Design, Synthesis, and Antifungal Evaluation of Novel Pyrazole-5-sulfonamide Derivatives for Plant Protection

To develop further novel environmentally friendly antifungal agents with high efficacy, a series of pyrazole-5-sulfonamide derivatives were designed and synthesized by using the active molecules synthesized in previous works as lead compounds. Their antifungal activities were evaluated in vitro against ten highly destructive plant pathogenic fungi. The bioassay results indicated that more than half of the target compounds displayed potent antifungal activities (inhibition rate ≥85%) against Valsa mali and Sclerotinia sclerotiorum at 20 mg/L. Among them, compound C22 exhibited significant broad-spectrum antifungal activities against V. mali, S. sclerotiorum, Rhizoctonia solani, Botrytis cinerea, and Trichoderma viride, with EC₅₀ values of 0.45, 0.49, 3.06, 0.57, and 1.43 mg/L, respectively. Moreover, compounds C21 and C22 exhibited remarkable protective effects on apple Valsa canker similar to tebuconazole (89.5%) at 50 mg/L. Preliminary antifungal mechanism investigations demonstrated that compound C22 may have inhibited V. mali mycelial growth by inducing oxidative damage to the mycelium and compromising the integrity of the cell membrane. Meanwhile, compounds C21 and C22 exhibited no obvious toxicity to worker bees (Apis mellifera ligustica). Taken together, these pyrazole-5-sulfonamide derivatives, particularly compound C22, possess huge potential to be developed as novel environmentally friendly fungicides with high efficacy.