Oak Tree Fungus Identification: Mushrooms That Grow on Oaks

Most fungus on an oak tree is a decay fungus fruiting on wood that is already dead inside the tree, a leaf disease that is mostly cosmetic, or a lichen that does no harm at all. A few signs do matter: black crust where bark falls away, honey-colored mushrooms at the base, and leaves bronzing and dropping in early summer. Where the growth appears tells you which group you have.

This oak tree fungus identification guide sorts what you can see on an oak by location, says what each one means for the tree, and covers the mushrooms foragers seek on and under oaks, including the deadly ones.

Key Takeaways

• A shelf or conk on the trunk means decay inside the wood. It is the sign, not the cause, and knocking it off changes nothing.
• Fungus at the base, such as Ganoderma, the weeping conk or honey mushrooms, means root or butt rot. Near a house, that is a job for an arborist.
• Black, burnt-looking crust where bark has sloughed off is hypoxylon canker. It attacks stressed trees and has no control once it reaches the trunk.
• Red oak leaves bronzing and dropping in early summer can be oak wilt, which kills red oaks in four to six weeks. Call your state forestry agency.
• Oaks host chicken of the woods, hen of the woods, chanterelles and boletes, but also jack-o'-lanterns, the deadly Galerina and the death cap. Every edible needs expert confirmation.

Oak Tree Fungus Identification Chart

Find the row that matches where the growth appears, then check the species named in it. Location is the best first clue.

For other host trees, see our tree mushroom identification chart.

Is a Mushroom on My Oak Tree a Sign the Tree Is Dying?

A mushroom or conk growing from an oak means the wood behind it is decaying, but the tree may still live for many years. Decay fungi digest wood that is already dead inside the tree: the heartwood, or the roots and butt. The conk is the sign of that decay, not its cause. MushroomExpert.com notes that taking chicken of the woods off a tree does not remove the fungus, because the fungus lives in the wood. Two things decide urgency: whether the decay is high on the trunk or at the base, and what the tree could hit if it failed. Our guide to mushrooms growing on trees explains how fruiting position maps to the type of rot.

Oak Tree Fungus on Bark

Fungus on oak bark falls into three groups: shelf fungi that signal heart rot, black crust that signals hypoxylon canker, and lichens that signal nothing at all. Clemson HGIC states that lichens are harmless and do not cause decline; they are simply more visible on trees with sparse foliage.

Shelf Fungi on the Trunk

Chicken of the woods is the most common trunk shelf on oaks. MushroomExpert.com describes Laetiporus sulphureus as parasitic and saprobic on living and dead oaks, causing a reddish brown, cubical heart rot. It fruits in summer and fall, often high on the tree, with bright yellow to orange caps of 5 to 25 cm and a yellow pore surface. The beefsteak fungus, Fistulina hepatica, is the other oak bracket people notice. MushroomExpert.com lists it as saprobic and sometimes weakly parasitic on oaks, causing brown rot, with a liver-red, tongue-shaped cap that bleeds reddish juice and grows low on the trunk or on stumps.

Black Crust Where Bark Falls Off

Black, crusty patches on an oak trunk are hypoxylon canker. Clemson HGIC names the fungus Biscogniauxia atropunctata, formerly Hypoxylon atropunctatum, on oak, hickory and pecan. Bark sloughs off and the crown thins. The Texas Plant Disease Diagnostic Lab gives the color sequence: a reddish brown to olive green crust, then a gray surface that flakes off after 6 to 12 months to reveal a black layer that looks burnt.

Hypoxylon attacks trees stressed by environmental extremes, and Clemson calls drought the most significant factor. Clemson states that there are no controls once infection has begun on the trunk, and the Texas lab says the brittle wood makes the tree a hazard to inspect and consider for removal. Prevention is the only tool: 1 inch of water per week in summer drought, a 3-inch mulch layer kept off the trunk, and no herbicides near the tree.

Oak Tree Fungus on Leaves

Leaf fungi on oaks are almost always cosmetic, and extension services rarely recommend spraying a mature tree. Powdery mildew is white powdery growth on both leaf surfaces that turns tan, then brown. Leaves may distort and drop early. Clemson concludes that control is usually not practical nor warranted.

Oak leaf blister is caused by Taphrina caerulescens, and Clemson notes that the red and black oak groups are especially susceptible. Leaves develop lighter green, blister-like patches that turn brown, and heavy infections can drop leaves in midsummer. Clemson says it is rarely severe enough to require control. Rake up fallen leaves; fungicides work only before bud break on small, heavily infected trees.

Fungus at the Base of an Oak Tree

Fungus at the base of an oak is the group to take seriously, because it usually means root or butt rot, and a tree with rotted roots can fall without warning. Three fungi account for most finds, plus hen of the woods, a slow butt rot fungus covered with the edibles below.

