In Vitro Antifungal Activity of the Diterpenoid 7α-Hydroxy-8(17)-labden-15-oic Acid and Its Derivatives against Botrytis cinerea

We investigated the inhibitory effect of the natural diterpenoids, 7α-hydroxy-8(17)-labden-15-oic acid (salvic acid, 1), 7α-acetanoyloxy-8(17)-labden-15-oic acid (acetylsalvic acid, 2) and the hemisynthetic diterpenoids 7α-acyloxy-8(17)-labden-15-oic acids derivatives, 7α-propanoyloxy-8(17)-labden-15-oic acid (propanoylsalvic acid, 3), 7α-butanoyloxy-8(17)-labden-15-oic acid (butanoylsalvic acid, 4) and 7α-isopentanoyloxy-8(17)-labden-15-oic acid (isopentanoylsalvic acid, 5), against Botrytis cinerea. Diterpenoid fungitoxicity was assessed using the radial growth test method. All diterpenoids, with the exception of isopentenoylsalvic acid, inhibited the mycelial growth of B. cinerea in solid media. Shortest side-chain diterpenoids were more effective than the derivatives with longer chains in the inhibition of B. cinerea mycelial growth. The results suggest that hydrophobicity and structural features would be important factors in the antifungal effect of these diterpenoids. Studies on a possible action mechanism of natural diterpenoids, salvic acid and acetylsalvic acid, showed that these diterpenoids exerted their effect by a different mechanism. Salvic acid did not alter cytoplasmic membrane or cause respiratory chain inhibition. Instead, acetylsalvic acid affected the cytoplasmic membrane producing leakage of 260-nm absorbing compounds.