Plasma activated water as sustainable method to control bacterial leaf spot of tomato

Plasma Activated Water (PAW) represents a promising tool for the improvement of plants growth [1]; moreover, the antimicrobial activity of PAW has been reported and correlated to the production of nitrates, nitrites and hydrogen peroxides and the reduction of pH [2]. These reactive species have an important role in plant defense responses, involving either hypersensitive reaction and systemic acquired resistance (SAR) and also, being bactericidal agents. Plant treatment with PAW might represent an innovative alternative in the control of plant diseases due to bacterial pathogens [3]. This study was aimed to assess the PAW efficacy in planta as resistance inducer using tomato-Xanthomonas vesicatoria (Xv) model. Sterile distilled water have been treated with plasma by means of a nanopulsed dielectric barrier discharge (DBD). The electrodes of the plasma source consist of alluminum foils, covered by 1 mm thick polystyrene as dielectric; one of the polystyrene dielectrics is shaped with edges in order to contain the water to be treated. The plasma forms in the 1 mm region between the liquid surface and the upper dielectric layer. Ten minutes treatment time with a peak voltage of 19 kV and a pulse repetition frequency of 1,000 Hz induced in plasma treated water the presence of nitrates, nitrites and peroxides and a change of pH. Under greenhouse condition, plants of two tomato cultivars, “Moneymaker” and “VF-10”, were grown in pots containing a mixture of peat and vermiculite as substrate until they reached the 3rd - 4th leaf stage. They were then soaked from the roots for 10 min in PAW, SDW (positive control) and in a solution of acibenzolar-S methyl (resistance inducer, as negative control) six-days before the inoculation with the pathogen. Experimental inoculation with Xv strain IPV-BO 2684 was carried out by spraying the pathogen aqueous suspension (ca. 1107 CFU/mL) on tomato leaves. The phytopathometric assessments were conducted 21 days after pathogen inoculation. The tomato plants showed a statistically significantly lower number of leaf spots with respect to those pretreated with SDW. The relative protection of PAW related to negative control was calculated as approx. 30% to 40%, depending on tomato cultivar assayed. No phytotoxicity effect has been observed. These results highlighted an apparent ability of PAW to induce a plant defense response; in fact, the preventive application at the root apparatus, provoked a reduction of the disease severity (number of leaf spots) implying the activation of plant defence responses.