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Abstract BACKGROUND The discovery of antimicrobial ingredients from natural products could be an effective way to create novel fungicides. Rubia cordifolia L., a traditional Chinese herb, may have antimicrobial effects on plant pathogens according to our previous screening study. RESULTS Rubia cordifolia L. extracts had moderate inhibitory effects on apple Valsa canker ( Valsa mali ) and tomato grey mould ( Botrytis cinerea ) at a concentration of 10 mg mL −1 . With the use of bioguided isolation methods, eight compounds ( 1–8 ) were obtained, including the new compound 2,2,6‐trimethyl‐6‐(4‐methylphenyl)‐tetrahydropyrano‐ 3‐ol ( 7 ), and seven quinone derivatives. Two compounds, mollugin ( 1 ) and 1,3,6‐trihydroxy‐2‐methylanthraquinone ( 6 ), were found to exhibit outstanding antifungal activities against V. mali and Phytophthora capsici Leon. The half maximal effective concentration (EC 50 ) of compound 1 and compound 6 against V. mali were 79.08 and 81.78 μg mL −1 , respectively, and the EC 50 of compound 6 against P. capsici was 4.86 μg mL −1 . Compound 1 also showed excellent activity against tobacco mosaic virus (TMV). The inactive, inductive, protective and curative activities against TMV were 84.29%, 83.38%, 86.81%, and 60.02%, respectively, at a concentration of 500 μg mL −1 , which were all close to or greater than that of the positive control (100 μg mL −1 chitosan oligosaccharide, COS). CONCLUSION Mollugin and 1,3,6‐trihydroxy‐2‐methylanthraquinone are potentially valuable active compounds that lay a foundation for research on botanical fungicide products derived from R. cordifolia L. and provide lead structures for quinone derivative synthesis and structural modification. © 2024 Society of Chemical Industry.
Wang et al. (Wed,) studied this question.