Non-target effects of essential oils on selected beneficial bacteria

Beneficial microorganisms and plant essential oils are key components of sustainable agriculture, but knowledge of their compatibility is limited. We investigated the effect of 23 essential oils on three beneficial bacterial species (Bacillus cereus, Bacillus velezensis and Priestia megaterium) that are commercially used as biostimulants and biocontrol antagonists. Essential oils were tested at a dilution of 1:10, the highest economical use rate anticipated in crop protection applications. Seventeen of the 23 essential oils had no inhibitory activity against any of the three bacterial species. Onion essential oil had strong activity against B. velezensis and no activity against B. cereus or P. megaterium. Palmarosa and pimenta essential oils were active against B. velezensis and inactive against B. cereus and P. megaterium. Coriander essential oil was inactive against B. velezensis, active against B. cereus, and strongly active against P. megaterium. Oregano essential oil was active against the three bacterial species while garlic essential oil was active against B. cereus and strongly active against B. velezensis and P. megaterium. Allyl disulfide, carvacrol, eugenol, and geraniol, the major constituents of garlic, oregano, pimenta leaf, and palmarosa/coriander essential oils, respectively, were active or strongly active against at least two of the three bacterial species suggesting their contribution to the antimicrobial activity of these essential oils. These findings indicate that most of the essential oils tested are compatible with beneficial microbes and can easily be integrated with beneficial microbes in organic cropping systems. Further studies should investigate how to harness the pesticidal properties of the six active essential oils while limiting their detrimental effect on beneficial microbes.