Novel Aromatic Carboxamides Containing Ether Group for the Control of <i>Rhizoctonia solani</i>

In the ongoing search for more potent succinate dehydrogenase inhibitors (SDHIs) against Rhizoctonia solani, 30 novel aromatic carboxamides containing an ether group were designed and synthesized. Most compounds, especially those with a pyrazole ring, showed excellent in vitro antifungal activity. Notably, compound 14b stood out as the most promising candidate, with a half maximal effective concentration (EC₅₀) value of 0.019 µg/mL, about 24 times more potent than boscalid (EC₅₀ = 0.464 µg/mL) and three times more effective than fluxapyroxad (EC₅₀ = 0.056 µg/mL). Furthermore, the detached leaf assay revealed that 14b significantly suppressed R. solani growth on rice leaves, with protective (90.28%) and curative (88.84%) efficacy at 100 µg/mL, outperforming boscalid (71.73% and 70.33%, respectively) and fluxapyroxad (85.98% and 83.49%, respectively), indicating its potential for effective application in real-world agricultural settings. In addition, the succinate dehydrogenase (SDH) inhibition assay showed that 14b inhibited SDH with a half maximal inhibitory concentration (IC₅₀) of 2.41 µM, stronger than boscalid (4.34 µM) and fluxapyroxad (3.90 µM). Further molecular docking suggested 14b shared a similar antifungal mechanism of action with fluxapyroxad. These results highlighted compound 14b as a promising SDHI fungicide candidate for controlling R. solani, deserving further study.