The disclosure relates to a method for the pre or post-harvest treatment of plant products that consist of the application of an at least partially salified form of phosphonic acid and an essential oil or one or more terpene agent, chosen from clove oil, thyme oil, oregano oil, cinnamon oil, peppermint oil, eugenol, isoeugenol, L-carvone, thymol, geraniol, carvacrol, cinnamaldehyde and/or mixtures thereof, at ambient temperature. The phosphonic acid may be prepared in advanced or formed in situ. The method may be used as a fungicidal or bactericidal treatment and may further contain organic solvents and emulsifiers.