Ganoderma Conks

A hard shelf at the root flare with a varnished reddish brown top and a white underside is Ganoderma. Eastern varnished oak conks were long called Ganoderma lucidum and now sit in the Ganoderma lucidum group, mainly G. sessile and G. curtisii. The Texas Plant Disease Diagnostic Lab names maples, oaks and honeylocusts as particularly susceptible. The fungus enters through wounds on roots and the lower trunk, and decay may extend from a few inches to several feet up the butt. Affected trees are prone to blowing down, so the lab's advice is that once a conk appears, a trained professional should inspect the tree for structural integrity and cut it down if necessary. No control is known.

Weeping Conk

A yellow to orange lump at the soil line that beads amber liquid is the weeping conk, Inonotus dryadeus. Purdue Extension calls it one of the more common decay fungi its lab sees on declining oaks. The conk forms on exposed roots, at the soil line or on the lower trunk and is renewed each year. Purdue warns that infected trees may show no external signs while root rot progresses, then fail unexpectedly in a windstorm.

Honey Mushrooms

Clusters of honey-colored gilled mushrooms at the root collar in late summer and fall are Armillaria, which Clemson lists as a cause of slow decline in oaks. MushroomExpert.com describes Armillaria mellea as pathogenic on hardwoods, causing a white, pulpy rot and spreading from tree to tree by long black rhizomorphs, the shoestring cords under loose bark. Clemson advises removing diseased trees with as much root as possible, and says no effective chemical controls exist. Our honey mushroom identification guide covers the species and their deadly look-alike.

Any base conk within falling distance of a house or driveway deserves a professional inspection, because root decay is hidden.

Oak Wilt: The Fungus That Kills Oaks

Oak wilt is a vascular disease caused by the fungus Bretziella fagacearum, and it kills red oaks in as little as four to six weeks. The Texas A&M Forest Service explains that the fungus invades the water-conducting vessels, the tree plugs them in response, and the leaves run out of water. There is no cure.

Symptoms by Oak Group

Red oaks are extremely susceptible. Leaves bronze, branches flag brown or red, the tree drops its leaves, and death follows in 4 to 6 weeks with 100 percent mortality. Only infected red oaks form the fungal mats under the bark whose rotting-fruit smell draws the sap beetles that spread the disease. Live oaks are intermediate. Their classic symptom is veinal necrosis: veins turn yellow, then brown, while the rest of the leaf stays green. They die in 3 to 6 months, with about 5 to 15 percent surviving untreated, and their connected roots move the fungus about 75 feet per year. White oaks are less susceptible but not immune, and they decline far more slowly.

How It Spreads and What to Do

Sap-feeding beetles carry spores from fungal mats on red oaks to fresh wounds on other oaks, and the fungus also travels through root connections between neighbors. It occurs well beyond Texas. Texas A&M Forest Service's advice:

• Do not prune or wound oaks from February through June unless there is an immediate safety concern. Dates differ by state. Paint every wound immediately, at any time of year.
• Do not move firewood. Dead infected red oaks should be burned, chipped or buried soon after discovery and never stored as firewood, because red oak firewood can still form fungal mats.
• Trench to break root connections, at least 100 feet beyond the drip line of symptomatic trees and at least 4 feet deep.
• Preventive injection of propiconazole into the root flares of healthy oaks near an expanding center protects only the treated tree and does not stop root spread.
• Confirm it with a lab sample, then contact your state forestry agency and a certified arborist.

Can You Treat Oak Tree Fungus?

No spray cures wood decay, root rot, hypoxylon canker or oak wilt in an oak, and extension services say so plainly. The Texas lab states that no control is known for Ganoderma rot. Clemson says no effective chemical controls exist for Armillaria and none for hypoxylon on the trunk. Texas A&M Forest Service says there is no cure for oak wilt.

What arborists actually do is different from spraying. They identify the fungus, often with a sample to a university diagnostic lab. They inspect the trunk and root flare and recommend removal if decay is extensive. And they reduce stress with the watering, mulching and pruning practices above.

Edible Mushrooms That Grow on Oaks

Oaks host several of the most sought-after edible mushrooms in North America, and every one of them needs confirmation from an experienced identifier before anyone eats it.

On the Wood

Chicken of the woods fruits on living and dead oaks in overlapping orange shelves with a yellow pore surface and no gills, and the related Laetiporus cincinnatus forms pinkish orange rosettes with a whitish pore surface at the base. Hen of the woods is a gray-brown rosette that MushroomExpert.com describes as weakly parasitic on living oaks, causing a white butt rot and often returning to the same spot for years; its flesh does not bruise, unlike the black-bruising Meripilus sumstinei. Oyster mushrooms, Pleurotus ostreatus, fruit in shelf-like clusters on dead hardwood in cool weather.

In the Soil Under Oaks

Chanterelles partner with oak roots. MushroomExpert.com lists the smooth chanterelle, Cantharellus lateritius, as mycorrhizal with oaks east of the Rockies, usually appearing in July. It is egg-yolk yellow with a smooth or shallowly wrinkled underside and an apricot odor, and it grows from soil, alone or scattered, which is the first thing that separates it and the golden chanterelle from their poisonous look-alike below. Boletes do the same. MushroomExpert.com describes Boletus variipes, in the Boletus edulis group, as mycorrhizal with hardwoods, especially oaks, with a white pore surface that does not bruise and a netted stem.

