N'-(4-fluorophenyl)-2-pyridinecarbohydrazide targets succinate dehydrogenase to control citrus blue mold

In this study, N'-(4-fluorophenyl)-2-pyridinecarbohydrazide (2-FPH), N'-(4-fluorophenyl)-3-pyridinecarbohydrazide (3-FPH), and N'-(4-fluorophenyl)-4-pyridinecarbohydrazide (4-FPH), which showed considerable antifungal activities against the prochloraz-resistant Penicillium italicum strain (PiR2), were designed and synthesized. Among these, 2-FPH displayed the strongest antifungal activity against PiR2 growth, with minimum inhibitory concentrations (MIC) of 12.5 μg/mL and minimum fungicidal concentrations (MFC) of 25 μg/mL. In vivo antifungal and hemolytic toxicity tests showed that 2-FPH effectively prevented PiR2 infection of citrus fruits, but exhibited exceptionally low hemolytic toxicity. At equivalent concentrations, its biological safety and control efficacy were far superior to those of prochloraz. According to morphological findings, 2-FPH increased cell membrane permeability, caused cell swelling and morphological abnormalities, disrupted structural integrity, and decreased chitin content in cell walls. Physiological, biochemical, and molecular biology investigations indicated that 2-FPH binds to succinate dehydrogenase (SDH), inhibits its enzymatic activity, and suppresses associated gene expression. This decreases intracellular ATP levels, disrupts normal mitochondrial function, and triggers a substantial increase in reactive oxygen species (ROS). In summary, 2-FPH shows promising potential as a possible fungicide and succinate dehydrogenase inhibitor for managing citrus blue mold.