4-Propylphenol Alters Membrane Integrity in Fungi Isolated from Walnut Anthracnose and Brown Spot

Walnut anthracnose (<i>Colletotrichum gloeosporioides</i> and <i>C. siamense</i>) and brown spot (<i>Alternaria alternata</i>) cause severe yield losses globally. Conventional fungicides face the challenges of pathogen resistance and environmental toxicity. This study evaluates 4-propylphenol, a plant-derived phenolic compound, as an eco-friendly alternative against key fungal pathogens of walnut. In vitro assays determined EC₅₀ values against target pathogens (29.11-31.89 mg·L^-1) via mycelial growth inhibition and conidial germination suppression (EC₅₀ = 55.04-71.85 mg·L^-1). Mechanistic analyses confirmed membrane disruption through propidium iodide staining (9.5-to-14.0-fold fluorescence intensity increase), DNA leakage (77.82-85.15% at 250 mg·L^-1), and protein efflux (58.10-66.49%). In field trials, we implemented a phenology-driven strategy: 100 mg·L^-1 ground/canopy spray at flowering to reduce primary inoculum, followed by 400 mg·L^-1 canopy application at fruiting. This protocol achieved 86.67% control efficacy against disease complexes with negligible phytotoxicity (SPAD variation < 5%). 4-propylphenol provides a sustainable solution through membrane-targeting action, effectively overcoming fungicide resistance in woody crops.