Antifungal properties of terpenoids in Picea abies against Heterobasidion parviporum

The antifungal properties of 14 major oleoresin‐constitutive terpenoids of Norway spruce (Picea abies) against Heterobasidion parviporum were evaluated in vitro at three gradient concentrations, 0.1, 0.2 and 0.4 μmol/cm², on potato dextrose agar medium. Eight monoterpene hydrocarbons (+)‐ and (?)‐α‐pinene, (?)‐β‐pinene, (+)‐3‐carene, myrcene, (+)‐ and (?)‐limonene, terpinolene; four oxygenated monoterpenes α‐terpineol, terpinen‐4‐ol, 1,8‐cineole, bornyl acetate; and two resin acids abietic acid and dehydroabietic acid were selected. Abietic and dehydroabietic acids showed the highest antifungal activities at all concentrations. Among oxygenated monoterpenes, bornyl acetate and α‐terpineol showed antifungal activity at the highest concentration. Among monoterpene hydrocarbons, (+)‐α‐pinene showed similar activity to terpinen‐4‐ol and 1,8‐cineole at the highest concentration; however, it was lower than α‐terpineol and bornyl acetate. Other monoterpene hydrocarbons inhibited mycelial growth by <10%. Re‐extraction of medium surfaces after the test period revealed that most α‐terpineol and terpinen‐4‐ol remained unchanged on the surface but monoterpene hydrocarbons completely disappeared. However, notable fungal transformed products were observed on surfaces applied with 1,8‐cineole and bornyl acetate. Thus, mycelial growth inhibition of monoterpenoids might be caused by complex functions such as biodegradation and/or detoxification by interaction between mycelium and compound. These results provide a basis for future studies considering the role of chemodiversity in the comprehensive chemical defence of P. abies.