Design, synthesis, antifungal activity, and phloem mobility of novel aromatic amide phenylthiazole‐4‐carboxylic acid derivatives as potential succinate dehydrogenase inhibitors

Abstract BACKGROUND Although succinate dehydrogenase inhibitors (SDHIs) display potent antifungal activity, the majority possess weak phloem mobility, limiting their efficacy against root and vascular diseases. To develop novel SDHI candidates with phloem mobility, 36 aromatic (heterocyclic) amide phenylthiazole‐4‐carboxylic acid derivatives were synthesized and subjected to antifungal evaluation. RESULTS In vitro bioassays showed that compounds E11 and E21 exhibited high antifungal activities against Sclerotinia sclerotiorum , with median effective concentration (EC 50 ) values of 0.34 and 0.19 μg mL −1 , respectively, outperforming boscalid (EC 50 = 0.65 μg mL −1 ). Further in vivo experiments and field trials indicated that E11 and E21 displayed stronger antifungal efficacy against sclerotinia stem rot in rapeseed compared with boscalid. Meanwhile, E11 and E21 showed no obvious effects on rapeseed germination and radicle growth at 200 μg mL −1 . Phloem mobility studies revealed that compounds E1–E36 exhibited different degrees of phloem mobility in Ricinus communis L., with E21 achieving the greatest systemic accumulation in stems and leaves of rape seedlings within 24 h. Further antifungal mechanism studies indicated that compounds E11 and E21 had a similar mode of action to boscalid. CONCLUSION These findings suggest that compounds E11 and E21 serve as promising phloem‐mobile SDHI candidates for controlling sclerotinia stem rot in rapeseed. © 2026 Society of Chemical Industry.