Have you ever had a mouth ulcer (also known as a canker sore)? These small irritations are not exactly pleasant, but they generally subside quickly enough without causing you any further harm. This is not always the case when one of your backyard trees has a canker sore. This is the general term applied when a tree becomes irritated due to a fungus or environmental factors. The cankers themselves might be nodules on the surface of the branches or trunk, a discolouration, or a small sunken patch. While you might need to take care of your own canker sores until they fade away (such as avoiding spicy food), your response to your tree's canker sores needs to be rather more drastic if you hope to save the tree.
The direct causes of the issue might be difficult to determine. While it could well be a fungal infection, the tree might also be demonstrating stress due to a variety of environmental factors (such as a nutritional deficiency, or even being poisoned by overuse of insecticides and weed killers in the vicinity). Environmental factors (if suspected) can often be overcome by amending the behaviour thought to be the cause. Fungal infections are another matter.
Whether it's caused by a fungal infection or environmental causes, the tree's ability to deal with a canker sore will depend on its health and the subsequent strength of its immune response system. Younger trees might be affected by cankers and will repel them much like you do with your own version of the condition. Older trees might become overwhelmed and can eventually succumb to the problem. So how can you prevent the canker from adversely affected the tree's health?
The patient might need amputation. Remove the affected branches and dispose of them in your household rubbish (not in your compost bin, where any fungal infections can be allowed to spread when the compost is used). Sacrificing these branches can save the remaining tree. Be sure to sterilise your pruning tools after removing the branches in question, so that cross-contamination of the fungal infection is not an issue.
Even when a tree has succumbed (or is at least looking decidedly unwell), it shouldn't be left as is. Call in the tree removalists. You need to get that tree out of there, since there are some fungal infections that can remain in the dead wood, potentially threatening the rest of your trees.
Ideally, a tree will be able to fight off a canker sore. But for the times when this isn't the case, remember that a speedy amputation can be necessary.