Synergistic interaction of natamycin with carboxymethyl chitosan for controlling Alternata alternara, a cause of black spot rot in postharvest jujube fruit

Black spot rot of jujube, caused by Alternata alternara, is an important disease affecting jujube tree and fruit production. In this study, the mixtures of natamycin (NATA) and carboxymethyl chitosan (CMCS) were investigated for their combined and/or individual bioactivity against A. alternara and the quality maintenance of postharvest jujube fruit. The combined effectiveness between NATA and CMCS was calculated by Wadley's method, and the results indicated synergistic interactions (SR ≥ 1.5) between NATA and CMCS up to the ratios of 1: 100 and 1: 500. Then, an emulsion was developed by mixing NATA and CMCS and was characterized by determining the particle size and the zeta potential. The results suggested that the emulsion has great physical stability after storage for 60 d at 25 °C. Furthermore, the emulsion was applied to the postharvest treatment of fresh jujube fruit. The results indicated that the treatment significantly decreased the fruit respiration rate and ethylene production, reduced postharvest natural decay, promoted fruit firmness, and delayed loss of titratable acidity and vitamin C during the storage 40 d at 10 °C, maintaining a better quality of the jujube fruit. Finally, high-throughput sequencing was used to determine the sequences of the ITS2 gene in ITS5-1737F variable regions between the treated and control jujube fruit. The results showed that the emulsion reduced the entire fungal counts and altered the absolute abundance of plant pathogens, including Alternaria species, in the jujube fruit. Taken together, these results suggested that the formulation incorporating NATA and CMCS represents an alternative to control postharvest diseases and to improve the shelf life quality of jujube fruit.