The splitting bark looks like normal expansion growth. Mine's the same. The stuff on the twigs not so much. That looks like gall mite. I've never experienced it so can't offer a treatment suggestion. But that sort of thing can vary from year to year.

I have 2 plum trees that are just over 20 years old and I don't expect them to last much longer. Typically stone fruit trees like plums and peaches may last 15-20 years with some care and good luck. Mine have been going down hill over the past 5 years mainly due to borer pests. The pic below shows how over time I've had to chop off limbs that had died from borers. This may be their last year since I do not use sprays on them...my 2 varieties of plums are Shiro and Yellow Egg.

linda_reb,

I notice a large stump nearby with holes drilled in it. How long ago was that tree cut?

If the plumb was growing under that canopy of that large tree, and that was restricting sunlight partially to totally, the tree might not have looked as good as it could've. The plumb might improve now that it's getting more light.

Are you treating that stump with any chemicals?