Discovery of Novel Pyrazole Amides as Potent Fungicide Candidates and Evaluation of Their Mode of Action

A rational molecule design strategy based on scaffold hopping was applied to discover novel leads, and then a series of novel pyrazole amide derivatives were designed, synthesized, characterized, and evaluated for their antifungal activities. Bioassay results indicated that some target compounds such as S3, S12, and S26 showed good in vivo antifungal activities; among them, S26 exhibited commendable in vivo protective activity with an 89% inhibition rate against Botrytis cinerea on cucumber at 100 μg/mL that is comparable to positive controls boscalid, isopyrazam, and fluxapyroxad. Microscopy observations suggested that S26 affects the normal fungal growth. Fluorescence quenching analysis and SDH (succinate dehydrogenase) enzymatic inhibition studies validated that S26 may not be an SDH inhibitor. Based on induction of plant defense responses testing, S26 enhanced the accumulation of RBOH, WRKY6, WRKY30, PR1, and PAL defense-related genes expression and the defense-associated enzyme phenylalanine ammonia lyase (PAL) expression on cucumber. These findings support that S26 not only displayed direct fungicidal activity but also exhibited plant innate immunity stimulation activity, and it could be used as a promising plant defense-related fungicide candidate.