Discovery of acylhydrazide‐modified usnic acid derivatives as novel antifungal agents: Design, synthesis, and preliminary mechanistic study

Abstract BACKGROUND Reactive oxygen species (ROS) induce apoptosis by disrupting cellular function and membrane integrity, establishing ROS‐induced mechanism treatment appears to be a powerful tactic for the development of novel fungicides. However, the limited diversity of molecular scaffolds highlights an urgent need for more potent ROS‐inducing agents. RESULTS Using natural product modification and active substructure splicing strategies, we designed and synthesized 40 novel usnic acid derivatives—with usnic acid as the core structure, amino acids as flexible linkers, and acylhydrazides as substituents—and evaluated their antifungal activity. Among these, compound A24 , which incorporated a 3,4‐difluorophenyl moiety, exhibited significant antifungal activity, with EC 50 values of 2.298 μg mL −1 against Thanatephorus cucumeris ( T. cucumeris ) and 3.349 μg mL −1 against Alternaria solani ( A. solani ). The pot experiments showed that A24 , at a concentration of 200 μg mL −1 , conferred both notable protective and curative effects against rice sheath blight (caused by T. cucumeris ), achieving a curative activity of 77.83% and outperforming the commercial fungicide thifluzamide (73.17%). Interestingly, mechanistic studies demonstrated that A24 could suppress the pathogen's peroxidase and catalase activities, thereby perturbing cellular redox homeostasis, eliciting reactive oxygen species accumulation and mitochondrial membrane potential disruption, and ultimately inducing mycelial malformation and death. CONCLUSION These findings position A24 as a highly promising antifungal candidate via inducing fungal ROS accumulation, combining potent efficacy with a favorable safety profile. This study not only advances the development of natural product‐derived fungicides but also provides a novel structural scaffold for ROS‐inducing antifungal agents. © 2026 Society of Chemical Industry.