In between rainstorms, you may be out for a walk and notice mushrooms popping up from the ground. Mushrooms are a common sight in fertile garden soil, on outdoor woodpiles, in green lawns, and fallen trees. This may contribute to the misplaced sense of normalcy when you see them growing on tree trunks or root flares.
However, you should be alarmed when you see mushrooms at the base or conks (shelf-like fruiting bodies) on a tree.
Really? Why?
Mushrooms and conks are the reproductive part of fungi. The fungi growing on your tree require rotting organic matter (wood) to thrive. Therefore, when you see conks on the trunk of a tree, it is an indication of decay within the tree. If the mushrooms are at the base of a tree on the root flare, it is a sign of root rot. (But if the mushrooms are growing only on the soil, you need not worry.)
How did this conk get on my tree?
Most wood decay in limbs and trunks is the result of infection by airborne fungal spores, which can be carried by wind or insects to wood exposed by injury. Injuries include natural branch breakage, pruning wounds, vandalism, and damage from machinery or construction. That’s why arborists always try to make the smallest cut possible when pruning trees. The removal of big branches creates a large exposed area of the tree where decay fungi spores can land and enter the tree.
But this is the first time I’ve seen a conk on my tree, does that mean the decay has just started?
Not necessarily. In climates where it is moist all year, mushrooms are visible year-round. But in California, mushrooms disappear during dry weather and reappear when moisture levels and humidity improve. That’s why you see so many of them in the fall after it rains. All mushrooms and some bracket fungi are annual (appearing and disappearing seasonally), but many conks are perennial and grow by adding a new spore-bearing layer each year.
Yikes! What should I do?
You should have your tree evaluated by a certified arborist. And knowing that the fungus is only visible during the rainy season, it’s best to have an arborist see the tree as soon as you see the conk or mushroom appear. That way they can ID what type of fungi it is.
Does that mean my tree is doomed and needs to be removed?
Not necessarily. However, the tree will need a Tree Risk Assessment performed to determine how extensive the decay is and what risks are present. Wood decay fungi can infect trunks and limbs, making them more susceptible to failure in adverse conditions such as wind or heavy rain, and unless there is an open cavity, conk, or mushroom present, it is possible for internal decay to be present but not visible. The good news is that many trees with decay do not need to be removed. A lot depends on the species and size of the tree, type of decay, and how much structural wood has been compromised. Of course, having high-value targets (such as homes, fences, or areas where people walk often) within the fall zone significantly raises the risk.
Is there a treatment for decay?
Unfortunately, there is no treatment that will eradicate wood decay fungi. The good news is that the tree has mechanisms in place to keep decay in check, and although trees don’t “heal” like humans, they do try to minimize the damage by producing new and more resilient wound wood tissue to wall off the spread of decay. When the tree is young and vigorous, this task is easier to accomplish, but as the tree ages its vigor and resiliency decline, making it more difficult. Wood decay is generally associated with old, damaged, or compromised trees.
What can I do to help my tree contain the decay? 
The best way to help your tree is to give it good cultural care to include the following:
- Protect the root zone from damage from construction
- Keep the lawnmower from hitting the tops of exposed roots
- Avoid damaging thin-barked trees with a weed whacker
- Make sure the tree has access to water
- Remove dead or diseased limbs
- Make proper pruning cuts at the branch collar to avoid leaving either stubs or flush cuts and make small cuts to decrease the surface area for decay fungi to land on
- Do not paint pruning cuts with wound dressing as they can create an environment for decay by trapping moisture under the dressing.
- Consult with an ISA Certified arborist about any other specific ways you can help your tree.
In closing, enjoy your neighborhood walks this wet winter and keep your eyes out for colorful and distinctive mushrooms and conks! But now you know, that mushrooms are not always as harmless as they appear. If you are concerned about fruiting bodies on or near your trees just call our office to arrange for one of our arborists to provide an evaluation.