Plum Tree (silver leaf disease)

Quite tricky this Sazg there is silver leaf and false silver leaf that affect plums and cherries in particular.Both conditions result in a silver downy like surface to the leaves.So if you cannot find this then not silver leaf.
Silver leaf - there is die back to affected branches when you cut through branches no sap or sign of healthy growth often brown staining evident.Cut back affected branches well into new growth and seal cuts with tree paint burn branches as they can spread infection.
False Silver leaf - there is no evidence of die back so if you cut into branch remains healthy in appearance.No pruning required just good soaking of roots with water and a foliar feed.Good luck if you able to post photo will know if silver leaf but you should be able to see tell tale signs.

If the tree had silvery sheen on the leaves when they last appeared that is a good indication that it may have silver leaf. To confirm this, cut into a branch that looks sick, and if you see dark brown staining in the wood (particularly visible if you dip it in water) then it could well be silver leaf.
Did you prune it heavily last year? Silver leaf infection is most likely to enter the tree through an old pruning wound that was not treated with Arbrex or similar.