Plasma activated water as sustainable tool to control bacterial disease severity on tomato plants

Tomato bacterial spot caused by Xanthomonas vesicatoria (Xv) represent a constant threat to tomato commercial production. Bacterial plant disease control is nowadays limited to copper- or streptomycin-based bactericides, but occurrence of pathogenic strains tolerant to these compounds significantly reduces their effectiveness [1]. As an alternative to copper- or streptomycin-based compounds, few resistance inducers and biocontrol agents are presently used to manage these plant diseases, while prophylaxis remains the most important control measure. In this study, the efficacy of plasma activated water (PAW) as resistance inducer in the control of plant diseases caused by bacterial pathogens was investigated. To assess the PAW efficacy as resistance inducer, tomato plants experimentally inoculated with Xv were used as model. Sterile distilled water (SDW) have been treated with plasma by means of a nanosecond pulsed dielectric barrier discharge (DBD) source, operating at atmospheric pressure and ambient air [2]. PAW was then assayed in greenhouse trials to evaluate its ability to induce resistance in tomato plants against the pathogen. Plants of the tomato cultivars Moneymaker and VF-10, were grown in pots under greenhouse condition until they reached the 3rd - 4th leaf stage. The roots of tomato plants were soaked for 10 min in 500 ml of PAW, SDW (negative control) and acibenzolar-S methyl (a known resistance inducer, used as positive 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. 107 CFU/mL) on tomato leaves. The phytopathometric assessments were carried out 21 days after pathogen inoculation by counting the number of spots on tomato leaves. The plants pretreated with PAW showed a significantly lower number of leaf spots compared to those pretreated with SDW. The relative protection of PAW with respect to the negative control was estimated in the range of 30%-40%, depending on the tomato cultivar. No phytotoxicity effect was observed. These results highlighted that the preventive application of PAW at the root apparatus was able to reduce the disease severity (number of leaf spots) implying the activation of plant defence responses. Further studies on kinetics of defense related genes expression confirmed the ability of PAW as elicitor of the plant immune system as pal, loxf and erf1 genes resulted significantly upregulated after PAW application.