The Potential of Ozone Nanobubbles in Suppressing Cucumber Powdery Mildew

The efficacy of ozone nanobubbles (O 3 -NBs) as a foliar treatment to suppress cucumber powdery mildew was evaluated in a controlled bioassay under optimal and suboptimal relative humidity (RH) conditions. O 3 -NB solutions were generated at three concentrations (0.54 ± 0.08, 1.16 ± 0.23, and 2.08 ± 0.22 mg liter −1 ) and compared with oxygen nanobubbles (O 2 -NBs) and distilled water controls. In a 20-day growth chamber bioassay under three RH conditions (≤40%, 65 to 73%, and ≥95% RH), leaves treated with 0.5 mg liter −1 O 3 -NBs exhibited suppression of powdery mildew by approximately 20 ± 3.2% and 33 ± 4.6% compared with the control at moderate and low humidity. However, efficacy declined under high relative humidity (≥95% RH). A 16-week hydroponic study confirmed that foliar application of O 3 -NBs at 0.5 to 2 mg liter −1 caused no adverse effects on cucumber growth, yield, or physical appearance. Although efficacy was lower than typically reported for conventional fungicides, O 3 -NBs offer a potentially safer, residue-free alternative for integrated pest management programs, particularly during withholding periods where chemical fungicides cannot be used.