Antimicrobial activity of essential oils and pure oil compounds against soilborne pathogens of vegetables

The antimicrobial effects of essential oils and pure oil compounds on mycelial growth of soilborne pathogens causing vegetable diseases in Australia was investigated. Fourteen essential oils and four pure oil compounds were evaluated using contact in vitro bioassays to determine optimum concentrations which can inhibit growth or kill pathogen mycelium. Three essential oils (thyme, clove bud and origanum) and all four pure oil compounds showed broad-spectrum and dose-dependant inhibitory and/or biocidal activity against mycelium of key soilborne pathogens. These treatments also showed antimicrobial activity against Trichoderma atroviride, which is a beneficial soil fungus. In pots, thyme, clove bud and origanum oils applied pre-planting as 5 % aqueous emulsions (5.0 ml/150 cm3) controlled Rhizoctonia solani AG2.1 infection on broccoli seedlings. However clove bud and origanum oils were phytotoxic at 10 % in soil. This study showed that essential oils have potential for controlling soilborne diseases by reducing the spread and viability of pathogen mycelium in soil. This information will be useful for the development of new and existing products which contain plant extracts for use to manage soilborne vegetable diseases in the field.