Fungitoxic activity of eugenol against Botryitis cinerea may be mediated by fungal peroxidase.

Plant essential oils have shown antifungal activity against several plant pathogenic fungi in pre- and post-harvest. This activity is due to their monoterpenic constituents like eugenol. However, eugenol mechanism of action is not completely clear. We studied the effect of eugenol on Botrytis cinerea (strain B05.10). The effective concentration of eugenol in inhibiting mycelium growth and conidia germination was 1-2 mM and the minimum effective dose of eugenol to protect grape leaves from B. cinerea infection was about 10 mM. Experimental data showed that eugenol causes a release in the B. cinerea culture filtrate of potassium ions and of intracellular material, indicating a damage to cell membrane possibly due to an oxidative stress. However, in comparison to untreated control, no accumulation of reactive oxygen species (measured by xylan orange assay) was observed in B. cinerea mycelia treated with eugenol, while the concentration of antioxidant molecules such as glutathione and cysteine decreased. Since B. cinerea produces high level of peroxidase activity and eugenol is a good competitive substrate of cellular antioxidants, the eugenol activity may be ascribed to the release of hydroxyl radical, a product of the reaction of eugenol with hydrogen peroxide. Moreover, the oxidized eugenol showed the same fungitoxic activity of the not oxidized eugenol.