Nano-emulsified savory and thyme formulation show limited efficacy to suppress Pectobacterium carotovorum subsp. carotovorum compared with pure oil

Abstract

Herbal volatile oils suggested as preserver coatings for potato (Solanum tuberosum L.) tubers against soft rot, a postharvest bacterial disease. The antibacterial potential of savory (Satureja khuzistanica Jamzad) and thyme (Zataria multiflora Boiss) stable nano-emulsified oil prepared with tween 80 and lecithin was evaluated towards Pectobacterium carotovorum subsp. carotovorum (Pcc). For this purpose, the antimicrobial activity of the nanoemulsion was compared with that of the pure oil by growth inhibition zone using well diffusion assay, minimum inhibitory and bactericidal concentrations (MIC and MBC) using microbroth dilution procedure, time-kill assay and potato tuber decay trial. The preventative efficacy of both optimized nanoemulsions was similar or reduced compared with the bulk oil on Pcc in plate assay. Savory and thyme nano-formulations displayed antibacterial capability against Pcc with a MIC of 0.59 and 0.29 g L−1, and MBC of 1.19 and 0.59 g L−1, respectively. The colony counts for Pcc were reduced to 0 in the range of 3–6 h based on time kill curves of MIC value. S. khuzistanica and Z. multiflora oils were as or more effective than nano-emulsions in the growth kinetic studies, although the viable cells number of Pcc was prevented at the MIC for both bulk and nano-formulations. The nano-encapsulated savory and thyme oil decreased disease development on intact potato tubers by 29.12 and 20.44 % in comparison with the control in the maceration assay. The data reported here, indicated that the usage of S. khuzistanica and Z. multiflora oil decreased soft rot incidence 1.82 and 2.18 times more than the nano-formulations, respectively. It seems that application of emulsified oil does not always more functional than bulk oil in the field of plant pathogens management.