Sick Apple Tree

Marisa starts with ...
Hi,
I have two young apple trees (a dorset gold and an anna) which are about 4.5 feet tall and bearing about 8 apples each. Recently the dorset gold has started dying down the trunk. I pulled off the fruit to give the tree more of a chance at survival but have no idea what is causing this. I have checked for collar and root rot and found nothing but the truck of the tree is now going a yellowish colour and feels a little spongy. (See attached pic or part of the truck). The leaves are still green and the fruit was developing well. Any ideas what could be causing this and how to fix it? Id really like to keep the tree or at least prevent it from spreading to my other healthy trees.
Thanks
Marisa

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Picture: 1