Bitter pit is a physiological disease of apples which causes the fruit to become deeply pitted with necrotic tissue. While still technically edible, apples with bitter pit are unsightly and not very tasty, so they can be difficult or impossible to sell at market, potentially representing a catastrophic loss for the grower. This condition has been studied since the 1800s, and it has proved difficult to fully understand or eradicate, much to the frustration of many orchard owners.
This condition is classified as a post-harvest disease, meaning that it usually shows up after the fruit is collected and put into storage, although sometimes fruit will display early signs when it is picked. Bitter pit starts from the inside out, typically with the core of the apple first, as the tissue slowly turns brown, dessicated, and rotten, and the outside of the fruit develops distinctive sunken spots with dead tissue underneath.
The cause of bitter pit is not fully understood. It is believed to be linked with calcium deficiency, and many apple growers use calcium sprays to reduce the incidence of bitter pit. However, apples from trees with plentiful supplies of calcium have also been known to develop bitter pit, so clearly it is influenced by a number of factors. Varietals like Baldwins, Delicious, and Gravensteins are among the most susceptible to bitter pit.
Water supplies available to the tree may also have something to do with bitter pit. If a tree experiences extremely hot, dry weather in the summer months, the fruit appears more likely to be diseased. As a result, agricultural associations strongly recommend watering routinely and heavily in hot weather, and keeping an eye on the moisture level in the soil even when the weather isn't hot. Thinning the crop to promote the development of healthier fruit is also recommended; better to have a moderate crop of really good apples than a bumper crop of poor fruit.
Bitter pit is also believed to be linked to the treatment an apple tree receives while it is dormant. Traditionally, trees are pruned and fertilized during the dormant season to promote healthy, even growth in the spring. Over pruning and excessive applications of nitrogen can cause bitter bit, as can excessive thinning of trees when the fruit starts to set in the fall. Furthermore, tree damage can also contribute to the development to bitter pit, presumably because such damage interferes with the tree's ability to distribute nutrients to the fruit.