Inactivation of Pythium aphanidermatum Zoospores Using Cold Plasma and Ozone under Laboratory Conditions

Abstract

A risk associated with recycling irrigation water is the spread of pathogens such as Pythium aphanidermatum in recirculating horticultural systems. The objective of this study was to quantify zoospore inactivation in response to cold plasma, ozone, and oxygenation across a range of oxidation-reduction potential (ORP), dissolved oxygen (DO), and exposure conditions. Deionized water inoculated with P. aphanidermatum zoospores (2500 zoospores·mL −1 ) was treated using cold plasma, ozone, or oxygenation systems. Three experiments evaluated zoospore survival following short-term exposure to treated water for 10 min or 2 h, a single pass of inoculated water through each treatment system, and continuous treatment during recirculation with sampling at increasing ORP levels. In short-term exposure assays, ozone reduced colony forming units (CFUs) beginning at 700 mV and cold plasma at 800 mV, and both treatments resulted in no detectable CFUs at 900 mV. Oxygenation did not reduce CFUs at ambient, saturated, or supersaturated DO levels (5.1, 10.5, and 18.3 mg·L −1 , respectively). With single-pass treatment, ozone reduced CFUs by 100%, cold plasma reduced CFUs by 68.4%, and oxygenation had no significant effect. This corresponded to dissolved ozone concentrations of 0.57 mg·L −1 for ozone and 0.46 mg·L −1 for cold plasma, whereas oxygenation produced no detectable ozone. Under recirculating conditions, CFUs reached nondetectable levels at ∼400 mV after about two tank turnovers for ozone and four to five tank turnovers for cold plasma, corresponding to cumulative ozone doses of 1.1 and 2.1 mg·L −1 , respectively. The effectiveness of ozone and cold plasma for pathogen control depends on system configuration and how the treatment is applied rather than defining dose response on achieving a target ORP alone. Dissolved oxygen is known to be an important determinant of root health in liquid hydroponic culture, but increasing DO alone did not control Pythium .