Comparative Efficacy of × Hesperotropsis leylandii, Platycladus orientalis, and Juniperus communis Essential Oils Against Four Phytophthora Species Common in Nurseries and Forests

Phytophthora species are devastating oomycete pathogens affecting agriculture, horticulture, and natural ecosystems globally, primarily spread through the international trade of nursery stock. While synthetic fungicides remain a primary control method, there is an increasing demand for sustainable biocontrol agents. This study evaluated the inhibitory potential of essential oils from three Cupressaceae species, × Hesperotropsis leylandii (needles), Platycladus orientalis (needles), and Juniperus communis (ripe berries), against four common nursery and forest pathogens: P. cactorum, P. plurivora, P. pseudocitrophthora, and P. × cambivora. The essential oils were evaluated at concentrations of 0.1%, 0.25%, and 0.5% (v/v), while the commercial fungicide Infinito® (propamocarb + fluopicolide) served as a positive control. Results demonstrated significant dose-dependent inhibition across all treatments, with the 0.5% concentration yielding the highest efficacy. Notably, essential oil from J. communis achieved 90.2% inhibition against P. × cambivora, while Pl. orientalis essential oil reached 82.8% and 73.1% inhibition relative to the Infinito® effect against P. × cambivora and P. cactorum, respectively, underscoring the potential antimicrobial properties of these coniferous essential oils compared to the tested synthetic standard. Although the chemical analysis revealed that all investigated essential oils (× H. leylandii, Pl. orientalis, and J. communis) contained significant quantities of α-pinene (32.11, 16.01, and 32.29%, respectively), their chemical compositions differed. Namely, GC analysis revealed the presence of δ-3-carene in × H. leylandii and Pl. orientalis (18.51 and 37.98%, respectively), while cedran-8-ol was detected in significant quantity in Pl. orientalis (19.96%). Sabinene and myrcene (18.52 and 14.57%, respectively), besides α-pinene, were most abundant in J. communis essential oil. The observed differences in the evaluated activity might be due to the determined essential oils’ chemical composition.