Vapour-phase efficacy of selected essential oils individually and in combination against Aspergillus flavus, A. niger, Fusarium proliferatum, and Curvularia lunata
Keywords:grains, essential oil vapour, storage fungi, mycelial growth, synergism, Thailand
Grain storage plays a crucial role in ensuring food security to Thai farmers so sustainable protection methods against deleterious microorganisms, such as fungi, are necessary. Essential oils (EOs) have demonstrated broad-spectrum fumigant antifungal activity against most storage fungi that are problematic in Thailand. Four storage fungi (Aspergillus flavus, A. niger, Curvularia lunata and Fusarium proliferatum) were isolated from dried rice and corn grains (stored for at least six months). EOs were extracted by hydrodistillation from clove buds (Syzygium aromaticum), fruit peel and leaves of makrut lime (Citrus hystrix), eucalyptus leaves (Eucalyptus sp.) and lemongrass stems (Cymbopogon citratus). The fungi inoculated on PDA in plastic cups were exposed to each EO vapour originating from paper disc attached in the lids in an inverted position. The minimum inhibitory concentration (MIC) for each EO was determined. Selected MICs were combined in a binary manner and similarly tested against the fungi. Fractional inhibitory concentration indices (FICI) were determined for each combination. Lemongrass and makrut lime leaf EOs were the most effective with MICs of 0.09 µL/mL against Curvularia lunata and 0.19-0.28 µL/mL against A. flavus, A. niger and F. proliferatum. Eucalyptus oil produced the least effective vapour (MIC 0.56-0.74 µL/mL) against all tested pathogens. A combination of lemongrass and makrut lime leaf EOs was partially synergistic against A. niger (FICI=0.75) but was fully synergistic against the other three fungi tested (FICI=0.5). The EOs from lemongrass and makrut lime leaf have potential to suppress the growth of the four grain-storage fungi tested.