The tree is in its 3rd year. It has started to fruit this year. The disease(?) started from the middle top section. The 3 photos say it all. The 3 photos are all of the one tree.
Plum Tree (Jubilee)
Certified GKH Gardening Expert
Drought or over fertilizing could cause leaves to turn brown and die off.
Cankers, Leaf spot and Fire Blight can also be causes.
Leaves began to wilt, brown and fall to the ground.
Leaf spots can cover the entire leaves with a brown discoloration. The infected leaves turn brown and drop off.
Fire Blight first attacks flowers and shoots and then can spread to the branches and leaves.
If the trunk becomes infected the tree can die.
Make sure the ground is clear of dead plant material and when watering you do not splash onto the tree itself.
Prune any dead plant material from the tree.
Using a fungicide can help as a preventative.
You will need to examine the tree closely to make a determination of the issue.
I've listed some links that will help you.