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Inhibitory Effects of Essential Oils for Controlling Phytophthora capsici

Essential oils (EOs) were studied in vitro and in vivo for inhibiting Phytophthora capsici. Mycelial growth of P. capsici was examined on EO-amended media or after exposing it to EO volatiles. The efficacy of EOs was determined by estimating the effective concentration for 50% inhibition of P. capsici mycelial growth (EC₅₀). Among 14 tested commercial products, oregano, palmarosa, and red thyme EOs had the lowest EC₅₀ values (<0.15 μg/ml) for inhibiting the production and germination of sporangia and zoospores, and mycelial growth of P. capsici. The EOs had the same range of effect on inhibiting some mutant P. capsici isolates resistant to fluopicolide and zoxamide. P. capsici population in soil was reduced by the three EOs. Zucchini (Cucurbita pepo) fruit were protected against P. capsici infection when they were sprayed with red thyme (0.1 μg/ml) or oregano and palmarosa (0.2 μg/ml) EOs. Zucchini seedling emergence was affected by oregano, but not by red thyme. Zucchini seedlings survived in P. capsici–infested soil treated with red thyme at 0.1 μg/ml, while all of the nontreated seedlings died. These results taken together suggest that oregano, red thyme, and palmarosa EOs may be potential components for integrated management of P. capsici.