In this study, antifungal activity of essential oils of Cymbopogon citratus and Ocimum basilicum and two fungicides Mancozeb and Metalaxyl-Mancozeb in six different concentrations were investigated for controlling three species of Phytophthora, including P. capsici, P. drechsleri and P. melonis on pepper, cucumber and melon in vitro and greenhouse conditions, respectively. In vitro, the median effective concentration (EC50) values (ppm) of plant essential oils and fungicides were measured. In greenhouse, soil infested with Phytophthora species was treated by adding 50 ml of essential oils and fungicides (100 ppm). Disease severity was determined after 28 days. Among two tested plant essential oils, C. citratus had the lowest EC50 values (ppm) for inhibition the mycelial growth of P. capsici (31.473), P. melonis (33.097) and P. drechsleri (69.112), respectively. The mean EC50 values for Metalaxyl-Mancozeb on these fungi were 20.869, 20.055 and 17.702, respectively. Chemical analysis of plant essential oils by GC-MS showed that, among 42 compounds identified from C. citratus, two compounds β-Geranial (α-Citral) (39.16%) and z-citral (30.95%) were the most abundant. Under greenhouse conditions, Metalaxyl-Mancozeb caused the greatest reduction in disease severity, 84.2%, 86.8% and 92.1% on melon, cucumber and pepper, respectively. The C. citratus essential oil reduced disease severity from 47.4% to 60.5% compared to the untreated control (p ≤ 0.05). Essential oils of O. basilicum had the lowest effects on pathogens in vitro and greenhouse conditions. These results show essential oils may contribute to the development of new antifungal agents to protect the crops from Phytophthora diseases.
