That sprout is almost certainly from the rootstock.

Tallclover,

I planted 2 Satsuma and 1 Shiro 6 ft apart last year (I plan to keep them short). Came back from a vacation last week. One Satsuma looks just like your tree. I am sure it's dead. I suspected the cause is underground. I will dig it up for an autopsy. The other two nearby are alive and well.

Mine is a seedling rootstock so I don't think there will be any sprouting if it's truly dead.

Mamuang, a friend said it might be fire blight. I'll send in a sample to WSU and see what they say it is.

do you have gophers???

Is the graft above ground? Looks like tree died above graft and rootstock is sprouting maybe fruit nut or harvestman can tell what happened. I would just be guessing. I think tree is over unless you want to graft to root stock Sorry but don't give up

Tallcover,

Please let me know what WSU says. I've seen fire blight before. It did not look like this.

We have voles. Usually my neighbor cats took care of voles (and moles). I don't see those cats much this year. Hard to believe that voles would damage only one tree when a few others are so nearby.

Maybe, I should call UMass Extension services. They usually do not offer much help for backyard gardeners. I'll try to call them Monday.

Not fireblight. That affects apples and pears, not plums.

Perhaps graft failure as has been said.

Best to start over.