Some pathogens don’t just damage trees; they make removal dangerous. Understanding which diseases compromise structural integrity helps identify trees that require special handling. The difference between a healthy tree and a disease-compromised tree can be the difference between routine removal and fatal failure.
Ganoderma Root Rot
The silent killer of mature trees.
The Pathogen: Ganoderma species (particularly G. applanatum and G. lucidum) are white rot fungi that decay lignin, the structural component of wood.
How It Spreads:
- Airborne spores infect wounds and exposed roots
- Infected root systems contact healthy ones
- Movement through contaminated soil
Disease Progression:
- Initial infection at wounds or root damage
- Decay spreads through root system
- Progression into trunk base (butt rot)
- Years of silent decay before visible symptoms
Visible Signs:
- Conks (shelf fungi) at trunk base: Hard, woody brackets often with white edges
- Crown thinning or dieback
- Unusual lean development
- Sudden failure without warning
Removal Implications:
Extreme Caution Required. By the time conks appear, internal decay is typically advanced. Trees may look sound externally while being hollow.
Assessment Tools:
- Resistograph drilling reveals decay extent
- Sonic tomography maps internal structure
- Mallet sounding can detect hollow areas
No-Climb Considerations: Severe butt rot may prevent safe climbing. The tree cannot be loaded at the base without risk of failure.
Felling Unpredictability: Decayed hinge wood doesn’t hold. Felling direction control may be compromised.
Armillaria Root Disease (Shoestring Root Rot)
The most widespread root disease.
The Pathogen: Armillaria species, particularly A. mellea, attack a vast range of tree species.
Distinctive Features:
- Black, shoestring-like rhizomorphs spreading through soil
- White mycelial fans under bark at trunk base
- Honey-colored mushrooms at trunk base in fall (some species)
- Sweet, mushroom odor at infected sites
Spread Mechanisms:
- Rhizomorphs grow through soil to contact healthy roots
- Root contact between adjacent trees
- Colonization of stumps as food bases for further spread
Removal Implications:
Root System Compromise: The primary structural support system may be significantly decayed. Trees can blow over in moderate wind.
Assessment Approach:
- Excavate root collar to examine root plate condition
- Look for white mycelial fans under bark
- Assess lean changes indicating root failure
Stump Considerations: Armillaria colonizes stumps and uses them to attack adjacent trees. Stump grinding or removal with fungicide treatment may slow spread.
Emerald Ash Borer: The Glass Tree Problem
EAB transforms wood structure in ways that create extreme hazard.
The Pest: Emerald Ash Borer (Agrilus planipennis), an invasive beetle from Asia. Larvae feed under bark, girdling trees.
Wood Changes After Death:
Ash trees killed by EAB undergo rapid structural change.
Year 1-2 After Death:
- Wood begins drying
- Some structural integrity remains
- Branches begin failing
Year 2-3 After Death:
- Wood becomes increasingly brittle
- “Glass-like” failure without warning
- Branches shatter rather than bend
- Trunk sections can snap from climbing loads
The “Glass Tree” Industry Term: Professional arborists now use “glass tree” to describe EAB-killed ash because of the shattering failure mode. Wood doesn’t give warning before breaking.
No-Climb Policy:
Industry standard now prohibits climbing EAB-killed ash. The risk of sudden, unwarned failure is too high.
Alternative Approaches:
- Bucket truck access only
- Crane removal
- Controlled felling from ground if conditions allow
- Mechanical felling with operator protection
Identification:
- D-shaped exit holes in bark (1/8 inch)
- Serpentine galleries under bark
- Woodpecker damage from birds feeding on larvae
- Crown dieback progressing from top down
- Epicormic sprouting (stress response)
Dutch Elm Disease
Vascular wilt affecting all American and European elms.
The Pathogen: Ophiostoma ulmi and O. novo-ulmi fungi, spread by bark beetles and root grafts.
Disease Signs:
- Flagging (individual branches wilting and browning)
- Brown streaking in sapwood when bark is peeled
- Rapid crown death in susceptible varieties
- Disease progression over weeks to months
Removal Implications:
Contagion Prevention: Quick removal of infected elms reduces beetle breeding habitat and inoculum for new infections.
Timing: Remove infected elms during dormant season when beetles are inactive.
Disposal: Infected wood should be burned, buried, or chipped and composted. Don’t leave elm logs where beetles can emerge.
Sanitation: Clean equipment between elm jobs to prevent spreading fungal spores.
Thousand Cankers Disease of Walnut
Emerging threat to valuable black walnut.
The Complex: Walnut twig beetle carrying Geosmithia morbida fungus. Beetles create galleries; fungus causes cankers.
Distribution: Currently concentrated in western states but spreading eastward.
Signs:
- Crown dieback starting at branch tips
- Numerous small bark cankers
- Beetle exit holes (smaller than pinhead)
- Branch flagging
Removal Implications:
Quarantine Concerns: Movement of walnut wood from affected areas is regulated. Check quarantine status before moving material.
Disposal: Regulations may require chipping, burning, or burial of infected material.
Disease Assessment Before Removal
Pre-work evaluation identifies hidden hazards.
Visual Survey:
- Fungal fruiting bodies (conks, mushrooms)
- Crown condition (dieback patterns)
- Trunk condition (wounds, cracks, cankers)
- Lean changes suggesting root failure
Physical Assessment:
- Mallet sounding for hollow areas
- Increment boring for decay detection
- Root collar excavation
Instrumental Assessment:
- Resistograph decay mapping
- Sonic tomography
- Aerial inspection for crown conditions
Communicating Disease Hazards to Clients
Help clients understand why disease affects removal approach and cost.
Explanation Points:
- Why climbing may not be safe
- Why equipment is needed
- Why costs exceed standard removal
- Why immediate removal may be recommended
Documentation:
- Photograph disease indicators
- Reference specific conditions in proposals
- Document declining conditions over time if not immediately removed
Disease-compromised trees don’t get better with time. Delayed removal usually means increased risk and cost.
Sources:
- Ganoderma biology: University extension plant pathology resources
- Armillaria research: USDA Forest Service forest pathology
- EAB guidelines: USDA APHIS Emerald Ash Borer program
- DED management: Regional cooperative extension publications