Comparison of Nanoemulsions Prepared with Cinnamon, Eucalyptus, Mountain Pepper, and Palmarosa Essential Oils for Antifungal Application

Essential oils (EOs) have demonstrated antifungal activity; however, their low solubility in water and physical instability under environmental conditions limit their agricultural applications. To overcome these challenges, oil-in-water nanoemulsions of EOs (EO-NEs) were developed using ultrasonic dispersion. The main chemical constituents present in the EOs were quantified using GC–MS. The EOs-NEs, containing cinnamon (CEO-NE), eucalyptus (EEO-NE), mountain pepper (MPEO-NE), or palmarosa (PEO-NE) as the oil phase and water with Tween 80 surfactant as the aqueous phase, were formulated and characterized. The EO-NEs with 0.5% w/v Tween 80 were evaluated for various nanoemulsion characteristics, including hydrodynamic diameter, PDI, zeta potential, surface tension, storage stability, and centrifuge stability. Tween 80 reduced the surface tension of the EO-NEs, and the zeta potential of the formulations was slightly negative, indicating oil droplet stabilization through steric hindrance due to the non-ionic surfactant. Additionally, the EO-NEs remained stable at room temperature for 90 days, maintaining a hydrodynamic diameter below 200 nm and PDI < 0.3, without phase separation. Accelerated stability testing using a Lumisizer confirmed that the formulations were relatively stable against separation phenomena. In vitro tests demonstrated the antifungal efficacy of the EO-NEs, with 0.5 mg/mL of CEO-NE, MPEO-NE, and PEO-NE completely inhibiting the mycelial growth of Colletotrichum lindemuthianum, the fungus responsible for the most aggressive foliar disease in beans. Additionally, spore germination of the pathogen was entirely inhibited with 1.0 mg/mL of CEO-NE. These results indicate that EO-NEs are stable and have the potential for improving the applicability of EOs in agricultural applications.