Question
Hi Jim- I live on the hot and humid gulf coast. I have an 25' ash tree that has had some kind of black looking substance growing in a depression in the bark which has been that way for a few years. I spotted some mushrooms,like oyster mushrooms, growing on the tree today and there is a few small branches that have died high up on that same side. Is the tree is totally doomed?

Answer
No it sounds like the tree may have a decay fungi on the inside of the tree. What you are seeing are the fruiting bodies of fungi.

Decay fungi enter the tree trunk through wounds and will over time eat away at the woody center part of the tree. They do not infect living cells so they are not really a health problem with the tree.
There is not really anything that can be done about the fungi growing in the trunk. It is not a problem unless you start to have the large limbs break and these are hollow. This would mean the decay has grown up into the trunk and maybe come a problem of breakage of large limbs and the trunk.  This will not be a real problem for many many years.

I would recommend that you fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb of fertilizer per inch of trunk diameter scattered around the tree and watered in good. Apply just before a rain storm and you will not have to water.

Not anything that can be applied directly to the fungi to control it since it is growing deep in the woody cells of the trunk. But you can keep the tree healthy by fertilizing and the tree will live for many, many years even IF it becomes hollow.