SDH-IN-18 (Compound 3a) is a succinate dehydrogenase (SDH) inhibitor with an IC50 of 8.70 mg/L. It exhibits potent antifungal activity, particularly against Rhizoctonia solani (EC50 0.48 mg/L) and Sclerotinia sclerotiorum (EC50 1.4 mg/L), by disrupting fungal morphology and reproduction, making it a valuable tool for antifungal mechanism and agricultural fungicide research.