Bark Splitting and Falling on Mature Live Oaks: Hypoxylon Canker

Identification

Grooved trunks with sloughed bark revealing powdery then crusty fungal mats (stromata) indicate Hypoxylon canker. The fungus invades drought-stressed or construction-damaged oaks.

Key Facts

• No cure once symptoms appear; fungicides are ineffective (disease is internal).
• Cankers signal severe stress (drought, root damage, compaction).
• Tree may survive for years but decline progressively.

Management for Valued Trees

• Water deeply: 1 inch per week during summer drought (use a pan to measure).
• Fertilize: 13-13-13 at 1 lb per inch trunk diameter, spread to 20% beyond drip line, water in. Repeat in fall.
• Do NOT prune live limbs—the tree needs all foliage for energy.
• Remove dead limbs to reduce spore spread.

Prevention via stress reduction is the only effective strategy.