Plum (Prunus domestica L.) is an important economic fruit in Xinjiang. However, it often suffers from postharvest black spot disease caused by Alternaria alternata. The fruit’s climacteric nature and sensitivity to mechanical damage increase its vulnerability during harvest and handling.
Latent Infection by A. alternata
A. alternata can infect fruit without showing symptoms. It penetrates the fruit or attaches to the surface during development. Symptoms appear after harvest or when fruit matures, as natural defenses decline. Under favorable storage conditions, the pathogen activates, causing decay and reducing fruit quality.
Chitosan Oligosaccharides (COS)
COS are natural, safe, and environmentally friendly oligosaccharides derived from chitosan. They are water-soluble and help delay fruit ripening, maintain quality, and enhance disease resistance.
Experimental Setup
We treated Xinjiang ‘France’ plums with chitosan oligosaccharide (COS) at 0.05%, 0.10%, and 0.20% concentrations during four growth stages: fruit set, enlargement, color change, and ripening. Water-sprayed fruit served as the control. Treated plums were inoculated with A. alternata and stored at 1 ± 1 °C with 90–95% relative humidity for 90 days. We collected samples every 15 days to measure disease incidence, lesion size, and biochemical indicators. We also tested COS antifungal activity in vitro.
Key Findings
1. Direct Antifungal Effect
COS strongly inhibited A. alternata mycelial growth and spore germination. Treated hyphae appeared rough, uneven, and partially collapsed.
2. Boosted Fruit Defense
COS increased activities of defense enzymes, including SOD, CAT, CHT, GLU, PAL, 4CL, and C4H.
3. Increased Secondary Metabolites
COS promoted the accumulation of total phenolics, flavonoids, and lignin. These compounds strengthened the fruit’s structural and biochemical defenses.
4. Controlled Reactive Oxygen Species
H₂O₂ levels rose quickly in early storage to trigger defense responses. Later, levels remained low to prevent oxidative damage.
5. Reduced Disease Incidence
COS-treated plums had lower disease incidence and smaller lesions after A. alternata infection.
Conclusion
Preharvest COS spraying enhances plum resistance to black spot disease. It works by directly inhibiting the pathogen and activating host defenses. This method helps maintain fruit quality, reduce postharvest losses, and extend storage life. COS spraying provides a sustainable solution for plum production and postharvest management.
