On the effectiveness of dielectric barrier discharge treatment against phytopathogenic fungi

The inhibitory effect of a square wave modulated plasma (duty cycle 20%) Volume Dielectric Barrier Discharge (VDBD) at atmospheric pressure in ambient air was evaluated on conidial germination of different fungal species: Botrytis cinerea, Monilinia fructicola, Aspergillus carbonarius, Fusarium graminearum and Alternaria alternata. Several factors potentially influencing the efficacy of plasma treatment were studied, such as total treatment time, applied voltage, and agarized medium composition. As previously observed the inhibitory effect of the treatment progressively increased with the extension of the time of exposure to VDBD and decreased with the complexity of cellular structures of the analyzed fungal species. The uniformity of the plasma, depending on the applied voltage, must be taken into consideration since in filamentary plasma. Factors that could be crucial are the selected applied voltage but also the samples under processing. In in vitro experiments, one crucial point is the response of agarized media (Water Agar (WA), Glucose Agar (AG), Malt Extract Agar (MEA), and Potato Dextrose Agar (PDA)) to the electrical applied field that can strongly influencing plasma generation. To monitor those possible effects, charge-voltage characteristics were monitored for each experiment, showing a different behaviour for different media. To better understand this point agarized media morphology was also analyzed cryo-Scanning Electron Microscopy (cryo-SEM) images and correlated to the different plasma electrical responses in terms of electrical impedance and Charge transfer characteristics in the plasma phase. In the tested experimental conditions, complete inhibition of conidial germination was achieved with different treatment times. For instance, 5 s of total treatment time, inhibited the conidial germination of B. cinerea completely, while 20 s were necessary for both M. fructicola and F. graminearum instead, the total inhibition of conidial germination for A. alternata was reached at 60 s of VDBD treatment. Aspergillus carbonarius was completely inhibited after 180 s of treatment, although the conidial germination was affected by the agarized media used. Optical emission spectroscopy of N2 molecular bands in order to derive information on gas plasma temperature and the reduced electric field (E/N) was also derived. Nevertheless, we must point out that in a side experiment performed in humid air, the comparison between E/N derived from the discharge and direct measurements of E-Field obtained using the Electric Field Induced Second Harmonic generation (E-Fish) showed that in certain conditions the assumed local thermal equilibrium is not satisfied inducing an error in an estimation of plasma-induced E-Field.