Tree Disease

Fungus on Trees: Removal and Treatment

Laetiporus, a bright orange bracket fungus, fruiting from an oak trunk as a sign of internal decay

Here is the answer most pages will not give you: most fungi on trees cannot be treated with a spray, and no fungicide on a shelf will cure a tree that has wood decay. The fungus lives inside the wood, where nothing you apply to the outside reaches it, and the mushroom or conk you can see is the last stage of the process rather than the first.

That is not the whole story though. Some fungal diseases of trees genuinely are treatable, and treating them works well when the timing is right. The useful question is which kind you are dealing with, and that is a question of identification before it is a question of treatment.

How my view of fungi on trees has changed

When I first became interested in mushrooms and found these large conks or ephemeral fruiting bodies on trees, I dismissed the idea that they could be bad. Like so much old natural history information, I was sure this was outdated, a fungiphobic point of view. Through working more in the world of arboriculture, my views have evolved, shifting to a more cautious view that true wood decay fungi are having some impact.

From there I have circled back to relying on the best available data: on the different types of decay and how each one impacts trees, on how aggressive a local species may be (it can vary widely depending on the tree host), and on the location of the decay, both inside the tree (sap rot, canker rot, heart rot) and on the tree generally (roots, lower trunk, upper trunk, limbs).

Then there are the somewhat overlooked foliar fungi on trees. Powdery mildews, scabs, tar spots, sooty mold, and anthracnose are all common names for various fungi that can be found on leaves. Just as with the wood decay fungi, some are more innocuous than others, while some can become persistent issues that impact a tree's health. Once again, proper identification is key to understanding cultural controls and treatment options.

How to get rid of fungus on a tree, step by step

The order matters more than the product. This is the sequence I work through on site, and each step rests on published research rather than habit. The studies are listed with the sources further down this page.

  • Step 1. Check that it is a fungus that can harm the tree at all. The gray, green, or orange crust on bark is usually lichen, and University of California IPM describes algae, lichens, and mosses on bark as generally harmless, with no control needed to protect the tree. A black sooty film on leaves and twigs is sooty mold, which grows on the honeydew left by aphids, scale, and whiteflies and does not infect the plant. You manage the insects and wash it off, and no fungicide is involved.
  • Step 2. Work out where the fungus lives: on the leaves and young shoots, inside the wood, or in the roots. Everything after this depends on that answer, because a material can reach the first and cannot reach the other two.
  • Step 3. If it is on the leaves, treat on the pathogen's calendar rather than yours. Peach leaf curl is treated after leaf fall while the tree is dormant, and the UC IPM guidance is blunt that treating after symptoms appear will not be effective. Powdery mildew is the reverse case, where potassium bicarbonate and horticultural oils can eradicate an existing infection while sulfur only protects before the disease appears. On a tall sycamore or ash with anthracnose, UC IPM notes that complete spray coverage is difficult and may not be justified, so raking the fallen leaves and pruning out infected twigs in winter is the practical treatment.
  • Step 4. If it is fruiting from the trunk or a limb, leave the conk or mushroom where it is until it has been identified. It is the best evidence of which organism is inside, and cutting it off removes none of the fungus, which is spread through the wood behind it.
  • Step 5. Do not paint, seal, fill, or scrape the area. Shigo and Shortle of the US Forest Service tested wound dressings for thirteen years and dissected 275 treated trees, and reported that no material prevented decay and that the individual tree mattered more than the treatment. UC IPM's position today is the same: wound dressings do not hasten closure or prevent decay.
  • Step 6. Where a canker or decay is confined to one limb, have that limb removed with a correct cut, just outside the branch bark ridge and collar, with no stub and no flush cut. Trees do not heal decayed wood; they wall it off, which is the compartmentalization model Shigo and Marx published in 1977, and the collar holds the tissue that does the walling. Most wood decay got in through a wound or a poor pruning cut in the first place.
  • Step 7. If it is at the base or in the roots, change the water and the grade before anything else. Keep irrigation off the trunk, and pull soil and mulch back until the root collar is exposed and can dry. In a California trial, Baumgartner found that excavating the root collars of grapevines infected with Armillaria mellea made the mycelial fans recede and restored yield in one vineyard, and did nothing in a second vineyard where the collars filled back in with soil. It works only while the collar stays clear.
  • Step 8. Find out how much sound wood is left. Decay is a structural problem before it is a cosmetic one, so a tree with a conk within reach of a house, a driveway, or a play area needs a tree risk assessment, not a product.

