Rheum palmatum Ethanol Extract Suppresses Strawberry Gray Mold Caused by Botrytis cinerea

Gray mold (Botrytis cinerea) is a destructive strawberry disease that is increasingly difficult to control because of fungicide resistance. We evaluated a 70% ethanol extract of Rheum palmatum rhizome and root (30%, w/v) and its constituents as natural antifungal agents. UPLC-ESI-QTOF-MS identified stilbenes and anthraquinones as the major components; of eight isolated compounds, the stilbene rhapontigenin (23 mg/L) was selected as a quality-control marker. The extract completely inhibited B. cinerea mycelial growth at ≥0.5% (v/v) (LC50 = 0.26%, v/v). In a detached-fruit assay, rhapontigenin, rhapontin and physcion (100 mg/L) each reduced disease by about 50%, exceeding catechin gallate, chrysophanol and emodin (31-36%). A suspension formulation of the extract achieved 66% control of gray mold in a greenhouse trial. R. palmatum extracts, standardized by rhapontigenin, are promising natural fungicides for strawberry gray mold management.