One of the first keys to identifying shelf fungi growing on a trunk is to turn it over and see if it has gills, pores, or "teeth" on the underside.
Generally, fungi grow on decaying wood, so it could mean that part of that trunk is dead. Oak trees commonly can have fungi actively growing on dead branches, which when they get too weak and consumed by the fungi, fall off in a windstorm. I suppose the same could be true with sumac, but I'm not that familiar with sumac and its parasites. What's safe to say is that you can't expect the parts of the trunk covered with fungi to come back to life and resprout. Sumac is a tree that you can coppice, so if you wish to save the tree, I would hack off all of the infected parts and see what comes back in the spring.
Location: Wellington, New Zealand. Temperate, coastal, sandy, windy,
posted 4 years ago
I don't know anything about sumac, but as John says, that tree looks like a gonner to me, with cracking, peeling bark colonised by fungi.
Not to worry, it coppices!
I'd take the whole tree right down below the rot.
It's hard to tell from the photo, but the mulch looks like it's thick, and right up against the trunk.
If that's the case, in the future make sure mulch doesn't touch the trunk as many plants will get collar-rot from the constant damp.
When tree trunks are mulched tightly against the bark like this one is, there is greater opportunity for fungus, bugs and rodents to attack the bark. I can't be sure that this has anything to do with this particular tree. Did it experience physical damage? If the organic mulch were moved back a bit, the bark would remain dry and sun lit. A mulch of rounded stones could fill the void if a level surface is desired.
Ooops. I just realized that Leila already addressed the mulch issue. Well that's two of us and hundreds of other gardeners who will no doubt think the same thing when they see the photo. Now, go around your yard and pull the mulch away from other trees and bushes. If you see problems, take a photo but no other action until it has had a chance to get some sun and dry out. The loss of this one won't seem so bad if you can prevent further problems down the road.
My wager would be it cracked in the winter, that is what that sort of damages tends to be. As far as the fungus, 99% of fungus that will be creating 'mushrooms' like that are growing on wood that was already dead, they are not the reason it died. That all being said, I wouldn't be optimistic for your tree, sorry.
Just from observation I've seen plenty of wild Sumac that just seems to up and die when they get to about that size, so it may be that they are short lived trees. Perhaps you'll get a few suckers to come up.
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