Your apples have “watercore,” a physiological disorder which develops while the fruit is on the tree. Multiple theories have been proposed about the causal factors, among them freeze damage or other temperature extremes; the watering regimen; and more. Evidence of watercore seldom appears on the exterior of the fruit.  It is most often recognized when the fruit is cut open after harvest. Affected fruits don’t store well.

Certain apple cultivars are more susceptible to watercore than others, among them Rambo, Jonathan, Delicious, Stayman, Arkansas (Mammoth Black Twig), and, in the northwest, especially Winesap.

Although most people consider watercore a defect of the apples, especially here in the US, some retail markets, such as in Japan, consider apples with watercore to be worth a premium price. The affected portions of the apple have a higher than normal sugar content and is very juicy.

See “Market Diseases of Apples, Pears, and Quinces” http://postharvest.tfrec.wsu.edu/marketdiseases/watercore.html. And if you are interested in the various causal theories of watercore, and more, see “Understanding Watercore” http://postharvest.tfrec.wsu.edu/pages/N3I4A. And here’s an old newspaper story in The Seattle Times about a King apple with watercore: http://community.seattletimes.nwsource.com/archive/?date=19941120&slug=1943107.

Enjoy your fruit!

In home gardens, so many factors are potential triggers for watercore that it's difficult, perhaps impossible, to point your finger at just one or two.

In home gardens, watercore in apples occurs only occasionally. Some gardeners never see it, whereas others see it just once during their tree's lifetime. Although you may consider watercore unattractive, it doesn't decrease fruit quality if the apples are used before the tissue breaks down.

Home garden management is in stark contrast to that in commercial orchards where growers manage their trees closely and fine tune their practices on a regular basis with professional laboratory tests of both soil and foliage with the goal of obtaining the best yield possible from a healthy, vigorous tree. They pay for numerous professional tests to evaluate the results from the current year in order to fine tune the program for the following year. This publication explains the process of determining the amounts of fertilizer needed for growing commercial apples based on soil and foliage tests: : https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/em8712.pdf.

After you read the above publication, you will likely realize that home gardening techniques are very casual. But one general recommendation for home garden soils here in much of the northwest that may help is to lime soil every several years to add calcium and, at the same time, help adjust soil pH.