Inhibitory Effects and Transcriptomic Insights of L-Arginine Against Diaporthe nobilis

Diaporthe nobilis causes postharvest decay in Chinese chestnuts (Castanea mollissima Blume). This study evaluated the effects of exogenous L-arginine (L-Arg) on fungal growth, fruit decay, and transcriptional responses using a single D. nobilis isolate. In a preliminary in vitro screen of nine amino acids at 30 mM, L-Arg showed the strongest inhibition of mycelial growth. L-Arg inhibited mycelial growth in a concentration-dependent manner in vitro. In the single fruit assay, the 2.5–20 mM groups had lower observed disease incidence and disease indices than the water control, whereas the 30 mM group had values close to the control. Transcriptomic analysis of mycelia grown on potato dextrose agar (PDA) showed lower transcript levels for several genes annotated as encoding membrane transporters, cell wall-degrading enzymes (CWDEs), and proteins involved in secondary metabolism under L-Arg treatment. These findings describe transcriptional responses under in vitro culture conditions. Without functional or biochemical validation, they do not establish reduced CWDE activity or suppression of virulence during host infection. The contrasting responses of fungal growth and fruit decay indicate that stronger inhibition in vitro does not necessarily translate into better disease control. Further studies across isolates, together with measurements during infection and functional validation, are needed to assess the mechanisms and practical value of L-Arg for postharvest chestnut protection.