It can show up when the fruit is still on the tree towards the end of the season, but most commonly appears during storage. The first signs are widespread shadowy spots showing just below the fruit skin. These form into dark, slightly sunken areas or pits, up to a few millimetres in diameter. Directly beneath each pit a small patch of the fruit tissue turns brown, and similar brown patches eventually develop scattered through the apple flesh. When the condition is severe it affects the entire apple, giving it an unpleasant bitter taste.
Hail damage causes similar symptoms on the skin, but without brown spots inside the fruit.
The condition develops when the movement of calcium into growing fruits is disrupted, and various complex factors can bring this about. Water stress at critical times in the growing season appears to be a major factor connected with bitter pit, and high applications of nitrogen-rich fertilisers and manures also seem to aggravate the condition. As an essential plant nutrient, calcium does not move very readily within the apple tree. In conditions when it is needed rapidly, it is directed to leaves and shoots in preference to the fruit. Bitter pit is usually worse in seasons with wide fluctuations in rainfall and temperature.