Investigate the adverse effects of foliarly applied antimicrobial nanoemulsion (carvacrol) on spinach

Abstract Encapsulated essential oils are being developed as natural antimicrobials and insecticides for use in sustainable agriculture applications. Although these plant-based pesticides are considered to be more environmentally friendly, they can have adverse effects on agricultural crops, thereby reducing yields. Our aim was therefore to establish the potential phytotoxicity of carvacrol nanoemulsions on a commercially important crop (spinach, Spinacia oleracea). Carvacrol oil-in-water nanoemulsions were fabricated by homogenizing 10 wt% oil phase (1:1 carvacrol and medium chain triglycerides) with 90% aqueous phase (1% quillaja saponin in 5 mM sodium citrate buffer, pH 3.5). Nanoemulsions were sprayed onto the spinach leaves by foliar exposure for three days and then the health status of the plants was assessed. At relatively low carvacrol concentrations (0.005 to 0.5%), the plants remained healthy, but at the highest concentration employed (5%) there was a significant reduction in the biomass and chlorophyll content of the spinach, as well as an increase in electrolyte leakage and malondialdehyde formation. We demonstrated that this strong phytotoxicity was due to the carvacrol oil, rather than due to other constituents in the nanoemulsions. Our results show that there is a critical threshold concentration for use of essential oil nanoemulsions as pesticides on crop plants.