Tree fungus shows up in several distinct forms: shelf-like brackets jutting from the trunk, mushrooms clustered at the base, powdery white coatings on leaves, cracked and sunken patches of bark, and blackened masses bulging from the wood. The specific appearance depends on the type of fungus and where it’s attacking the tree. Learning to recognize these forms helps you spot problems early, since some are cosmetic nuisances while others signal that a tree is rotting from the inside out.
Brackets, Conks, and Shelf Fungi
The most recognizable type of tree fungus is the bracket or conk, a hard, shelf-like structure that grows directly out of the trunk or a large branch. These range from a few inches across to well over a foot wide, and they often have concentric rings on their upper surface, similar to growth rings on a stump. The top may be smooth, velvety, or cracked depending on the species, while the underside is typically flat and covered in tiny pores rather than the gills you’d see on a grocery-store mushroom.
Color varies enormously. Tinder conks are gray to black and shaped like a horse’s hoof, with a cream-colored underside that darkens with age. Artist’s conks are flat, woody shelves with a gray banded top and a bright white underside that bruises brown when scratched. Turkey tail, one of the most common, is thin and flexible with velvety concentric bands that can include brown, tan, gray, white, red, yellow, and even purple on a single specimen. Chaga looks nothing like a typical bracket. It appears as a rough, blackened, deeply cracked mass on birch trees, almost like a chunk of charcoal. Cut it open and you’ll find a hard, orange-brown interior.
Some conks are perennial, adding a new layer each year and growing larger over time. Others are annual, appearing after rain and disappearing within weeks. A perennial conk on a trunk is a strong indicator that internal wood decay has been underway for years.
Mushrooms at the Base or Along Roots
Clusters of fleshy mushrooms sprouting from the soil right at the base of a tree, or along surface roots, point to root and butt rot. These often appear after rain in fall or winter. Unlike conks, they’re soft, short-lived, and look more like the mushrooms you’d find in a forest clearing, with caps and stems. Honey fungus is a common example, producing golden-brown mushroom clusters around infected trees.
These mushrooms are the visible fruiting bodies of a fungus that may have been decaying roots and the lower trunk for a long time. By the time you see them, significant internal damage is likely. Other signs of root decay include the base of the trunk looking flattened on one side, or the lower trunk developing a swollen, bottle-shaped profile. You might also notice cracked or heaving soil near the roots. Trees with advanced root rot can fail structurally with little warning, so mushrooms at the base are one of the more serious signs to watch for.
Cankers on Bark
Cankers are localized dead areas on branches or the trunk where fungal infection has killed the tissue beneath the bark. They look like sunken, discolored patches, often oval or elongated, and the bark over them may crack, peel away, or stay tightly attached to the dead wood underneath. Some cankers ooze sap or gummy resin that dries into amber-colored blobs along the crack lines.
One telltale sign is what arborists call “sooty canker.” If you peel back the damaged bark, you’ll find a black, powdery fungal growth underneath, almost like soot. This dark discoloration sits just under the inner bark and extends only shallowly into the wood, but it disrupts the tree’s ability to transport water and nutrients in that area. Over time, the branch beyond the canker may die back entirely, leaving bare, leafless limbs while the rest of the tree still looks healthy.
Powdery Mildew and Leaf Spots
Not all tree fungus attacks the wood. Foliar fungi target leaves, and the most recognizable is powdery mildew. It looks exactly like its name suggests: white to gray powdery spots, blotches, or felt-like mats on the surface of leaves. An infected tree can look like someone dusted it with baby powder or draped it in thin cobwebs. Leaves at the infection site may cup, twist, or curl. When the infection is severe, leaves turn yellow and drop early. In some species, the tissue around the infection turns purple or reddish.
In late summer, look closely at the white mats and you may spot tiny round balls shifting from orange to black. These are the fungal reproductive structures preparing to overwinter. Come spring, new shoots growing from infected buds emerge already covered in velvety white fungal growth. Powdery mildew rarely kills a tree, but repeated heavy infections stress it and reduce its ability to photosynthesize.
Other foliar fungi cause distinct spot patterns on leaves. Anthracnose creates irregular brown or black lesions, often following leaf veins. Rust fungi produce raised orange or reddish-brown pustules on the undersides of leaves. These are mostly cosmetic problems for established trees, though they can weaken young or already-stressed specimens over multiple seasons.
Galls and Abnormal Growths
Fungal galls are lumpy, abnormal growths on branches or twigs where the tree’s own tissue has been hijacked by infection. They range from small round bumps to large, irregularly shaped knots. Cedar-apple rust, for instance, creates golf-ball-sized galls on cedar trees that sprout bright orange, gelatinous tentacles during wet spring weather. The texture of galls varies from smooth to rough to spiny, and their color may differ sharply from the surrounding healthy bark.
Galls on stems and twigs cause visible swelling or knot-like deformities. While many galls are caused by insects rather than fungi, fungal galls tend to be woodier and more integrated into the branch tissue, making them harder to remove cleanly.
How to Tell Fungus From Lichen
One of the most common misidentifications is confusing lichen with tree fungus. Lichen grows on bark as flat, crusty patches (sometimes resembling spray paint), raised leaf-like lobes, or bushy, branching structures that look like tiny shrubs or beards. Colors range from gray-green to bright orange-red. Unlike fungus, lichen does not penetrate or feed on the tree. It’s completely harmless, using the bark only as a surface to cling to.
Lichen tends to show up more on mature or slow-growing trees because the bark stays in place long enough for it to establish. This creates a misleading pattern: a declining tree may be covered in lichen, leading people to blame the lichen for the decline. But the lichen is a bystander, not a cause. During dry weather, lichen shrivels and looks faded or dull, then plumps back up and becomes more vivid when it rains. Fungal brackets and conks, by contrast, maintain their shape regardless of moisture.
Green patches on bark are also frequently moss or algae rather than fungus. Algae may have a moist or slightly slimy appearance, and both algae and moss tend to be green to blue-green. None of these require treatment.
When Fungus Signals Danger
The location of the fungus matters as much as its appearance. Conks or mushrooms on the trunk or at the base indicate internal structural decay. A tree with large brackets on the main trunk, mushrooms at the root zone, thinning leaf cover, and dead branches in the canopy is showing multiple signs that its structural integrity is compromised. Cracked or heaving soil around the base adds to the concern.
Fungal growth higher up on limbs, or limited to leaves, is generally less urgent. Cankers on individual branches can often be managed by removing the affected limb. But when the trunk itself or the root plate is involved, the stakes change, because that’s the part of the tree holding everything upright. A tree can look green and leafy in its canopy while harboring extensive internal rot that only reveals itself through the fruiting bodies on the outside. If you spot conks, mushrooms, or major cankers on the trunk or base, having the tree evaluated by a certified arborist is the practical next step.