The objective of this study was to investigate the antifungal activity and synergistic effect of cinnamaldehyde combined with antioxidants against wood decay fungi. Five antioxidants, propyl gallate, octyl gallate, quercetin, eugenol and catechin were tested against various wood decay fungi. Octyl gallate and eugenol were found to be the only two antioxidants processed antifungal activities. IC50 values of octyl gallate were 0.47 and 0.04 mM against L. betulina and L. sulphureus, respectively. The IC50 values of eugenol were 0.37 and 0.25 mM against L. betulina and L. sulphureus, respectively. The synergistic effects were also found on the combinations of octyl gallate-cinnamaldehyde and eugenol-cinnamaldehyde. The combination of either using octyl gallate with cinnamaldehyde or eugenol with cinnamaldehyde greatly reduced the concentrations to achieve the inhibitory effect that a higher concentration was needed by octyl gallate, eugenol or cinnamaldehyde alone. The antifungal action of octyl gallate could be attributed to its pyrogallol group functioning as an attached moiety to the hydrophilic portion of the membrane surface and the octyl moiety interfering with the hydrophobic interior surfaces of the membrane. Meanwhile, the synergism of cinnamaldehyde with octyl gallate or eugenol could be due to the interference of fungal cell wall synthesis and destruction on cell wall and membrane plus the additional radical scavenging effect. Results also suggested that antioxidant with fungicidal effect might be a better candidate than pure antioxidant for the system of fungicide/antioxidant.

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