The aim of evaluating electrolysed oxidising water (EOW) for controlling botrytis (Botrytis cinerea), powdery mildew (Sphaerotheca pannosa) and downy mildew (Peronospora sparsa) using two greenhouse rose varieties was to verify this new technology’s effectiveness by contrast with traditional chemical handling. Two concentrations of hypochlorous acid in EOW were produced during electrolysis in a P1200 cell and applied at three application volumes using stationary hydraulic pressure equipment. The commercial rose varieties (Rosa sp) used here were the Orlando and Versilia varieties. The EOW used was slightly acidic type C, pH 5.0, having high-oxide-reduction potential (ORP) +850 millivolts (mV). Two types of EOW were evaluated for this study: EOW C-50 (50 ppm HOCl) and EOW C-75 (75 ppm HOCl). The test results showed the feasibility of EOW as a fungicide at the volumes tested here, providing significant control of powdery mildew which has high incidence due to climatic conditions. The best disease control was achieved by applying the highest volume; however, some curled leaflets appeared with this application.