There are numerous fungi that cause wood rots. All the wood rot fungi are saprophytic,which means they only break down what is already dead.
One group of wood rots are called shelf fungi since the reproductive structure sticks out like a shelf on a wall. The shelf fungi are divided into two groups. One group's fruiting structure is soft and can usually be quickly replaced if damaged or destroyed. The other group's reproductive structure is hard. They are conks and are sometimes called artistic fungi since writings/pictures can be drawn on the lower surface and these drawings last for years. The conks grow slowly and produce growth rings just like most trees. Some decay organisms make mushrooms/toadstools while some make puffballs. Some decay fungi make a "fuzzy-like" structure on the out side of the tree/wood. They may be in a variety of colors. Wood rots break down only dead woody material. Therefore, a tree may be rotting internally and still be able to maintain a full canopy of foliage since phloem, cambium, and sapwood are still active. Some trees show no external symptoms. Other trees may have openings and hollows in a branch or trunk which is another symptom of wood rot.
Varies with the pathogen causing the decay.
Rotting trees can be unsafe, especially when they overhang homes, other structures, as well as play areas. One must decide the risk level one is willing to take with his or her life and the lives of their loved ones. If the tree will not cause damage or loss of life when it falls, leave the tree standing. If damage or loss of life is a possibility, consider removing the tree as soon as possible.