This weeping willow tree has been healthy up to about a year ago when I started noticing some branches dieing but I didn't think much of it. Now about half of the tree appears to be dead and is covered with this very hard fungus. I don't know if this fungus caused the death or simply took over because the tree died. I would like help identifying the illness and any suggestions on how to save the tree.
The willow is a very fast growing tree that can pretty much handle all conditions, rugged temperatures and soil types. Fast growing, hardy, opportunistic trees are considered 'weed trees'. This is not a scientific term it is just an informal 'group' including Poplars (Aspens, Cottonwood), Locust, Birch, Alders, Russian Olive and some of the Prunus species. All fast growing, opportunistic plants usually doing best in the worst conditions, the coldest winters. The trade off for fast growth, fast shade, fast screening and being indifferent to poor soils, drainage is a short life span and hosting lots of diseases, insects and in this case mushrooms.
This tree looks like it is getting close to the end of its life. But it is still healthy 'for its age'. Certainly needs some professional pruning as it is common large branches to just die and will come down in the next wind storm. Mushrooms on trees are pretty normal especially an older tree. And I am guessing this side is the north side of your tree where moisture can linger longer...like moss, mushrooms grow on the north side of trees.
There ARE shelf mushrooms like this that are able to kill a tree, however. They create a White Rot`or a Brown Rot killing a tree within a few years. There are 5 or 6 kinds of shelf mushrooms that can do this; ranging in color from white, to grey, splotched with brown or having red, yellow, orange colors.
You need to get an ID on those mushrooms and we might be able to help; there are some great mycologists on this site that could give a tentative ID. But you need to go find a certified arborist to positively ID the mushrooms. They will be able to tell you what you can do to prolong this tree's life, recommend professional tree pruners or tree removers. Call your Cooperative Extension Service of your nearest University (usually only one University per state becomes the host of this cooperative). I would start there as the services are free or very inexpensive. `
You asked what you can do to save the tree. Willows are very tough as stormy mentions but the old wood on this tree is done for. A common practice for centuries is called coppicing which is the practice if inducing new growth from the base of the trunk by cutting the existing growth.
Seeing as the existing growth is done for call an arborist and have it cut to the ground but do not grub out the root system. By next spring you should see some new growth and in a few years it will have regrown.
There is nothing you can do - that's one of the bracket fungi. Its hard to tell which without a close up shot, but its most likely Daedaleopsis confragosa, which is very common on dead or dying willows. Once bracket or shelf fungus appears on a trunk, the die is cast, and the tree is on its way out - you may want to consider having it removed before it falls, if it's likely to cause injury or damage when it does so. Link below to information re bracket fungus (and yes, they are hard, unlike other types of fungal growths such as mushrooms) and the fact they mean death to the tree - by the time the fungal growth is on the outside, the heartwood has been infected for some time.