I have a flavor king pluot tree that is doing great. Just 10 feet away is a flavor queen that has been having the same problem for years now. The fruit starts out looking pretty good, then the pit rots out from the inside. We noticed that some of the branches have no leaves on them, so we cut them off. I've included a picture of a freshly cut branch. it looks like there's some dark spots in the wood.

I'm not sure if this is a disease or maybe a pest that eats the inside of the fruit.

I live in the west side of San Jose California. Plant Hardiness Zone: 9b (25 to 30 °F/-3.9 to -1.1 °C)

Any thoughts?

Thanks.