Bitter rot of apple is caused by

The bitter rot of apples is caused by Colletotrichum gloeosporoides.
Bitter rot on apple and pear fruit is caused by the pathogenic fungi in the Colletrotrichum genus, specifically those species in the C. acutatum and C. gleosporioides species complexes.
It is identified by sunken circular lesions with conical intrusions into the apple flesh that appear V-shaped when the apple is cut in half through the center of the lesion. It is one of the most devastating diseases of apple fruit in regions with warm wet weather.