Formulated plant extracts and oils were investigated for control of diseases caused by Phytophthora spp. Soil infested with chlamydospores of Phytophthora nicotianae was treated by incorporating 1, 5, and 10% aqueous emulsions of formulations containing clove oil, neem oil, pepper extract and mustard oil, cassia extract, synthetic cinnamon oil, or the fungicide metalaxyl. Population densities of P. nicotianae were determined at 0 (before treatment), 1, 3, 7, 14, and 21 days after treatment. Treatment of the soil with 5 and 10% aqueous emulsions resulted in significant (P < 0.05) differences among treatment mean values at each assay date. After 1 day, population densities were reduced to below the limit of detection (<0.04 CFU/cm3) in soil treated with 10% aqueous emulsions of two pepper extract-mustard oil formulations and two cassia extract formulations, and near the limit of detection for a synthetic cinnamon oil formulation. Over time, populations of P. nicotianae were detected in the assay; however, after 21 days, populations of P. nicotianae in soil treated with one of the pepper extract-mustard oil formulations still were not detected. Formulations of clove oil, another pepper extract-mustard oil combination, the two cassia extracts, and the synthetic cinnamon oil reduced populations 98.4 to 99.9% after 21 days compared with the nontreated control soil. The neem oil formulation and metalaxyl did not reduce pathogen populations at any rate tested. In the greenhouse after 35 days, 10% aqueous emulsions of a pepper extract-mustard oil formulation, a cassia extract, and the synthetic cinnamon oil formulation suppressed disease development in periwinkle 93.0 to 96.7% compared with the nontreated infested soil. The observed reduction in the pathogen population and significantly more healthy plants in the greenhouse indicates that these formulations of plant extracts and oils could have important roles in biologically based management strategies for control of diseases caused by P. nicotianae.