You do get a leaf curl thingie on plum types. (Not too technical for you am I?
) Our damson gets it - if you uncurl the leaf there's some sort of grub thing inside - quite gross. I usually ignore it (I'm like that - most things cure themselves and/or the plant survives it). It is unsightly but it doesn't stop the tree producing heaps of fruit. No doubt there's a chemical you could use but I'd rather not.