁻¹ air, and for trans-c, 96.15 μL L⁻¹ air. The in vitro checkerboard assay showed synergy at two concentrations viz. 240.38 μL L⁻¹ air of MEO +24.03 μL L⁻¹ air of trans-c (N2) and 240.38 μL L⁻¹ air of MEO + 12.015 μL L⁻¹ air of trans-c (O2). The In vivo fumigation with N2 and O2 combinations showed complete inhibition of C4 growth on lemons till seven days post-treatment at 24 ± 2 °C. The vapours showed no significant changes in the weight loss, ascorbic acid content, pH and total soluble solids of treated lemons. Propidium iodide staining and ergosterol content analyses showed that the vapour treatments damaged the fungal plasma membrane, which led to cytoplasmic material leakage. SEM analysis revealed the vapour treated C4 mycelia to be rough, broken and squashed. Overall, our results indicate that the MEO and trans-c vapours irreversibly damage the plasma membrane of A. fijiensis and could be exploited for the postharvest storage of lemons.