In a word, YES!
Consider the appearance of mushrooms on a tree's trunk or root flares (the part of the roots coming directly off the trunk) an urgent problem. While some mushrooms are harmless, others are a sign that there is decay in your tree’s roots or trunk — or both. And when there is decay, there is often risk that the tree will break or fall over. Depending on the size of the tree, this could spell T-R-O-U-B-L-E for whoever or whatever is underneath it.
The mushrooms illustrated below are considered deadly to a tree.
If you see any of these types of mushrooms (or others) at the base, on the trunk, or on the ground around your tree, DON'T REMOVE THEM! (They will be helpful for identification purposes.) Call a Certified Arborist immediately. The arborist you call should have a Resistograph, an invaluable tool when a tree has decay. This machine will tell you how much decay there is and how deep it goes into the tree or its roots. This information is critical to making a decision about whether or not the tree should be removed.