I have two plum trees planted side-by-side. One seems fine but the other has branches where all the foliage has wilted & died. It seems to be spreading to other, nearby branches. I think it might be some sort of vascular disease since the remaining parts of the tree seem fine. Can you identify the culprit and/or suggest treatment or corrective action? Other plum trees (3) in the orchard seem unaffected. The attached photos are from different parts of the affected tree.