| Summary: | Rice sheath blight, caused by <i>Rhizoctonia solani</i>, is a globally important rice disease and the increasing resistance of this pathogen highlights the need for new active compounds against rice sheath blight. In this study, natural <i>β</i>-carboline alkaloids were optimized to obtain a series of 1,2,4,9-tetrahydro-3-thia-9-aza-fluorene derivatives and evaluated for their fungicidal activity and mode of action against <i>R. solani</i>. Of these compounds, <b>18</b> exhibited significant in vitro fungicidal activity against <i>R. solani</i>, with an EC<sub>50</sub> value of 2.35 μg/mL, and was more active than validamycin A. In vivo bioassay also demonstrated that <b>18</b> displayed superior protective and curative activities as compared to validamycin A. Mechanistically, <b>18</b> not only induced the loss of mitochondrial membrane potential and accumulation of reactive oxygen species, but also interfered with DNA synthesis. Therefore, compound <b>18</b> displayed pronounced in vitro and in vivo fungicidal activity against <i>R. solani</i> and could be used as a potential candidate for the control of rice sheath blight.
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