Plasma activated water (PAW) for grapevine yellows disease management

Plasma activated water (PAW) is produced treating distilled water with atmospheric pressure plasmas and possesses significant antimicrobial efficacy due to the synergistic effect of reactive oxygen and nitrogen species (RONS) produced by plasma treatment [1]. 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 due to bacterial pathogens and phytoplasmas (bacteria lacking the cell wall). In this study, the efficacy of PAW was tested in vivo in vineyards on phytoplasma infected grapevine plants showing symptoms associated to the grapevine yellows disease. This is among the most economically relevant infectious diseases affecting vineyards in the European continent, able to reduce up to 100% the wine production in Italy and in the other main viticultural areas located in France, Spain and South Eastern Europe countries. Sterile deionized water was exposed to a nanosecond pulsed dielectric barrier discharge, operating in ambient air for 10 min treatment with a peak voltage of 19 kV and a pulse repetition frequency of 1 kHz, which induced in PAW the presence of nitrates, nitrites and peroxides, and a pH decrease [2]. During 2015 preliminary trials using PAW were carried out in five Glera and one Chardonnay vineyards on grapevine plants preliminary tested to verify the presence and identity of phytoplasmas in both symptomatic and asymptomatic plants. Experiments were performed injecting in three different times of the year (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 untreated plants were used as control to verify the evolution of the disease. The liquids were injected into the plant vascular tissues 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 treated and untreated plants did not show symptom reduction. Analyses carried out in September 2015 on samples from PAW treated plants showed a 62% reduction of phytoplasma positive plants; however a 33% reduction of phytoplasma positive plants was detected also in SDW treated grapevines. No phytotoxicity was observed in the phytoplasma-free plants treated with PAW. These results are promising and have prompted us to enlarge the trials increasing the number of tested grapevine plants to achieve statistically evaluable data.