What killed my new plum?

We just planted a new shiro plum tree a couple of months ago. It's a pretty big tree- 7' tall and trunk is several inches wide. I thought it was slow to bud because of the crazy weather, warm and then snow and warm and snow again.

Well as of this week, all the leaf buds are dead and the branches are starting to turn brown and dead. I looked closer and there are fine white threads, like webs, along the branches.

What bug is this and is it too late to do anything about it?

I also have several plums in the vicinity, another newly planted one and one that is a few years old and very robust. Should I worry about them too? They are leafed out and looking good, though the snow killed all the blossoms so no plums for us this year.

First is that you said the tree was planted at mixed weather,
so I estimate the wet soil conditions weakened your plum tree so much, that the following invasion of spider mites made it coming to its end...

(If it is a grafted plum also the root stock could not be suitable one for your area.)

A better guess is without photos not possible.

Jenny said, " I looked closer and there are fine white threads, like webs, along the branches.

My suggestion is that this is spider mites like See Hes suggested.

Make a spray solution of 3 tablespoons of dish soap and one gallon of water.  You might consider spraying all your trees so that the spider mites don't spread to your other trees.

Then spray the soap solution on infested plant leaves weekly, as needed. Try to get both sides of the leaves.