The trees I see this on, and where

I work in the Sierra Nevada foothills, and the fungal problems that reach me track the species that grow here. On the oak side that is blue oak and interior live oak on the dry slopes above Placerville and Shingle Springs, valley oak on the deeper soils around Folsom and Sacramento, and California black oak through the middle elevations toward Camino and Pollock Pines. On the conifer side it is ponderosa pine, gray pine, sugar pine, incense cedar, and Douglas fir.

That matters because the same question has different answers on different hosts. Armillaria mellea at the root crown of a valley oak is a different prognosis from Heterobasidion in a ponderosa pine, and charcoal canker on a drought stressed blue oak is less a fungus problem than a water problem wearing a fungal mask. If you are reading this from a different climate and a different flora, the biology below still holds, but the species and the odds will not be yours.

Most of the fungi people ask me about are on oaks. The oak disease guide goes through them one at a time, with photos of specimens from oaks in this region.

The fungal problems that can be treated

These are mostly foliar and shoot diseases, the ones that live on or just inside the leaves and young twigs where a material can actually reach them:

  • Peach leaf curl, which is treatable and is entirely about timing. It is managed during dormancy, before bud swell, and an application made after the leaves have distorted in spring accomplishes nothing at all that season.
  • Anthracnose on sycamore, ash, and oak, where repeated defoliation is weakening the tree.
  • Powdery mildew, shot hole, and the rusts, on ornamentals and fruit trees.
  • Some canker pathogens, where the response is as much pruning and sanitation as anything applied.

For all of these, treatment is only half the job. The reason a tree is losing to a foliar pathogen year after year is usually the growing conditions, so correcting water, drainage, airflow, and mulch does more for the long run than repeating an application every spring.

The fungal problems that cannot

These are the ones people most often arrive hoping to spray, and they do not work that way:

  • Wood decay fungi, the group that causes heart rot and butt rot. The fungus is already established in the wood, and there is no product that penetrates sound timber to reach it.
  • Root diseases such as Armillaria and Phytophthora. These are managed culturally, through drainage, irrigation, and grade, rather than cured. There is no labeled treatment that eradicates Armillaria from a landscape tree.
  • Vascular wilts, which move inside the water conducting tissue where surface applications do not follow.

Being told this early is worth money. The common situation I get called into is someone who has spent two seasons and several hundred dollars treating the wrong category of problem, while the actual cause, usually a site or water issue, went on working uninterrupted the entire time.

What kills fungi on trees, and what the trials actually show

Three claims come up constantly, and each has real research behind it, just not the research people assume.

Baking soda is the first. The source is a Cornell University trial published in Plant Disease in 1992, in which weekly sprays of sodium bicarbonate with one percent horticultural oil significantly controlled powdery mildew and black spot on roses, and the combination outperformed either ingredient alone against the mildew. That is a result about a fungus growing on the surface of a leaf. It says nothing about a bracket fungus on an oak, and I know of no published trial in which baking soda, vinegar, bleach, or hydrogen peroxide applied to bark stopped decay inside a trunk.

Phosphonate, sold as phosphorous acid, is the second. Garbelotto and colleagues at UC Berkeley showed in controlled experiments that phosphonate, injected or sprayed on the bark with a penetrating surfactant, suppressed the sudden oak death pathogen in coast live oak, while soil drenches were ineffective. It is the clearest case where something applied to a trunk reaches a trunk disease, and it comes with two limits. Phytophthora is a water mold rather than a true fungus, which is why the chemistry works on it and not on decay fungi. And it is used to protect a tree before infection or very early in it, not to reverse a canker that has already girdled the trunk.

Biological control is the third, and given how much of my work is fungal, it is the one I find most interesting. Schubert, Fink, and Schwarze treated 1,431 pruning wounds on 159 urban trees of six species with a selected strain of Trichoderma atroviride, a fungus that competes with decay fungi, and recorded an 82.3 percent preventive effect against the decay fungi they tested, compared with untreated wounds. That is prevention at a fresh wound, with one strain in one formulation. It is not a cure for decay already established in a trunk, and it has not become standard practice in California tree care.

So what does treatment look like when there is no spray?

It looks like management, and it is more effective than it sounds. For a tree with wood decay the work runs in this order:

  • Identify the organism. Not the category, the organism. This decides everything that follows and it is the step most often skipped.
  • Establish how much sound wood is left and where. Decay matters because of what it does to strength, so the question is structural as much as biological.
  • Find and correct whatever let the fungus in. Decay fungi are opportunists, and most of them entered through a wound, a bad pruning cut, a buried root collar, or roots damaged during construction.
  • Reduce what the tree has to carry, where that helps. Sometimes weight reduction buys a decade on a tree that would otherwise be removed.
  • Set a monitoring interval, in writing, so change gets measured against a baseline rather than remembered.
  • Remove it if the structure no longer justifies keeping it near a target. That is a real outcome and an honest report says so.

