My knee jerk was some type of leaf curl. Is this the only place on the tree? I've seen some of the petioles of my own plum leaves do something similar, can't tell if that is one large grown from petiole, or a ruffle from the leaf. I'm losing one of the oriental purple plums that my grandmother planted. I'm pruning back heavily this year. Any sign of disease gets a snip and a sanitize.
The fruit tree keepers can answer this much more fully, though!
In the meantime, hope that helps a little!