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.
It is more common in young, vigorously growing trees. In mature trees it is most likely to occur in larger fruits, and in fruits picked before they are fully mature. Prevention and control Bitter pit can usually be prevented with careful management. The aim should be to maintain conditions that allow steady, uniform growth throughout the season as this will avoid rapid fluctuations of calcium in the trees. Keep the tree mulched, applying the mulch early in the season when soil moisture levels are still high.