Question
We have a mature pecan tree with a large (18" in diameter) mushroom like fungus at the base of the trunk.  We have been advised that the tree needs to come down because this is an indication of root rot.  Otherwise, the tree appears fine, except for some minimal die back way out on the limbs. Do you think the tree needs to come down?

Answer
No not unless there are large limbs that are breaking off that are hollow. This is the fruiting body of a decay fungus. Decay fungi eat away at the woody part of the tree. These are the non living cells in the center of the trunk that most call "wood". The living cells are just under the bark and are called the cambium. As long as the foliage is green and not thinning out the tree is ok. Over time the decay fungi will hollow the trunk and weaken the strength. This could cause the tree to break during a high wind storm. BUT a tree this old has with stood many wind storms. The first signs of a problem with strength will be the large limbs breaking off and these will be either hollow or decayed in the center. If this is not happening I would not cut the tree.
To increase the health of the tree I would fertilize it with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around under the tree and watered in good. Do this now and again in the spring.