Fungi are a vital part of global ecology. Without the work of these organisms, organic matter would accumulate and soil would cease to be fertile. But that doesn’t mean each and every fungus is a welcome sight, especially when found growing on your trees.
There are a number of fungal species that produce white fruiting bodies on deciduous and coniferous trees. Mushrooms are a common example of a fungal fruiting body and are often the first sign that something is amiss with your tree’s health.
In this article, I’ll describe the most common white tree fungus and what to expect if you find them growing on your property. I’ll also provide some ways to prevent fungal growth on landscape trees in the first place.
What Is White Fungus Growing On Trees?
It’s important to remember that mushrooms are not fungi in their entirety. Instead, it’s best to think of mushrooms as the ‘flowers’ of much larger organisms. When mushrooms grow out of a log or tree trunk, it means that there is far more fungal activity within the wood itself.
If you notice what looks like white mold growing on your trees, however, there’s a good chance it’s a type of powdery mildew fungus. Powdery mildew does not penetrate the wood the way many other fungi do.
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Is White Tree Fungus Harmful?
As long as it is not ingested, white tree fungus is not harmful to people or animals within the area. You can safely co-exist with nearly all types of fungus in your yard and garden.
If you have children or pets who may be tempted to get up and close to the fungus, I do recommend identifying the species to determine whether it is toxic or not.
Identifying White Tree Fungi
Identifying the fungus on your tree will help you determine the severity of the infection and whether or not other trees in the area are at risk.
I already mentioned that there are several fungi species that produce white growths on trees. Fortunately, most cases can be narrowed down to a handful of common varieties.
Trametes Versicolor – Turkey Tail
Turkey tails are some of the most common mushrooms found growing on deciduous and coniferous trees around the world. You can find these unique fungal growths in a variety of forest biomes and, occasionally, landscaped areas.
These are stemless mushrooms that form fan-like structures. Most bodies are between 1 and 4 inches across and often grow stacked atop each other. The texture of turkey tails is generally described as leathery.
It’s relatively easy to see how Trametes versicolor got its common name. The mushrooms boast multi-colored sections that resemble a classic Thanksgiving turkey tail.
Turkey tail mushrooms are extremely prolific. The majority of tree species are at least somewhat susceptible to this fungus and there’s little you can do to stop its spread to dead or damaged wood.
Armillaria Mellea – Oak Root Fungus
Oak root fungus, also known as honey fungus, is a soil-borne pathogen that generally attacks the roots and lower trunks of infected trees. This fungus is widespread throughout Europe and parts of North America.
Despite its common name, this fungus does not just affect oak trees. Armillaria mellea can cause root rot in a variety of ornamental and crop plants, including a number of tree species.
Cases of oak root fungus generally go up during wet summers or when trees are excessively irrigated. According to the University of California, it can spread between plants when the soil is disturbed or when the roots of a healthy tree come into contact with diseased plant matter nearby. Fungal bodies can live in the soil for a significant time after the original host has died.
The stemmed mushrooms of this fungus are white or yellow and usually form in large clusters at the base of infected trees. However, symptoms often occur long before mushrooms appear and can include wilting, yellowing leaves, leaf loss, and dying limbs.
Pleurotus Ostreatus – Oyster Mushroom
Pleurotus ostreatus is a popular, edible fungus commonly known as an oyster mushroom. It’s found throughout the entire world, typically growing in temperate and subtropical forests.
Oyster mushrooms have very short (almost non-existent) stems and form shelf-like clusters. The caps tend to be medium brown but many specimens fade over time until they are entirely white.
These mushrooms grow on a number of hardwood tree species but are notably prevalent on beech and aspen wood. According to the University of California, internal decay can extend 10 feet in either direction from the mushrooms themselves. While this fungus feeds on dead or damaged wood it does not necessarily infect and kill its hosts outright.
Oyster mushrooms are well-known for their culinary uses. However, there are several other fungi whose mushrooms look almost identical to the oyster mushroom. Do not forage any mushrooms without verification.
Schizophyllum Commune – Split Gill Fungus
Split gill fungus is another incredibly widespread species that affects trees of all types. This fungus is prevalent throughout every continent except Antarctica.
The mushrooms of this fungus can emerge as fan-shaped brackets or circles attached to the tree at a central point. Most specimens resemble coral.
