Design, Bioactivity, and Action Mechanism of Pyridinecarbaldehyde Phenylhydrazone Derivatives with Broad-Spectrum Antifungal Activity

Replacing old pesticides with new pesticide varieties has been the main means to solve pesticide resistance. Therefore, it is necessary to research and develop new antifungal agents for plant protection. In this study, a series of pyridinecarbaldehyde phenylhydrazone derivatives were designed and evaluated for their inhibition activity on plant pathogenic fungi to search for novel fungicide candidates. Picolinaldehyde phenylhydrazone (<b>1</b>) and nicotinaldehyde phenylhydrazone (<b>2</b>) were identified as promising antifungal lead scaffolds. The 4-fluorophenylhydrazone derivatives (<b>1a</b> and <b>2a</b>) of <b>1</b> and <b>2</b> showed highly effective and broad-spectrum inhibition activity in vitro on 11 phytopathogenic fungi with EC₅₀ values of 0.870-3.26 μg/mL, superior to the positive control carbendazim in most cases. The presence of the 4-fluorine atom on the phenyl showed a remarkable activity enhancement effect. Compound <b>1a</b> at 300 μg/mL provided almost complete protection against infection of <i>Alternaria solani</i> on tomatoes over the post-treatment 9 days and high safety to germination of plant seeds. Furthermore, <b>1a</b> showed strong inhibition activity with an IC₅₀ value of 0.506 μg/mL on succinate dehydrogenase in <i>A. solani</i>. Molecular docking showed that both <b>1a</b> and <b>2a</b> can well bind to the ubiquinone-binding region of SDH by the conventional hydrogen bond, carbon-hydrogen bond, π-π or π-amide interaction, π-alkyl interaction, X---F (X = N, C, or H) interaction, and van der Waal forces. Meanwhile, scanning and transmission electron analysis displayed that <b>1a</b> destroyed the morphology of mycelium and the structure of the cell membrane of <i>A. solani</i>. Fluorescent staining analysis revealed that <b>1a</b> changed the mitochondrial membrane potential and cell membrane permeability. Thus, pyridinecarbaldehyde phenylhydrazone compounds emerged as novel antifungal lead scaffolds, and <b>1a</b> and <b>2a</b> can be considered promising candidates for the development of new agricultural fungicides.