Cork spot and bitter pit are both physiological disorders of apples caused by low levels of calcium. Visually, cork spots develop anywhere on the fruit, will be relatively small in number per fruit, are sunken and somewhat diffuse spots about 1/2 inch in diameter. Beneath the spot will be a corky area extending into the flesh of the apple. Bitter pit, on the other hand, develop only on the half of the apple opposite the stem, are smaller and a bit more defined. They also will have a corky area immediately below the spot.
The difference in appearance allows a diagnosis, but the two disorders develop because of the timing of calcium deficiency. Early season deficiency, as might be caused by very low soil calcium levels, will cause cork spots. Late-season deficiency, which might result from late-summer drought, will not result in cork spots, but instead will cause bitter pit to develop.
When the number of fruit on a tree is low, vegetative growth often will compete with fruit to use the tree’s source of calcium. Usually, bitter pit will develop under these conditions.