Plasma activated water (PAW) for tomato plants and grapevine disease management

Plasma Activated Water (PAW) represents a promising tool for the improvement of plants growth, probably as a consequence of the presence of reactive oxygen and nitrogen species produced during plasma treatment. These reactive species might also have an important role in plant defence responses, involving both hypersensitive reaction and systemic acquired resistance; thus plant treatment with PAW could represent an innovative alternative in the control of plant diseases caused by bacterial pathogens and phytoplasmas (bacteria lacking the cell wall). In this study, the efficacy of PAW was tested in vivo on i) tomato plants, experimentally inoculated with Xanthomonas vesicatoria (Xv) under greenhouse conditions, and ii) phytoplasma infected grapevine plants in vineyards (grapevine yellows diseases, associated to the presence of phytoplasmas, are an economically relevant diseases affecting vineyards in the European continent). Sterile deionized water was treated by means of a nanosecond pulsed dielectric barrier discharge, operating in ambient air; 10 min treatment with a peak voltage of 19 kV and a pulse repetition frequency of 1 kHz induced in PAW the presence of nitrates, nitrites and peroxides, and a pH decrease. On tomato plants, roots were soaked in PAW before pathogen inoculation; as a consequence, a reduction of Xv disease severity was observed (relative protection of ca. 35%); moreover, PAW did not exert any phytotoxicity on tomato plants. Focusing on the grapevine plants, during 2015 preliminary trials using PAW were carried out in five Glera and one Chardonnay vineyards; nested PCR/RFLP assays were performed in October 2014 on symptomatic and asymptomatic plants to reveal the presence and identity of phytoplasmas. Experiments were performed, inoculating in three different times of the year (on April, June and July 2015) 10-20 ml of PAW or sterile distilled water (SDW, as control) on each selected phytoplasma-infected and phytoplasma-free (to investigate any possible phytotoxicity caused by PAW treatment,) plant; the remaining plants of the vineyards were used as untreated control to verify the natural behaviour of the disease. The liquids were injected into the plant vascular tissue by using specially adapted syringes, since with commercially available devices the injection time would have been too long and would have reduced PAW activity; injections were carried out at different daytimes to evaluate the grapevine absorbance ability. In a relevant number of Glera cases, the PAW treated symptomatic plants showed reduction of symptoms and a production increase, while the SDW and untreated plants did not show any symptom reduction. PCR/RFLP analyses carried out in September 2015 on leaves of PAW treated plants showed a reduction of phytoplasma positive plants (62%); it is worth noticing that a reduction (33%) of phytoplasma positive plants was observed also for plants treated with SDW. No phytotoxicity was observed in the phytoplasma-free plants treated with PAW. At this stage of the research results are very promising and have prompted us to enlarge the trials increasing the number of tested grapevine plants, in order to achieve an increase of statistically valuable data.