Your tree could be dying from the inside out.

Turkey Tail Mushroom on Tree
Credit: Steve Bender

I like mushrooms in soup, mushrooms in stir-fry, mushrooms in spaghetti sauce, and mushrooms on pizza. I do not like mushrooms on my tree. Not because I can't eat the tree. It's because the mushrooms probably are.

Mushrooms are the visible fruiting bodies of a fungus that may attack living tissue, but usually confines itself to feeding on dead organic matter, such as rotten wood. This is largely beneficial, as the breakdown of wood returns its constituent matter to the soil to enrich it. When you see mushrooms growing on a living tree, however, this is a warning sign that all is not well with that tree.

What you're looking at up top is a particularly beautiful form of shelf or bracket mushroom called a "turkey tail" for obvious reasons. Shelf mushrooms are shaped like pizzas (mmm, mushroom pizza!). Instead of having spore-bearing gills on their undersides, they have zillions of tiny holes. They can grow straight out from the side of a trunk or up from the ground, decomposing dead wood in a tree. The dead wood can be major roots or the heartwood in the center of the trunk that gives the tree its strength.

WATCH: Grumpy's Field Guide To Mushrooms

Why do some trees get shelf mushrooms and others don't? One cause is stress brought on by things like extended drought, compacted soil, and overly wet soil. Healthy, vigorous trees are less prone to infection, just like people.

Then there's physical damage. Take a close look at the base of the tree above. You'll see a cavity probably caused by a piece of landscape equipment. If you have really sharp eyes, you'll also notice thick plastic straps girdling the base of the trunk. These straps hold the root ball together when a balled-and-burlapped tree is planted. You should always cut the ones that circle the base of the trunk after planting, lest they strangle the tree when it grows. In this case, they weren't, roots died, and the turkey tails appeared.

Once this happens, you can't save the tree. Oh, you can cut off the shelf mushrooms, but they'll just regrow, because the fungus is inside. Eventually, they'll hollow out the tree, making it prone to falling in high wind. The best thing to do is take down the tree before it falls on you.

Can you plant a new tree in the same spot? Yep. Just keep it healthy, don't damage the trunk or roots, and don't pile up a volcano of mulch around the trunk.

There's the bell! Class dismissed.