How to identify fungus on a tree

Start by naming the structure, because the shape tells you a lot:

  • A soft, fleshy mushroom with a cap and stem, usually short lived and appearing after rain.
  • A conk or bracket, a hard, woody, shelf like growth attached directly to the bark that can persist for months or years.
  • A crust or a spreading white fan of tissue under the bark or at the soil line.
  • A powdery or fuzzy coating on leaves, which is usually a foliar issue like powdery mildew and is rarely a structural threat.

The soft mushrooms and the woody conks are the ones that matter most for the tree's future, because they are the fruiting bodies of fungi that may be feeding on wood.

Where the fungus is growing matters most

Location is the single most useful clue, because it tells you which part of the tree is being decayed and therefore what is at stake:

  • At the base or on the roots: this points to a root or butt rot. It is the most serious location, because decay here attacks the roots and lower trunk that hold the whole tree up, and it raises the risk of the tree uprooting.
  • On the trunk: this points to a stem or heart rot decaying the structural wood inside the trunk, which can weaken it well before the outside looks bad.
  • On branches, often at old wounds or branch junctions: this points to a canker rot or a branch decay, usually a more local concern.
  • On the leaves only: this is almost always a foliar problem rather than a structural one, though some leaf diseases become persistent enough to affect the tree's health.
Armillaria mellea, the honey colored oak root fungus, fruiting in a cluster at the base of a tree

Harmless, helpful, or harmful?

Not every fungus near a tree is bad news. Sorting them is the whole point of identification:

  • Helpful: many mushrooms that pop up in the soil around a tree are mycorrhizal, meaning they are partnered with the roots and are a sign of a healthy soil, not a disease.
  • Harmless: fungi fruiting on a dead stump, a fallen log, or long dead branches are usually saprotrophs, doing the useful work of recycling wood that is already dead.
  • Harmful: fungi fruiting from the living roots, trunk, or main limbs of a living tree, such as Armillaria at the base or a bracket on the trunk, are the ones that decay the tree's structural wood and deserve a closer look.

A useful rule from the field is that heart rot fungi tend to produce a few conks near branch junctions, while saprotrophs tend to scatter many conks randomly over dead wood. It is a clue, not a diagnosis, but it helps sort the serious from the routine.

Four fungi you can learn to recognize

Most fungi on trees cannot be safely named from a photograph. But a handful of local species are distinctive enough that recognizing them is realistic, and the photographs here are my own, taken during this work in this region.

Laetiporus, the western sulphur shelf

The bright orange and yellow brackets in the photograph at the top of this page, fruiting in overlapping shelves on oak trunks in late summer and autumn. It is unmistakable when fresh, and it matters because it causes a brown rot of the heartwood: the decayed wood turns brittle and cubical and fails suddenly rather than bending first. A mature oak with Laetiporus fruiting from the trunk deserves a structural assessment, not a wait and see.

Armillaria mellea, the oak root fungus

Honey colored mushrooms in a dense cluster at the base of the trunk, usually after the first soaking autumn rains, as in the photograph above. Armillaria mellea causes the aggressive root disease of this region, but its milder relatives fruit in the same honey colored clusters, so the mushrooms tell you the genus and not the species. One California detail is worth knowing: the black shoestring rhizomorphs people are taught to look for are actually weak evidence for this species here. In the California work, Armillaria mellea was mostly recovered from white mycelial fans under the bark of living trees, while the shoestrings on root surfaces mostly belonged to the milder species. The fan under the bark is the finding that matters.

Cryptoporus volvatus, the veiled polypore, sliced open to show the pore surface hidden behind its veil

Cryptoporus volvatus, the veiled polypore

A small rounded conk on conifers with a built in veil that hides its pore surface, shown sliced open above. Find it on a pine or fir and the fungus is the messenger rather than the culprit: it fruits on trees killed or dying from bark beetle attack, usually appearing within a year of the attack. It is effectively a flag that says check the neighboring conifers for beetles, and I have written about it separately.

The black carbonized stroma of Biscogniauxia, charcoal canker, erupting through oak bark

Biscogniauxia, charcoal canker

Not a mushroom at all to the eye: a black, crusty, carbonized sheet that looks like the bark was scorched, erupting on the trunks and limbs of drought stressed oaks. By the time the black stroma shows, that part of the tree is dead, so the real question it raises is water: the fungus succeeds on oaks already pushed past their drought tolerance, and the neighboring oaks on the same site are the ones a site visit is really about.

