Bitter pit is quite a common disorder on apples sometimes caused by a calcium deficiency in acidic soil.
Other causes can be excessive use of nitrogen, potassium and magnesium fertilisers.
The symptoms appear as shallow sunken pits 2-10 mm in diameter on the skin of the fruit, under which brown areas develop into the flesh, the fruit becomes quite bitter, hence the name bitter pit.
Symptoms usually begin four to six weeks after petal fall.
Fruit growing on vigorous, leafy, upright growing branches tend to have a greater potential to develop bitter pit than those that develop from spurs
Young trees that are just coming into fruit are often the most susceptible.
As mentioned above; a common cause is calcium deficiency sometimes induced by irregular watering, rather than the lack of calcium in the soil.
As prevention is often better than cure good plant care and attention such as regular watering, feeding and mulching can save trees becoming affected.
Foliar sprays of calcium nitrate fertiliser mixed to the manufacturer's specification can / should be applied at ten day intervals between Week 24 and 37
Note; never spray above 20°C (68°F).