Essential Oils of Thymus Species Against Phytophthora Species: A Structured Review and Novel In Vitro Evaluations

Phytophthora species are among the most destructive plant pathogens worldwide, causing severe losses in agricultural, forest, and natural ecosystems. In recent years, the management of Phytophthora diseases has increasingly shifted toward eco-sustainable strategies, with growing interest in plant-derived extracts, particularly essential oils, as low-risk alternatives to synthetic fungicides. In this study, a structured review was combined with new in vitro assays to assess the antifungal activity of essential oils from Thymus vulgaris (TV-EO) and T. serpyllum (TS-EO) against P. cinnamomi, P. drechsleri, P. cactorum, P. citrophthora, P. nicotianae, P. palmivora, and P. infestans. Literature searches were conducted in April 2025 using theWeb of Science and Scopus databases, following PRISMA guidelines, with the search term “Thymus” or “Thyme” and “Phytophthora”. Twenty studies included in the review demonstrated that the activity of Thymus essential oils against Phytophthora species was highly variable and shaped by chemotype, Thymus species, pathogen, and experimental setup. Additional in vitro assays further confirmed a clear dose-dependent inhibitory effect for both TV-EO and TS-EO. TS-EO consistently exhibited stronger activity than TV-EO, likely reflecting its carvacrol-rich chemotype, while thymol-based TV-EO showed lower but still significant inhibition depending on the pathogen species. Overall, these results highlight the potential of Thymus essential oils as eco-friendly tools for the management of Phytophthora diseases. However, the strong dependence on chemotype, pathogen species, and assay conditions underscores the need for standardized testing, detailed chemical characterization, and in vivo validation.