If what you found does not match any of these, that is normal. These four are recognizable; hundreds of others are not, and those come down to the microscope.

Why the name of the fungus changes the prognosis

This is the part that separates a real diagnosis from a guess, and it is the reason I take organisms to the microscope when the outside of the bark cannot settle the name. Decay fungi do not all attack wood the same way, and the type of decay changes how the tree is likely to fail.

Brown rot fungi consume the cellulose and hemicellulose and leave a modified lignin behind. The residual wood turns brittle and crumbles into cubes, and it tends to fail suddenly rather than bending first. White rot fungi degrade lignin along with the cellulose, and the wood left behind stays more fibrous and generally retains more ductility. Two trees with the same visible amount of decay can therefore carry very different amounts of risk depending on which organism is in there.

The same logic applies to whether a fungus is a pathogen at all. Some species on the list of things that fruit on a trunk are aggressive pathogens of living tissue, some only decay wood that has already died, and some are doing nothing to the tree whatsoever. Lumping them all together as fungi on trees and reaching for the same product is how good trees get removed and bad trees get kept.

Heart rot, butt rot, and what a conk is telling you

Heart rot is decay of the inner wood of the trunk or a major limb. Butt rot is the same process at the base and in the buttress roots, and it is the more serious of the two, because that is where the loads are highest and where a failure takes the whole tree rather than a branch.

A conk is a fruiting body, and it is a late sign. By the time a fungus has the resources to build a reproductive structure on the outside, it has been working through the wood inside for a considerable period. That is why a fresh crop of conks at the base of a mature tree deserves a proper assessment rather than a wait and see. It is also why the absence of conks proves very little; plenty of decayed trees never show a conk.

Are fungi on trees dangerous?

To you, almost never. Wood decay fungi are, with rare exceptions, not human pathogens and they do not spread to people or pets by being in the garden. The genuine caution is about eating them: several species that fruit on trees around here are toxic, including the jack o lantern that fruits at the base of oaks and is regularly mistaken for chanterelle. Never eat a wild mushroom on the strength of a photograph or an app.

To the tree, the danger is real but depends entirely on species, location, and extent. To your property, the danger is structural: a decayed tree within falling distance of a house, a driveway, or anywhere people gather is a risk question, and that is what a tree risk assessment answers.

How I identify what is on your tree

Where the fungus can be named in the field, I name it in the field. Where it cannot, I examine it under the microscope, and where there are no fruiting structures to examine, I culture it. That means isolating the organism onto malt extract agar or potato dextrose agar, then examining the spore and hyphal structures under the microscope, using stains such as lactophenol cotton blue and Congo red where the structures need contrast. Fungal taxonomy is what I have spent over fifteen years studying, and it is why people bring me the trees that are hardest to explain.

A site assessment with a written report starts at $500, and includes the identification, the evidence behind it, and a management plan. The site visit on its own is $250 and covers up to one hour. Where laboratory pathology is needed I will tell you the cost before that work begins. Where a treatment is appropriate, I can apply it under my California Qualified Applicator License. I am not a Pest Control Adviser, so writing the material recommendation is not my licensed function, and I will tell you that rather than blur it.

I run no removal crew, so nothing in my findings is steering you toward a cut. Where a report does point to root or soil work that I can perform myself, I say so plainly in it. Call (530) 391-6100.

University and arboricultural sources

Frequently Asked Questions

How do you get rid of fungus on a tree?

Start by placing it. A fungus on the leaves, such as powdery mildew, peach leaf curl, or anthracnose, is handled with correctly timed treatment, sanitation, and better growing conditions. A fungus fruiting from the trunk, a limb, or the base is already inside the wood, and no product removes it, so the work becomes identifying it, correcting water and grade, pruning out confined decay with proper cuts, and assessing how much sound wood remains. Lichen and moss on bark need nothing at all.

What kills fungi on trees?

On foliage, registered fungicides, potassium bicarbonate, and horticultural oils kill or suppress specific leaf diseases when the timing is right. Inside wood, nothing does. No fungicide, home remedy, or wound paint has been shown to stop established decay, and thirteen years of US Forest Service wound dressing trials found that no material prevented it. The best supported treatment applied to a trunk is phosphonate against Phytophthora, which is a water mold rather than a true fungus.

Does baking soda or vinegar kill fungi on trees?

Baking soda has trial evidence against powdery mildew and black spot on rose leaves, where a Cornell study found weekly sprays with horticultural oil effective. That is a surface disease on foliage. Neither baking soda nor vinegar reaches a decay fungus inside a trunk, and I know of no published trial showing that either one stops wood decay. Acetic acid is also the active ingredient in vinegar based weed killers, so it is a poor thing to pour around roots.

