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

Walnut anthracnose (Colletotrichum gloeosporioides and C. siamense) and brown spot (Alternaria alternata) 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.