Tree Cankers
The Cause
A canker is defined as a diseased, sunken lesion on woody tissue. There are three types of cankers; fungal, bacteria or frost. Frost cankers, actually physiological disorders, are cracks on the bark caused by freezing and thawing. Fungal and bacterial pathogens usually gain entry through frost cracks, pruning wounds, wind damage, or sun scald. Sun scald is another physiological condition that occurs on the south west side of tree trunks exposed to bright winter sunlight, most often affecting mayday and mountain ash trees. Fungal infections can spread into adjacent tissue, cut off sap flow and result in branch die-back. 
Cytospora canker sometimes infects older cotoneaster hedges where overcrowded stems create reduced light and poor air circulation; a perfect place for fungi to grow. Dead stems with sections of peeling bark could mean Cytospora has found a home. However, if you notice orange spots on dead stems, this is Nectria canker, a secondary rot organism that only colonizes tissue that is already dead. Some cankers are incorrectly diagnosed as fireblight, a bacteria which can kill off leaves and branches but may also create points of entry for fungal cankers to form afterwards. While possible on most trees, Cytospora cankers are most prevalent on mountain ash, poplar, spruce, and willow. Bacterial cankers, also called slime flux or wetwood, are most freqently observed on poplar trees.
Control
There is no chemical control for fungal or bacterial cankers. The only way to control them is to prune the branches back into healthy wood. In the case of hedges; remove the inside, affected branches and then the remaining branches will grow into the center because of the available light and air circulation. This is not a cure as the fungi or bacteria could spread within the sap of the plant and cause more cankers. Eventually, if it invades the main trunk, the tree may have to be removed. Be careful to clean pruning tools with a 10% bleach solution between each cut, as fungi or bacteria is spread by tools. It is difficult to diagnose cankers by describing them on the phone so bring in a branch to customer service for easier identification.
