Abstract: This study investigated the effect of preharvest application of chitosan oligosaccharides (COS) on the control of postharvest black spot disease in ‘French’ prune fruits. Prune fruits were sprayed with different concentrations of COS (0.05%, 0.10% and 0.20%) or water as a control at four developmental stages (fruit set, enlargement, color change and maturity). After preharvest COS treatment, the fruits were inoculated with Alternaria alternata and stored for 90 days at (1.0 ± 1.0) ℃ and a relative humidity of 90%-95%. Samples were taken every 15 days to measure relevant indicators. The in vitro inhibitory effect of COS on A. alternat was also tested. The results showed that COS significantly hindered the colony growth and spore germination of A. alternata. Scanning electron microscopic examination showed that the hyphae of A. alternata treated with COS exhibited a rough surface and were irregular in thickness with partial collapse, indicating notable disruption of the hyphal integrity. Notably, the preharvest application of 0.10% COS significantly inhibited the incidence of black spot disease and lesion progression in prune fruits inoculated with A. alternata during postharvest storage. Furthermore, preharvest COS treatment elevated the activities of superoxide dismutase (SOD), catalase (CAT), chitinase (CHT), β-1,3-glucanase (GLU), phenylalanine ammonia-lyase (PAL), 4-coumarate:coenzyme A ligase (4CL), and cinnamate-4-hydroxylase (C4H), and facilitated the accumulation of total phenols, flavonoids, and lignin along with a rapid surge in H2O2 levels during the early storage phase, which was maintained at lower levels in later stages. These findings suggest that the preharvest application of COS can enhance the black spot disease resistance of postharvest prune fruits by directly exerting an antifungal effect and inducing disease resistance. This study provides a new idea and theoretical basis for disease prevention and control in prune fruits.

Key words: chitosan oligosaccharides; preharvest spraying; prune; in vitro fungal inhibition; induced disease resistance