Trim the ragged edges of the cambium layer and hope that the cambium layer can fill in the wound. Diseased wood is harder to recognize than dead or damaged wood. Careful observation is the beginners best tool. The infected stem or branch is going to look different from the rest of the plant. Wilted or dying leaves are an obvious indication of trouble.
Some infections cause cankers to form at the point of infection.
The area changes color and is sunken. In some cases the fruiting structures of fungi can be seen. Take a sample of the affected branch to the local Cooperative Extension office if you need help. Cut into live wood if there is a fast moving infection that has not been isolated by a barrier layer. This type of infection is usually found on soft tissues that have not matured and hardened.