Novel Aromatic Carboxamides Containing Ether Group for the Control of Rhizoctonia solani

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.