Discovery and Mechanistic Investigation of Aminopyrimidine/Pyridine Hydrazone 4b-8 as a Potent Fungicide Candidate with Distinct Glucan-Associated Antifungal Mechanisms.

The limited diversity of fungicide modes of action and rising resistance highlight the need for novel agrochemicals. Here, aminopyrimidine/pyridine hydrazones were rationally designed through bioisosteric modification of lead compound L. Twenty-five derivatives were synthesized and evaluated, yielding multiple candidates with strong activity against major phytopathogenic fungi. Compound 4b-8 showed the best performance, with an EC50 of 0.3 μg/mL against Monilinia fructicola and >90% in vivo control efficacy against peach brown rot at 50 μg/mL, outperforming chlorothalonil. Mechanistic studies suggested that 4b-8 induces extensive ultrastructural damage, mitochondrial dysfunction, ATP depletion, and antioxidant system collapse in fungal hyphae. Transcriptomic profiling indicated substantial disruption of carbohydrate metabolism, particularly glucan-associated processes, which was consistent with the marked reduction in total β-glucan content. Collectively, the data demonstrate that 4b-8 exerts fungicidal activity through interference with glucan-related cellular processes, providing a promising lead with a mode of action distinct from commercial standards.