Hypoxylon Canker, Most of the Time

Hypoxylon is a fungus that lives quietly in a lot of healthy trees, oaks and pecans especially. It generally only takes over once the tree is already run down by something else.

Drought, root damage from construction or trenching, a buried root flare, or a few bad summers in a row can all do it. Once the tree loses the ability to hold the fungus back, it spreads under the bark, the bark blisters and falls away, and that powdery layer shows up on the wood underneath. Sometimes the trunk on that side looks like it has been sanded.

A tree trunk with a section of bark missing, showing the wood underneath

Real Arborwise example. Bark missing from a trunk. What sits underneath it, and how far around the trunk it runs, is what we would want to look at.

Why It Matters

It Usually Means the Tree Was Already Struggling

The canker itself is rarely the whole story. It tends to be the visible end of a stress problem that started years earlier. Wood under an active canker can also get brittle, which is why we look at how much of the trunk is involved before we say anything about keeping the tree.

There is no spray for it once it shows. What can be done is decide honestly whether the tree is worth keeping, and take the pressure off the trees around it so they don't follow.

Bark blistering or sloughing off in sections rather than flaking normally
A powdery tan, olive or silver-gray layer on the wood underneath
Dieback in the crown on the same side, or across the whole tree
Recent construction, trenching, grade changes or a long dry spell

Read more from people who study this

Texas A&M Forest Service on hypoxylon canker

Look, Photograph, and Leave the Bark Alone

Photograph the patch and the whole tree. One close shot of the wood under the bark, and one from far enough back to get the whole crown in.

Don't peel more bark off to see how far it goes. It won't tell you much and it opens up more of the tree.

If it has been dry, water slowly and deeply out at the edge of the branches, not against the trunk. That helps the trees around it more than it helps this one.

How Much of the Trunk, and What Is Left Above It

We would look at how far around the trunk the canker runs, what the crown looks like above it, and whether the root flare is buried. Those three things usually decide it.

What fraction of the trunk circumference is involved

Whether the crown above it is still holding leaves

The root flare and the soil around the base

What the neighboring trees are doing, and what can be done to keep them out of it

Sometimes the answer is that the tree has a few good years left and it is worth keeping. Sometimes it isn't. Either way you get a straight answer before anybody quotes a removal.

What Decay Does To Hardware

Decay changes what hardware can do. A cable has to pull against sound wood, so on a tree with real decay at the union, support usually will not hold the way you would want it to, and we would rather tell you that up front than sell you hardware. It is still your tree and still your call, and we will do the work either way. Sorting it out takes eyes on the tree, and that judgment is a good bit of what you are hiring, 20+ years navigating tricky tree situations. More on our cabling and bracing page.

Bark Peeling Off a Tree Trunk

Hypoxylon Canker, Most of the Time

It Usually Means the Tree Was Already Struggling

Look, Photograph, and Leave the Bark Alone

How Much of the Trunk, and What Is Left Above It

What Decay Does To Hardware

Not Sure This Matches Your Tree?