Use of plant-made essential oils as biological seed treatments in soybean

Soybean (Glycine max (L) Merrill) seed is commonly treated with chemical compounds to extend the cycle of the crop and protect the seed from early season pathogens. However, in organic and sustainable agriculture, chemical seed treatments are restricted and planting date therefore has to be delayed. Seed treatments based on essential plant oils emerge as an alternative because of their natural origin. Seed producers could benefit from these treatments because treated seed can be sold as grain instead of having to dispose the chemically treated seed. The objective of this study was to identify essential oils with antifungal and antibacterial properties to be used as seed treatment in soybean. To achieve this goal, essential oils were initially screened in vitro against common soybean pathogens. Efficacy of essential oils of anise, basil, bay, black pepper, caraway, cinnamon, clove, coriander, lavender, lemongrass, nutmeg, oregano, peppermint, rosemary, sage, savory, tea tree and thyme was tested against Pythium sp, Phomopsis sp. and Pseudomonas syringae pv. glycinea. Minimum concentration of selected essential oils capable of inhibiting pathogenic growth was determined. Results from in vitro experiments showed that cinnamon, clove, lemongrass and oregano essential oils controlled Pythium at concentrations as low as 200 [Mu]L L⁻¹ of the oils. Cinnamon, clove, oregano and savory controlled Phomopsis at concentrations as low as 400 [Mu]L L⁻¹ of the oils. Oregano and savory were able to control Pseudomonas at concentrations as low as 400 [Mu]L L⁻¹. Additionally, essential oils were tested for potential phytotoxic effects over soybean germination, at the minimum inhibitory concentration and at double of the minimum inhibitory concentration. No deleterious effects of the oils on seed germination were observed. Finally, infected seeds were treated with essential oils and efficacy of the oils as seed protectants was examined. Germination tests results did not show an advantage of seeds treated with essential oils over non-treated seeds (P\u3c0.05). Seed treatment could lead to a significantly higher plant stand when conditions are adverse for the seed. However, in our results, levels of seed infection were low and differences in germination were not observed (P\u3c0.05)