I have a flowering purple plum tree. For the last two years it has had
black knobby growths on the limbs. The number of these growths are
increasing and there is no sign of any type of bug involved. The tree
is healthy in all other respects and the growths remain on the limbs
all year. I cannot find anyone who knows what these are and if I need
to do anything to stop and/or remove these growths. Obviously they are
ugly but probably not fatal and do not spread to any other trees. Can
you give me a clue?
We can only guess from your description, and in order to get an accurate diagnosis you will need to take a sample (including both healthy and affected parts if possible) to a Master Gardener clinic. Click on the appropriate link for your local clinic through Master Gardeners / Washington State University Extension.
Meanwhile, for information about common diseases of plums in the Pacific Northwest, try searching Washington State University’s HortSense website.
The symptoms you describe are similar to 1. Crown gall, 2. Black canker and 3. Black knot. Click on those diseases for descriptions, photos, and control methods.
You can also take a look at pages like this one, on black knot of ornamental cherry and plum, from Morton Arboretum. See if the images resemble what you are seeing on your tree.