Japanese Maple Hypoxylon Canker: Fatal Fungal Diagnosis and Sanitation

Japanese maple (Acer palmatum) with hard, crusty black bark patches, bark separating from xylem, and scorched leaf dieback shows classic Hypoxylon canker—a secondary pathogen attacking stressed trees with no recovery.

Hypoxylon Canker Profile

• Pathogen: Hypoxylon spp. (ascomycete fungi)
• Role: Secondary—invades trees weakened by drought, compaction, construction damage, root loss, other diseases
• Signs: Black, crusty, patchy fungal stroma (fruiting structures) on bark; bark sloughing; white/tan spore dust
• Progression: Rapid once visible; girdles branches → canopy dieback → tree death

Why Treatment Fails

By the time black stroma appear, the fungus is extensively established in the vascular system. Fungicides cannot penetrate dead bark or reverse vascular occlusion. The underlying stressor (often undiagnosed) initiated the decline—Hypoxylon is the executioner, not the root cause.

Required Action: Sanitation Removal

1. Remove entire tree—no partial saving
2. Destroy wood (chip/burn)—don't store as firewood (spores spread)
3. Remove stump and major roots if possible
4. Do not replant Japanese maple in same site

Prevention for Remaining Maples

• Identify and correct stressors: irrigation, soil compaction, grade changes, chemical exposure
• Mulch 3-4 inches to root zone; avoid trunk contact
• Water deeply during drought (1" weekly)
• Protect from mechanical damage (mowers, string trimmers)
• Monitor for early stress signs: leaf scorch, early fall color, reduced growth

Hypoxylon is a sentinel—its presence signals a tree already beyond saving. Focus energy on protecting neighboring trees.