Canker in plum trees comes from an invasive fungus, which can eventually eat away at and kill your plum trees if left untreated. Cankers can develop on the large branches or even the trunk and enlarge to the point where they'll choke off and kill the part of the tree above them. Prevention is just as important as treatment when one of your plum trees has canker. Preventing the spread of canker is essential in not only saving the infected tree, but also preserving the health of surrounding plum and other fruit trees.
Diagnose canker in your plum trees by inspecting them for holes in the outer bark and rust-colored ruptures on the trees. The first sign of cankers is oozing light-amber gum near the infection point beginning in April or early May. Dieback will also occur on infected twigs during the spring and early summer, with gumming at the twig's base.
Remove infected branches completely from your plum tree, especially if the branches are dead from the cankers. Burn the infected wood or throw it in the trash.
Cut out small cankers in the summer. Cut away the bark and remove the infected areas using a sharp knife. Cut at least four inches below the canker, and try to keep the margins as smooth as possible.
Disinfect your pruning shears, knife and other tools used on the infected plum trees after each use. Place tools in a solution of one part bleach and four parts water. Wipe the tools down and dry them immediately to avoid corrosion.
Apply a wound dressing to the excised canker wounds. Then, inject a Bordeaux mixture into the plum trees to treat the cankers. A Bordeaux mixture is a combination of copper sulphate and hydrated lime that is effective at killing fungi.
Prevent further infection, do your summer pruning after harvest so that the weather is dry. Also, postpone your dormant pruning until January or February.