In vivo investigation on the effects of plasma activated water against plant pathogenic bacteria.

The use of Plasma Activated Water (PAW) on plants has been reported as a promising tool for the improvement of plants growth [1]; moreover, plasma treatment of water is known to induce the formation of nitrates, nitrites and peroxides and a change in pH, producing PAW with antimicrobial properties [2]. Indeed, conventional management of plant diseases caused by bacteria has been mainly focused on the use of copper compounds and a few biofungicides or, in the case of phytoplasmas, on insect vector chemical control and on infected plant rouging. Treatment of infected plants with PAW represents an innovative alternative in the control of plant diseases caused by phytopathogens. Fundamental investigations are needed to understand the antimicrobial activity of PAW on plants and the possible phytotoxicity induced by such a treatment. In this work, the antibacterial properties of PAW have been investigated with in vivo tests. Experiments were carried out using Sterile Distilled Water (SDW) treated for 10 minutes by means of a dielectric barrier discharge driven by nanosecond pulses with a peak voltage of 19 kV and a pulse repetition frequency of 1 kHz. PAW antibacterial effects were tested on two vegetal models: experimentally infected tomato plants and periwinkles micropropagated shoots. In the case of tomato plants, the roots were soaked for 10 min into PAW or into SDW (positive control); then, the plants were experimentally inoculated by spraying a water suspension of Xanthomonas vesicatoria (Xv) on the leaves and the disease severity was assessed 21 days after. A reduction of the number of leaf spots caused by Xv was observed in the tomatoes plants soaked in PAW compared to positive control. Moreover, no phytotoxicity effect has been detected on treated plants. In the second model, periwinkle micropropagated shoots, maintained in tubes containing MS agar-solidified medium [3] infected with ‘Candidatus phytoplasma asteris’ were treated adding 1 ml of PAW to the surface of the agar in the micropropagation tubes. Preliminary results indicate that treatment by PAW does not induce shoot toxicity and allows an improved detection of phytoplasma presence in shoots and also in liquid phytoplasma isolation medium [4], indeed, after direct PCR, amplicons of the expected lengths were obtained from all the treated shoot, while the use of nested-PCR was required to obtain specific primers amplification from untreated shoots. The reduced symptomatology and the lack of bacterial endophytic colony formation from liquid isolation medium inoculated with treated shoots, induce to speculate that PAW interacts with phytoplasmas and/or with endophytes viability.