Coumarin Decouples ROS Homoeostasis, Membrane Integrity, and HOG Signalling to Inhibit Alternaria alternata

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Alternaria alternata is a common pathogen responsible for leaf spots and postharvest decay in multiple crops, including sweet cherry. Coumarin, a plant‐derived compound, exhibits broad antifungal activity against plant pathogenic fungi including A. alternata . In this study, we found that coumarin significantly inhibited mycelial growth, conidiation, conidial germination, and infection of cherry tissues. Mechanistically, coumarin disrupted reactive oxygen species (ROS) homoeostasis, impaired membrane integrity, and inhibited ergosterol biosynthesis. Transcriptomic analysis revealed that coumarin dysregulated genes involved in membrane and sterol metabolism. Notably, coumarin activated the Hog1 (High osmolarity glycerol‐1) dependent stress response pathway, as evidenced by nuclear accumulation of AaHog1 and increased coumarin sensitivity of ∆ AaHog1 mutants. Furthermore, coumarin enhanced the efficacy of demethylation inhibitor (DMI) and dicarboximide (DCF) fungicides. Our findings demonstrate that coumarin acts through multiple antifungal mechanisms including HOG pathway activation, oxidative stress induction, and membrane disruption, highlighting its potential as a sustainable antifungal agent for controlling A. alternata .