Poisonous Mushrooms On and Under Oaks

Three dangerous mushrooms share oak habitat: the jack-o'-lantern on stumps and roots, the deadly Galerina on oak logs, and the death cap in the soil under oaks. Each has a well-known edible look-alike.

Jack-o'-Lantern

The jack-o'-lantern is poisonous and is the orange cluster people mistake for chanterelles. MushroomExpert.com describes Omphalotus illudens as saprobic in large clusters on stumps or buried roots, especially oaks, from late summer through fall east of the Rockies. Caps are bright to pumpkin orange, 3 to 20 cm, with true, sharp-edged gills running down the stem. Fruiting from buried roots, it can appear to grow from soil.

Deadly Galerina

The deadly Galerina grows on the same rotting logs as honey and oyster mushrooms, and it carries the same class of toxins as the death cap. MushroomExpert.com describes Galerina marginata as saprobic on rotting hardwood and conifer wood, usually in clusters, with a small sticky cap of 1.5 to 5 cm, a thin ring that may vanish, and a rusty brown spore print. The spore print is the test, because honey mushrooms drop a white print.

Death Cap

The death cap, Amanita phalloides, is deadly, and oaks are its partner trees. MushroomExpert.com lists it as mycorrhizal with coast live oak on the West Coast and various oaks in the East. It was introduced from Europe and is expanding on the West Coast and in the Mid-Atlantic states. A survey by Wolfe and colleagues (New Phytologist, 2010) mapped two ranges, California to British Columbia and Maryland to Maine. Sources differ on its eastern host trees, so do not assume any lawn is free of it.

The cap is 4 to 16 cm, dull green to olive, yellowish or brownish, occasionally white. Gills are white and free from the stem. There is a skirt-like white ring and a sack-like white volva at the base, which may sit underground. Dig up the whole base of any white-gilled mushroom under an oak, because the volva is the feature casual pickers miss.

Common Identification Mistakes

Calling Every Orange Cluster Chicken of the Woods

Chicken of the woods has pores and no gills. The jack-o'-lantern has sharp gills running down a stem. Turn the mushroom over before you decide.

Picking Honey Mushrooms Without a Spore Print

Deadly Galerina grows on the same logs with a similar cap color. A rusty brown print means Galerina. A white print is the minimum check.

Assuming Lawn Mushrooms Under an Oak Are Harmless

The death cap partners with oaks in lawns and parks, not only forests. Any mushroom with white gills, a ring and a sack at the base is deadly until an expert says otherwise.

Frequently Asked Questions

What are the balls on my oak tree?

Round balls on oak leaves and twigs are insect galls, not a fungus. Iowa State Extension says small wasps or midges cause most of them, and oak apple galls reach 2 inches across. Most galls do not seriously affect an established tree, and there is no cure once they form.

Should I remove mushrooms from my oak tree?

Removing a conk does nothing for the tree, because the fungus lives in the wood. Remove fruiting bodies only to keep children and pets away. The useful step is an arborist assessment, especially if the fungus is at the base.

Is fungus on an oak tree contagious?

It depends on the fungus. Decay fungi spread by airborne spores that only take hold in wounds or dead wood, so a conk on one tree does not doom its neighbor. Armillaria and oak wilt do move between oaks, through root contact and, for oak wilt, sap beetles.

What fungus grows on live oak trees?

Live oaks get oak wilt, which shows as veinal necrosis on the leaves and spreads through their connected roots, plus hypoxylon canker on stressed trees and Ganoderma butt rot at the base. In California, the death cap fruits in the soil under coast live oaks.

What does oak wilt look like on an oak tree?

On a red oak, leaves bronze and brown, branches flag, and the tree drops its leaves and dies within weeks in summer. On a live oak, the leaf veins yellow and then brown while the rest of the leaf stays green.

Can an oak tree survive with fungus growing on it?

Often, yes. Oaks live for decades with heart rot behind a shelf fungus, as long as enough sound wood remains. Root and butt rot fungi at the base are more serious, and hypoxylon canker and oak wilt usually mean the tree is dying.

What is the black fungus on my oak tree bark?

Black, crusty, burnt-looking patches where bark has fallen off are hypoxylon canker, caused by Biscogniauxia atropunctata on drought-stressed trees. There is no control once it is visible on the trunk, and the brittle wood makes the tree a hazard.

Is the orange fungus at the base of my oak tree chicken of the woods?

Not necessarily. A rosette with a whitish pore surface may be Laetiporus cincinnatus, while a dense clump with sharp orange gills is the poisonous jack-o'-lantern. A yellow-orange lump weeping amber drops is the weeping conk, a root rot fungus.

Final Safety Reminder

Oaks carry prized edibles and some of the deadliest mushrooms in North America on the same trunk and lawn. Never eat a wild mushroom based on a chart, a photo or an app, and have every find confirmed by an experienced identifier. Keep children and pets away from unknown mushrooms. A conk at the base is a reason to call an arborist, and summer leaf bronzing on a red oak is a reason to call your state forestry agency. For a first opinion, photograph the top, underside and base and check them with our free mushroom identifier, then with a local expert.