I noticed many of the leaves have lumps forming, mostly around the leaf edge. Turns out these are caused by #106 the Plum Leaf Gall Mite. Apparently this is harmless.  Wish I could say the same for the extreme damage done by #3 the Woodpigeon, which have pecked at most of the leaves except on outermost branches, and in the process either eaten or knocked off all but a handful of the many plums that were forming on the tree.