Hypoxylon canker (Biscogniauxia atropunctata, formerly Hypoxylon atropunctatum) is a fungal bark disease that attacks oaks and other hardwoods in decline. Unlike primary pathogens that attack healthy trees, Hypoxylon is an opportunist — it colonizes trees that are already stressed by drought, soil compaction, construction damage, root disturbance, or other factors. Once active, it girdles branches and stems rapidly, producing a characteristic silvery-gray bark appearance as the outer bark sloughs off. In Upstate SC, Hypoxylon canker is common on water oaks, willow oaks, and other stressed oaks in residential landscapes.
How to Identify Hypoxylon Canker
- Outer bark sloughing off in irregular patches to reveal a silvery-gray or tan powdery layer beneath
- The powdery layer (conidia) later darkens to tan-brown-black as the fungal stroma matures
- Affected areas have a distinct boundary between the healthy-looking bark and the diseased section
- Crown dieback often begins in the upper canopy or at branch tips, progressing downward
- Wood beneath infected bark is brown-stained and dead — no green cambium tissue
Hypoxylon as a Secondary Condition: Finding the Primary Stress
When our ISA Certified Arborist identifies Hypoxylon canker on an oak in Upstate SC, the first question is: what stressed the tree? Drought is the most common trigger — water oaks on poorly drained soils that experienced multiple dry summers are especially vulnerable. Construction within the drip line (soil compaction, root cutting, grade changes) is a close second. Root damage from sidewalk repair, utility trenching, or impervious surface expansion near established trees is a consistent factor. Soil compaction from heavy equipment or long-term foot traffic suppresses root function. Once the stress trigger is identified, it informs whether remaining healthy trees nearby are at similar risk — if soil compaction is the cause, other trees in the same area should be assessed.
How Fast Does Hypoxylon Kill?
Once Hypoxylon canker is active and expanding on an oak, it can kill individual branches in a single growing season. Whole-tree death from girdling typically takes 1–3 years, though trees with multiple active cankers on major scaffold branches can decline faster. There is no fungicide treatment that stops active Hypoxylon — once the stroma are visible, the affected tissue is dead. Management consists of removing and properly disposing of infected material to reduce spore load, reducing other stressors on the tree, and making a removal decision based on how much of the crown is already affected. Trees with Hypoxylon on more than 30–40% of the canopy branches are generally in a terminal decline pattern.
Hypoxylon on Water Oaks and Willow Oaks
Water oaks (Quercus nigra) are the most Hypoxylon-susceptible oak species in Upstate SC landscapes. They are short-lived relative to other oaks (60–80 years), typically planted in moist or wet sites, and show Armillaria susceptibility in addition to Hypoxylon. A water oak showing both Armillaria mushrooms at the base and Hypoxylon stroma in the mid-crown is a high-risk tree — two simultaneous decay pathways, one attacking the root system and one killing the crown. Willow oaks (Quercus phellos) are also commonly affected by Hypoxylon, particularly street trees under pavement and irrigation stress. These are among the trees where proactive risk assessment — before the tree becomes an emergency — is most valuable.