Non-thermal plasma as a new alternative technology for effective control of postharvest fungal pathogens

Several techniques can be used to preserve the quality of fruits and vegetables during postharvest storage. Non-thermal plasma, also known as cold plasma, and Plasma Activated Water (PAW) are currently proposed as novel promising tools against microbial contamination of produce, including major fungal pathogens responsible for postharvest decay. Atmospheric air plasma consists of a mixture of electrons, ions, radicals, stable and short living products, such as reactive oxygen and nitrogen species (RONS), and ultraviolet radiation. PAW is enriched in terms of RONS and could be successfully applied during postharvest stages. The effects of both direct and indirect applications of plasma on conidial germination of Botrytis cinerea, Monilinia fructicola, Aspergillus carbonarius, Penicillium italicum, Penicillium digitatum, and Alternaria alternata were evaluated using different plasma sources, i.e. Surface Dielectric Barrier Discharge (SDBD), Atmospheric Pressure Plasma Jet (APPJ), Volume Dielectric Barrier Discharge (VDBD). The complete spore inactivation was obtained after few seconds (VDBD) or minutes (SDBD and APPJ) of exposure. The efficacy of PAW was proved stable in its inhibitory effect for all the tested species with a persistence of at least 30 min after water activation. Major structural damages to conidia surface after plasma treatment were assessed by Scanning Electron Microscopy (SEM) analysis. An early etching and later perforation of cell walls up to complete cell disruption was observed. A decrease in viability of conidia and an increase in their membrane permeability was assessed by fluorescence-based assays. Differences in fungal sensitivity to plasma were observed, with A. carbonarius and A. alternata showing the lowest sensitivity to the treatments. Experiments on cherry fruits artificially inoculated with B. cinerea and M. fructicola and exposed to SDBD plasma demonstrated that the application of cold plasma on fruits may significantly extend their shelf life by direct inactivation of fungi and possible activation of plant defense responses.