Sodium Alginate–Carboxymethyl Cellulose Composite Coating Incorporating Natamycin Improves Disease Resistance and Preserves Postharvest Attributes of ‘Cat Chu’ Mango Fruit

Anthracnose, caused by Colletotrichum sp. isolate XCC1, is a major postharvest disease causing significant quality deterioration and economic losses in ‘Cat Chu’ mango during storage. This study evaluated the effectiveness of sodium alginate–carboxymethyl cellulose (SA-CMC) coating with natamycin for controlling anthracnose and maintaining postharvest fruit quality. Mango fruits were treated with the SA-CMC-Natamycin coating and stored under controlled conditions (25 ± 2 °C; RH = 60 ± 5%) to assess disease development, plant defense enzyme activities, and fruit quality attributes. Natamycin inhibited spore germination of Colletotrichum sp. isolate XCC1 with a Minimal Inhibitory Concentration (MIC) of 6.25 µg mL−1. The SA-CMC-Natamycin coating significantly reduced anthracnose development, resulting in a three-fold decrease in disease incidence and a 3.86-fold reduction in disease severity compared with the control on day 9 of storage. However, the persistence of the treatment was limited since no significant disease incidence reduction was observed after 15 days. The treatment also enhanced chitinase (CHI) and β-1,3-glucanase (GLU) activities and increased phenolic compound accumulation. In addition, the coating delayed fruit ripening by maintaining firmness, titratable acidity (TA), vitamin C, and chlorophyll while suppressing increases in color change and total soluble solids (TSS). These results demonstrate that SA-CMC-Natamycin coating is a promising eco-friendly strategy for controlling anthracnose and preserving postharvest quality of ‘Cat Chu’ mango.