What hypoxylon canker is
Hypoxylon canker is caused by an opportunistic fungus (Biscogniauxia, formerly Hypoxylon) that lives quietly on and in healthy oaks doing no harm. It only turns deadly when the tree's defenses are already down — from drought, root damage, soil compaction, construction injury, or another disease. In a stressed tree, the fungus colonizes the sapwood and bark, and the tree can decline and die surprisingly fast. It is best understood as a symptom of a tree that is already in serious trouble, not a primary attacker.
The tell-tale symptom: sloughing, silvery bark
The signature of hypoxylon is unmistakable once you've seen it: patches of bark slough off the trunk or large limbs to reveal a smooth, silvery-grey to tan fungal crust underneath, often dusted with brown or black spores that rub off like soot. As it matures the surface turns hard, crusty, and black. You'll also see rapid canopy dieback above the affected area. By the time the silvery crust is showing, that part of the tree is usually already dead or dying.
Why there's no cure — and what that means
There is no fungicide injection that cures hypoxylon canker the way propiconazole treats oak wilt. Once the fungus has colonized significant sapwood, that wood is gone. This is exactly why the disease matters: it makes drought stress in DFW oaks potentially fatal rather than just unsightly. The only effective “treatment” is to keep oaks vigorous enough that the fungus never gets the opening it needs — prevention is the entire strategy.
Why DFW oaks are at risk
Our post oaks and red oaks are especially vulnerable because DFW conditions stack the deck: long summer droughts, alkaline clay that stresses roots, and — the big one — construction. Post oaks in particular are exquisitely sensitive to soil disturbance; grade changes, trenching, compaction, and root loss during home building or landscaping send them into the slow stress spiral that hypoxylon finishes. Many “mysterious” post-oak deaths a few years after construction are stress decline ending in hypoxylon.
Prevention: keep oaks un-stressed
Everything that keeps an oak vigorous keeps hypoxylon out: deep, occasional watering through summer drought (especially for mature oaks people assume don't need it); a wide mulch ring instead of turf and string-trimmer wounds at the base; no grade changes, trenching, or compaction in the root zone; root-collar checks for buried flares; and prompt attention to any other disease or pest pressure. For high-value oaks under stress, a Plant Health Care program — deep-root fertilization and monitoring — is the practical insurance policy.
What to do if you see it
If you find sloughing, silvery bark on an oak, call an arborist promptly — not because the affected wood can be saved, but to assess how far it has gone, whether the tree is now a hazard (hypoxylon-killed wood becomes brittle and unpredictable), and whether the rest of the tree and your other oaks can be protected. Affected limbs over targets often need removal for safety, and the underlying stress — usually drought or root damage — needs correcting fast for any chance of saving the tree.