Should I cut the conk or mushrooms off my tree?

It will not hurt the tree and it will not help it. The conk is a fruiting body, and the organism is in the wood behind it, so removing it changes nothing about the decay. I would rather you left it in place and photographed it, because its shape, pore surface, and position are the fastest route to an identification. The exception is a toxic mushroom within reach of children or dogs, which is reasonable to remove for that reason alone.

Does a mushroom on my tree mean it is dying?

Not by itself. It depends on the species and the location. A mushroom in the surrounding soil is often a healthy mycorrhizal partner, and a fungus on a dead stump is usually just recycling dead wood. But a conk or cluster fruiting from the living roots, trunk, or main limbs usually means wood decay is already established inside, and that is worth having assessed.

How do you identify fungus on a tree?

By the structure, the location, and, when needed, the microscope. The shape tells you whether it is a soft mushroom, a woody conk, or a leaf mildew. The location on the tree tells you which wood is being decayed. For a firm identification of a pathogen, spores and fruiting body features are examined under magnification, since many species look alike to the naked eye.

Is a fungus at the base of my tree dangerous?

It can be, because the base and roots are where decay most threatens the tree's stability. A fungus fruiting at the root collar can indicate a root or butt rot that raises the risk of the tree uprooting. It is the location I take most seriously, and it is a good reason to have the tree looked at.

Is the green or gray growth on my tree bark a fungus?

Usually it is lichen, which is a partnership between a fungus and an alga living on the bark surface, or it is moss or algae. University of California IPM describes all three as generally harmless to trees, with no control needed. Heavy growth can point to shade, slow growth, or excess moisture, which are worth noticing, but the lichen itself is not the problem.

Can fungi on trees be treated?

Some of it. Foliar and shoot diseases such as peach leaf curl, anthracnose, powdery mildew, and the rusts respond to treatment when it is applied at the right point in the season. Wood decay fungi, Armillaria root rot, and vascular wilts do not, because the organism sits where nothing applied to the outside can reach it; Phytophthora, a water mold rather than a true fungus, is the one partial exception. Which category you have is the first thing worth establishing.

Can fungi on trees be removed?

Not in the way the phrase suggests. The visible conk or mushroom is only the fruiting body; the organism itself is distributed through the wood, and no amount of scraping, cutting the conk off, or sealing the spot removes it. What removal genuinely means in practice is pruning out an infected limb where the decay is confined to it, or removing the tree where the structure no longer justifies keeping it near a target. Between those two ends, the work is management: correcting what let the fungus in, reducing load, and monitoring. Anyone offering to remove the fungus itself from inside a living trunk is selling something that does not exist.

How do you treat fungus on a tree trunk?

Generally you do not, in the sense of applying something to it. A fungus fruiting on a trunk is a decay organism already inside the wood, and no product cures that. What you do instead is identify the species, work out how much sound wood is left and where, correct whatever let it in, reduce the load on the tree if that helps, and monitor it against a written baseline. Scraping the conk off changes nothing except your ability to track it.

Will a fungicide kill fungi on trees?

Not for decay. A fungicide can protect foliage and new shoots from infection, which is why it works on the treatable diseases and why timing matters so much there. It does not penetrate sound wood to reach a decay fungus that is already established, and no product sold for garden use does.

Can a tree recover from a fungus?

From a foliar disease, usually yes, especially once the growing conditions behind it are corrected. From wood decay, the tree does not recover the wood it has already lost, because decayed wood is not replaced. Trees do compartmentalize, walling decay off and growing sound wood around it, and plenty live for decades in that state. The question stops being cure and becomes whether the remaining structure is adequate for where the tree is standing.

What is the difference between white rot and brown rot?

Brown rot fungi consume cellulose and leave modified lignin, so the remaining wood becomes brittle, crumbles into cubes, and tends to fail suddenly. White rot fungi degrade lignin as well as cellulose, and the wood generally stays more fibrous and keeps more of its ability to bend before breaking. It is one of the clearest reasons that identifying the organism matters for judging risk.

Does a tree with conks need to be removed?

Not automatically. It needs assessing. Conks mean established internal decay, but the decisions turn on which fungus, where on the tree, how much sound wood remains, and what is within falling distance. Some trees with visible conks are retained safely for years with monitoring. Others are genuinely finished. That is a judgment, not a rule.

Ask a Tree or Mushroom Question

Not sure what your tree needs? Ask me a tree or mushroom question free, fifteen minutes, no obligation.

Request a Consultation(530) 391-6100
Text a Photo of Your Tree