Canker is a deadly disease caused by a fungus, affecting many kinds of stone fruit trees, such as plum, apple and pear trees. The infected parts of the wood turn dark as the fungus eats away at the fruit tree. To treat canker in fruit trees, you must first cut out the cankers. If you have several infected fruit trees, you can also apply a copper-based fungicide in late summer and early autumn. Cut out the cankers in May or June, when the fruit trees are growing and can better recover from the wounds.
Begin your cut about 2.5 cm (1 inch) outside the edge of the canker. Slice down at least 10 cm (4 inches) below the canker, but don't "dig" into the wood. Make clean, even cuts beneath the bark.
Cut away all the diseased wood until you see only healthy wood. Keep your margins at least 1.25 cm (1/2 inch) outside the canker's edges.
Repeat this process for all the other cankers. Apply canker paint to the wounds.
Spray all the infected fruit trees with a fixed-copper fungicide. Follow the directions on the label closely and repeat the application two more times during the autumn. Use a fungicide that contains 50 per cent actual copper.
Disinfect your knife or chisel before cutting and after cutting out each canker. Dip your cutting tool in a solution of one part bleach and four parts water or three parts denatured alcohol and one part water.
Don't cut out a canker if it is more than half of the branch's circumference. This will likely kill the branch, so you may be better off removing the branch altogether if you can.
Don't apply standard tree-wound compounds to canker-removal wounds, such as latex paints or water-asphalt emulsions, because these are not effective in healing the wounds. Use canker paint if you can find it or do not use wound paint at all.