While split gill mushrooms are unexceptionable from the top, the underside is easily identifiable. Each mushroom features tight, branching folds that are often mistaken for gills.
Split gill fungus is an opportunist that grows on dead or damaged wood. You can’t stop its spread but it typically won’t infect healthy trees.
Oxyporus Latemarginatus
Image Matt Welter by CC 4.0
You may know this tree fungus as a frothy piecrust, and it’s hard to think of a more apt description of its tell-tale appearance.
The first sign of Oxyporus Latemarginatus on most trees is a white, frothy growth covering the base of the trunk. In some cases, this growth will also extend onto the surface of the soil surrounding the tree.
This fungus generally starts in the roots or lower trunks of damaged trees. It can do serious damage to the sapwood before there are any visible signs of its presence. Depending on your area, frothy piecrust may be relatively rare in residential trees. I recommend checking all organic materials — e.g., cut lumber, wood mulch, etc. — for fungal growths to prevent introducing Oxyporus Latemarginatus to your property.
Treating White Fungi and Mushrooms on Trees
With the exception of powdery mildew, treating tree fungus generally isn’t possible. Instead, the focus should be on preventing further damage and promoting new, healthy growth. In some cases, tree removal is the best option.
Heart Wood Rot
Heartwood is the innermost part of a tree’s trunk and branches. This wood is technically nonliving but is vital to the structure and overall health of the tree.
When fungi infect a tree’s heartwood, it can quickly eat away at this material while showing minimal external signs. Often, the only symptom of heartwood rot visible from the outside is the presence of mushrooms.
Unfortunately, there’s no way to reverse the damage done by heartwood rot. Once you notice mushrooms coming from a tree’s heartwood, significant damage has been done to the interior.
Large trees should be frequently monitored for structural integrity — many trees affected by heartwood rot eventually fall or lose branches which can cause serious damage to nearby buildings and passersby. If the rot becomes too severe, the tree will need to be cut down.
Sap Wood Rot
Sapwood rot is fungal decay that typically affects a tree’s bark and cambium (the layer of wood that produces new outward growth). Unlike heartwood rot, this decay starts on the outside of the trunk or branch and works its way inward.
This type of rot is most often seen in wood that is near death and already dead. However, this does not mean that the entire tree must be dead. Sapwood rot can infect dead branches on a living tree and remove them without harming the rest of the tree.
With that said, signs of sapwood rot on living trees should be closely monitored. Because of the way this rot breaks down the tree’s structure, it can actually be more dangerous than heartwood rot when left unchecked.
The best way to control sapwood rot is to remove affected limbs as soon as possible. This can prevent the spread of fungi to otherwise healthy parts of the tree.
Powdery Mildew
Though powdery mildew can wreak havoc on small ornamentals and vegetable plants, it’s generally a non-issue when it comes to mature trees. It is, however, unattractive.
Young stems and leaves are the most susceptible to powdery mildew. Avoid fertilizing and pruning practices that will encourage new shoot production.
You should ensure there is plenty of space between trees for adequate air circulation. Pruning overcrowded branches will also improve circulation and reduce fungal growth within the tree’s limbs. Do not prune trees when they are actively growing.
Chemical fungicides can be used to treat powdery mildew before it becomes too severe. The University of Minnesota recommends the following active ingredients for powdery mildew control:
- Thiophanate methyl
- Chlorothalonil
- Sulfur
- Potassium bicarbonate
Preventing Tree Fungus
Prevention is always the best strategy when it comes to tree fungus. Above all else, maintaining the long-term health of your trees will help them fight off fungal infections in the future.
Throughout the life of your trees, monitor for fungal infections and remove affected limbs as early as possible to prevent their spread.
Avoid unnecessary pruning that will create large wounds in the tree bark and allow fungal spores to enter the heartwood. I suggest shaping new trees when they are young to improve structural integrity and airflow while also reducing the need for pruning later in life.
Be wary of excess moisture around the base of your trees. Do not allow organic mulch to come into direct contact with the trunks and remove dead plant tissue — e.g., fallen leaves — from around the base of trees. These materials tend to hold moisture and promote fungal growth.
Do not fertilize established trees unless specifically indicated by a soil test. Fertilizer is generally ineffective on larger trees and may encourage the growth of certain fungi within the soil.
