tm There is no way to determine how long until the tree fails structurally due to this invasion of the fungi. As you've noted, the tree can remain healthy even though this is happening. This is due to trees' ability to wall off infection internally, but in time, these walls can fail. The biggest factor in any decision you make is whether there are "targets", ie. houses, sandboxes where kids play, etc. directly underneath the trees. If the answer is no, it may be reasonable to live with them for a time to come before removal. It's really up to how much risk you can live with. +oM

You're right that it's a fungus - that appears to be stromata of Hypoxylon (recently reclassified as Biscogniauxia). This is an opportunistic fungus that is basically an indicator of a tree (or portion of the tree) that is dead or dying. The fungus is commonly present as a harmless resident in the inner bark of healthy trees, but will move into the sapwood once the stem is severely stressed or declining for some other reason. The spore-producing structures will grow and cause the bark to slough off in patches. If that's happening on the main stem, I would regard it as a sign that the tree is on its way out. Because it acts as a decay fungus, Hypoxylon can also cause the stem to become structurally unstable fairly quickly. Here is a link that might be useful: USFS - Hypoxylon canker of oaks

For what it's worth, I was sent a leaflet some time ago entitled "Resistance or Susceptibility of Certain Plants to Armillaria Root Rot, which lists a great many species tested by the University of California for resistance in Northern California conditions. The author is Dr. Robert D. Raabe, and it was published as "Leaflet 2591" by the Division of Agricultural Sciences, University of California, Revised August 1979. I'm not sure how one could go about getting a copy today, given the date, but you might try a Google search on the author's name, or contacting the University directly.

I have a large area prone to erosion under a huge Arizona ash, under which is complete shade. I needed something to fill in fairly quickly. I've got Kimberly Queen Fern (I think that's it). It borders on invasive the second year, but I need it to be there. It doesn't mind a little standing water, loves the shade, and just leaving the leaves there to rot works just fine as far as feeding it. If we go through a long dry spell, I water a little bit (I run a sprinkler for a hour every other week), but that's it. It's not that hard to control, either--just dig up or pull. I got mine pulled up from a friend's garden, not even in dirt and in a five gallon bucket that sat out for two days. I put down twenty five "plugs" and now I'd estimate the crowd at about 150 ferns. I've also had what I think is called mondo grass under there. It's the common variety, came with the house (I don't care for it, so I pulled most of it up.), but was thick and lush and happy. In the shade, it's very dark, almost black. Caladiums are fun in shade, but I have to either dig them up or replant every year, as they rot down here in 9. My other ground cover is an undesirable....Indian strawberry...I swear that stuff will take over the world.