Bitter pit is a physiological disorder resulting from calcium deficiency in the fruit. It can develop within the first month or two of cold storage as well as on the tree before harvest.
Low levels of calcium in the fruit are due to competition with shoots or high fruit load for calcium, which may be aggravated by weather conditions. Hot, dry weather in July or August tends to increase the incidence of bitter pit. Irregular irrigation may also increase bitter pit, however, summer deficit irrigation reduced size and bitter pit in 'Honeycrisp'. Heavy dormant-season pruning, overthinning, and excessive nitrogen fertilizer promote bitter pit. Injury to trunks, such as winter freezes, interferes with calcium movement. Bitter pit occurs most severely in years of light crops.
Cultivars very susceptible to bitter pit: Honeycrisp, Northern Spy, Gravenstein, Grimes Golden, and Baldwin. A cultivar moderately susceptible is Golden Delicious. Cultivars that are fairly resistant are Delicious, Gala, and Winesap. Rootstock can also influence the amount of bitter pit observed on the scion.
during irrigation season, avoid wide fluctuations in soil moisture
do not over-fertilize, as vigorously growing trees with over-sized fruit are more susceptible
do not over-prune in winter
do not over-thin in spring
keep trees at annual bearing through proper thinning and pollination practices
harvest at optimal timing because late harvested fruit is prone to bitter bit
calcium sprays (calcium chloride, calcium nitrate, STOP-IT, Nutri-Cal, Miracal, etc.) should be applied monthly throughout the growing season only if the above practices do not alleviate the problem
Here is a publication that will help: