I have a flowering plum tree that was place in a retaining wall by a lanscaper two years ago. I did some online research on tree disease because I noticed a some issues with the tree. It looks like it could be Black knot and or Bacterial canker. What do I do? Thanks for you question Laura, This is bad news…both these disorders are common to plums and both are dificult to control. Here’s some information from Cornell on Black Knot http://www.nysipm.cornell.edu/factsheets/treefruit/diseases/bk/bk.asp and on Bacterial Canker http://www.ehow.com/how_5786749_treat-canker-plum-trees.html The bottom line is that you’ll need to prune away the infected growth and spray during the growing season with a copper-based fungicide. We sell one from Bonide and you can see it here. It is available in a spray or dust (that can also be mixed with water for spraying). Repeat sprayings will be needed and even then either of these diseases will be hard to control completely but you might keep them under control.