Oak Tree Fungus Identification: Leaf, Bark, Root Signs

Leaf problems are the most visible and, for the most part, the least dangerous. From UGA Extension's Key to Diseases of Oaks in the Landscape (Williams-Woodward and Pearce, 2012):

• Oak leaf blister (Taphrina caerulescens): yellowish, blister-like bulges on the upper leaf surface, up to about half an inch across. Cosmetic; rake fallen leaves, no chemical control needed.
• Anthracnose: leaves and shoots brown and shrivel, with dead patches that follow the veins. Can cause twig dieback and branch cankers.
• Tubakia leaf spot: brown or reddish-brown blotches, well defined on young leaves.
• Powdery mildew: white, powdery growth on the upper side of leaves. Cosmetic.
• Sooty mold: a black crust or powder on leaves and stems. The fungus grows on sugary insect honeydew, so UGA's advice is to deal with the insects, not the mold.
• Fusiform rust: yellow to orange pustules and black hair-like clusters under the leaves; minor on oaks, serious on pines.

Trunk fungi deserve more attention, because they rot the wood the tree stands on.

• Hypoxylon canker: bark sloughs off to expose gray and black patches of fungal crust that are hard, tan to silver-gray outside and black inside (UGA Extension). It attacks trees already under stress; Yellowstone Landscape adds thinning crowns, undersized leaves, dead limbs and splitting bark.
• Heart rot / canker rot (Inonotus species): a white rot of the heartwood, with conks or mushrooms growing out of the trunk and branch dieback (UGA Extension).
• Not fungi but often mistaken for them: crown gall (large, rough galls, spongy at first then hard), bacterial wetwood (sour-smelling, water-soaked wood and vertical streaks down the bark) and bacterial leaf scorch (leaf edges burn brown with a reddish or yellow border, worse each year). All three are bacterial (UGA Extension).

• Armillaria root rot: crown dieback, slower growth and early leaf drop, with white fans of fungal growth between the bark and the wood at the base (UGA Extension). Yellowstone Landscape also lists mushrooms at the tree base as a sign; these are the fungus's fruiting bodies, typically honey-colored and clustered in late summer or fall (Wikipedia; see our honey mushroom identification page).
• Phytophthora root rot: reduced vigor, yellowing leaves and reddish-brown discolored inner bark; favored by poor drainage and planting too deep (UGA Extension).

UGA lists both as serious, able to kill the tree.

Oak wilt (Ceratocystis fagacearum in UGA's key) is the disease to rule out first in summer. University of Minnesota Extension describes:

• Leaves that turn brown from the edges inward while the centers stay green, with wilting that starts at the top of the crown and moves down quickly.
• Red oaks that can lose all their leaves and die in as little as four weeks; white oaks that decline branch by branch over 2 to 5 years or longer.
• Spore mats that form between bark and wood from April through July on red oaks that wilted the previous year; Yellowstone Landscape describes them as white, felt-like material under the bark.
• Spread by sap beetles attracted to fresh wounds, and through grafted roots, mostly within 30 feet of an infected tree and up to 50 feet.

Symptoms overlap with drought and other problems, so UMN recommends lab confirmation (in Minnesota, the university's Plant Disease Clinic). Other states' extension services run similar diagnostic labs.

It depends on which fungus and where it is. The management column of UGA's key, summarized:

• Leaf fungi (blister, mildew, anthracnose, leaf spot): usually yes. Rake and remove fallen leaves, prune lower branches to improve air flow, water in the morning and keep the tree watered and fed.
• Hypoxylon canker: remove infected limbs and reduce stress with watering and fertility; the fungus is opportunistic, so a vigorous tree resists it better.
• Armillaria root rot: no cure is given; UGA's advice is to increase vigor with irrigation and fertility.
• Heart rot: avoid new wounds and remove trees that have become hazardous.
• Oak wilt: infected trees are removed, and root connections are cut (UGA mentions trenching; Yellowstone Landscape describes trenches 4 to 5 feet deep) to protect neighbors. UMN reports that propiconazole injections can keep a healthy oak from showing symptoms for up to two years, but do not stop the fungus moving through the roots.

For a large tree near a house, have a certified arborist judge whether decay has made it unsafe; that is a structural question a photo cannot answer.

No single product treats oak fungi, and for many of them no spray is recommended at all. UGA says leaf blister needs no chemical control and sooty mold is managed by controlling insects; it mentions fungicides only for fusiform rust on nursery oaks. The one well-documented treatment for a lethal oak disease is preventive propiconazole injection against oak wilt in healthy trees, which lasts up to two years (UMN Extension). Yellowstone Landscape also mentions spring fungicide applications and injections for anthracnose on valuable trees. Get a diagnosis before paying for any treatment.

Our summary of the sources above, sorted by how urgently to act:

• Call an arborist or extension lab soon: leaves browning from the edges in summer on a red oak; sudden top-down wilting; white fans or mats under loose bark; mushrooms or conks on the trunk or at the base; gray-black crusts where bark has fallen off.
• Watch and improve care: anthracnose on a few lower branches; thinning crown after drought.
• Cosmetic, no treatment: blisters, white powder or black sooty film on leaves.

For the app, an arborist or a diagnostic lab, we suggest: a whole-tree shot showing where in the crown the symptoms are; both sides of an affected leaf next to a healthy one; any fungus on the bark in close-up with a coin for scale; and the base of the trunk. Note whether the tree is in the red oak or white oak group (our oak tree identification page shows how), because oak wilt behaves differently in each. NatureLens can name the oak species and many conks and mushrooms from a photo; a lab test is still the way to confirm oak wilt.

Research and drafting for this page used AI assistance. Every number, date and claim was then checked against the sources listed below, which were opened and read on 2026-09-25. Where a statement is our own advice rather than a sourced fact, the text says so.