| Summary: | The yield and quality of potato can be severely affected by bacterial ring rot, which is caused by <i>Clavibacter michiganensis</i> subsp. <i>sepedonicus</i> (Cms). Recently, using natural compounds to control bacteria has received more attention. In this study, five antibacterial compounds from ethyl acetate (EtOAc) extract of Fructus <i>Polygoni orientalis</i> (FPO) against Cms were isolated and the most active compound was screened. Five active compounds were identified as 3,3′-di-<i>O</i>-methylellagic acid (<b>1</b>), 3,3′-di-<i>O</i>-methylellagic acid-4-<i>O</i>-<i>β</i>-<i>D</i>-glucopyranoside (<b>2</b>), dihydroquercetin (<b>3</b>), protocatechuic acid (<b>4</b>) and quercetin (<b>5</b>). Compound <b>3</b> (dihydroquercetin, DHQ) was confirmed as the most active compound. The diameter of inhibition zone (DIZ), minimum inhibitory concentration (MIC), protective efficiency and curative efficiency of DHQ were 22.50 mm, 0.313 mg/mL, 84.49% and 79.63%, respectively, which exceeded these of thiophanate-methyl (TM) in antibacterial activity assays; this indicated that DHQ had satisfactory antibacterial activities against Cms in vitro and in vivo. Results of cell membrane damage assessments indicated that DHQ could reduce membrane potential (MP), disrupt the cell membrane integrity, and promote the leakage of nucleic acids and proteins. Overall, these findings suggested that DHQ could serve as a promising lead molecular against Cms, which could provide a basis for its further derivatization.
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