Evaluating the inhibitory effect of low-temperature volume barrier discharge 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 considered, including total treatment time, applied voltage, and agarized medium composition. As previously observed, the inhibitory effect of the treatments progressively increased with the total time of exposure to VDBD and decreased with the complexity of cellular structures of the analyzed fungal species. Another main factor influencing efficacy is the uniformity of the distribution of the plasma microdischarges. 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 could strongly influence plasma generation. To better understand this point, agarized media structure was analyzed through 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 total treatment time, inhibited completely the conidial germination of B. cinerea, while 20 s were required for both M. fructicola and F. graminearum instead, the total inhibition of conidial germination was shown after 60 and 180 s of treatment for A. alternata and A. carbonarius, respectively.