Hypoxylon Canker on Oak Trees in Texas: Signs, Causes and What to Do

After long, hot droughts, a lot of Central Texas homeowners notice the same thing on their oaks: bark sloughing off in sheets, and underneath, patches of tan, silver-gray or black crust. Very often that's hypoxylon canker.

What hypoxylon canker is

Hypoxylon canker is a fungus that commonly lives in oaks and other hardwoods without causing harm. It becomes a problem when the tree is severely stressed, most often by drought, heat, root damage or construction. When the tree's defenses drop, the fungus takes over the inner bark and sapwood.

That's why it shows up after dry years across Leander, Cedar Park and the Austin area. It's best understood as a sign the tree was already in serious trouble.

How to recognize it

• Bark falling off the trunk or large branches.
• Fungal crust underneath: tan or brownish at first, turning silver-gray, then black and hard over time.
• Dieback in the canopy, often starting at the top.
• Thin or wilting leaves in summer.

It's often confused with normal bark shedding (some trees shed bark naturally) or with oak wilt. Oak wilt usually shows in the leaves first, like yellow or brown veins on live oaks. Hypoxylon shows on the bark and wood.

Can a tree with hypoxylon canker be saved?

There's no cure once the fungus has established in the trunk. Whether the tree can be kept depends on how much of it is affected:

• A single affected branch on an otherwise healthy tree can often be removed, and the tree's stress reduced.
• Crust across a large part of the trunk usually means the tree is dying, and the wood is decaying and getting weaker.

Why it's a safety issue

The fungus decays the wood, and affected trees can become brittle quickly. Large limbs, or the whole tree, can fail. If an infected tree is near a house, driveway, fence, play area or street, plan to remove it soon rather than waiting.

Does it spread to my other trees?

The fungus is common in the environment, and healthy, unstressed trees generally resist it. The real risk to your other trees is the same stress that weakened the first one. Good care helps:

• Water deeply and slowly during drought (soaking the root zone, not the trunk).
• Mulch the root zone a few inches deep, kept away from the trunk.
• Avoid soil compaction, trenching and grade changes under the canopy.
• Remove deadwood and keep wounds clean.

Infected wood should be removed and not stacked as firewood next to other oaks.

Frequently asked questions

What does hypoxylon canker look like? Bark falling off oak trunks or limbs, revealing tan, gray or black crusty fungus underneath, often with dieback in the canopy.

Can hypoxylon canker be treated? There's no cure once it's established in the trunk. Reducing stress can help trees with only limited infection.

Is a tree with hypoxylon canker dangerous? It can be. The fungus decays wood, so affected trees can drop limbs or fail. Trees near structures or walkways should be evaluated soon.

Is hypoxylon canker the same as oak wilt? No. Oak wilt is a vascular disease that usually shows first in the leaves. Hypoxylon is a fungus that shows on the bark and wood of stressed trees.

Seeing bark peel off your oak?

Use the form below with photos of the trunk, the crusty patches and the whole tree. We'll tell you whether it can stay or should come down safely. Related: signs a tree is dying · tree service.