This study addresses the need for effective natural inhibitors of enzymatic browning in fresh-cut produce by exploring plant-derived compounds with activity against apple polyphenol oxidase (PPO). Extracts from 26 wild plant species were screened using thin-layer chromatography (TLC)-coupled bioautographic assays specifically adapted to apple PPO. Among these, D. fullonum showed clear inhibitory activity and was selected for further investigation. Extracts from different plant organs revealed that PPO inhibition was localized in the leaves, while stems and flower receptacles were inactive. Bioautography-guided fractionation of leaf extracts enabled the isolation of active fractions, and structural characterization by HRMS and NMR indicated that the activity is associated with the iridoid glycosides sylvestrosides III and IV. Antioxidant assays (DPPH, ABTS, CUPRAC) and TLC profiling demonstrated that PPO inhibition was not directly correlated with radical scavenging activity. In situ application of the leaf extract (0.5% w/v) on fresh-cut apple slices significantly delayed browning during refrigerated storage, showing comparable or improved performance relative to ascorbic acid at later stages. These results highlight D. fullonum as a promising source of natural antibrowning agents and demonstrate the utility of food-specific bioautographic assays for the targeted discovery of functional plant metabolites.