The effect of potassium tetraborate on germination of conidia of Colletotrichum gloeosporioides, and postharvest rot of mango were studied. An application of K2B4O7 to mango trees at flowering increased fruit set and decreased the incidence of anthracnose on harvested fruit. The effects of borate on the germination, nuclear division, endocytosis, and ultrastructure of conidia of C. gloeosporioides were studied using light, confocal, and transmission electron microscopy. The results showed that borate inhibited germination and germ tube elongation, delayed nuclear division, and impaired endocytosis of C. gloeosporioides conidia. Ultrastructural abnormalities also occurred in conidia treated with borate, and these included an increase in numbers of vacuoles, cytoplasmic disintegration, mitochondria degradation, and plasmolysis. These results suggest that borate can serve as a potential alternative to synthetic fungicides for the control of the postharvest disease of mango fruit caused by C. gloeosporioides.