See:
Apple (Malus spp.) - Bitter Pit
Note: For Anjou, lower calcium rates are recommended than those for correcting bitter pit in apple because there is a risk of marking the fruit.
Symptoms Depth and shape of the corking spots will give diagnostic clues as to the cause. Cork spot (calcium deficiency) has spots deeper into the fruit than a brown marmorated stink bug could reach with its mouthparts (1 cm or less from the surface). Stink bug caused cork spots are shallow and have irregular (but not square) shape. The boron deficiency spots tend to be square and flat-bottomed.