plum tree pests
From your description the damage looks like it may be caused by Leaf Curl Plum Aphid (Brachycaudus helichrysi) or the Mealy Plum Aphid (Hyalopterus pruni). There are a couple of other aphids that can cause the same type damage. The aphids will start damage on one limb or part thereof and as they multiple they will spread to other limbs. If you closely inspect the deformed / damaged leaves you should be able to see the tiny aphids, or the sticky residue that they deposit on the leaves. The damaged limb will probably not produce fruit this year. But if you control the aphids as recommended in the provided publication your plum trees should produce next year. The sooner you treat your tree the less damage you will have this year. Good luck!
Here is a publication you may find useful.
Hope this helps!