CCD-screened Rheum rhabarbarum extract combined with fatty acid ethyl esters reduces powdery mildew severity in cucumber: from antifungal testing to greenhouse validation

Powdery mildew caused by Sphaerotheca fuliginea is the most common and damaging fungal disease affecting cucumber crops, with significant impact on yield and quality. Growing concerns over fungicide resistance and environmental safety have driven interest in plant-based biocontrol alternatives. This study evaluated different formulations of Rheum rhabarbarum root extract to mitigate powdery mildew caused by S. fuliginea on Cucumis sativus L. Extract screening was performed using central composite design, targeting maximum total polyphenolic content and antioxidant capacity. Out of the twenty experimental variants tested, the extracts with the highest polyphenols concentration or antioxidant activity were subsequently assessed in vitro for antifungal activity against Fusarium oxysporum f. sp. lycopersici as a model pathogen. A formulation combining R. rhabarbarum root extract with fatty acid ethyl esters derived from natural oils was then advanced to greenhouse evaluation. Prophylactic foliar application of a tank mixture consisting of R. rhabarbarum root extract (8 L ha − 1 ) and fatty acid ethyl esters (4 L ha − 1 ) reduced powdery mildew severity in C. sativus seedlings by up to 78% relative to the untreated control and had a numerically higher biological efficacy than the tested reference fungicide. These findings suggest that experimentally screened plant-based formulations merit further investigation as potential biocontrol options for powdery mildew in cucumber production.