Fungal Bracket vs Decay Cavity Decision Tree
Why this matters
Two findings during a tree inspection signal internal decay, and the customer-facing decision depends on identifying what is happening inside the tree from external evidence alone. A fungal bracket (conk) on the trunk is a fruiting body of a wood-decay fungus that has already established and produced reproductive structures - usually meaning years of decay underneath. A visible cavity on the trunk is a wound that has either healed in, or is currently being colonized, or is the symptom of internal heart rot showing through a thin trunk wall. The same wood-loss percentage produces different risk profiles depending on species, location of the decay (compression face vs tension face), and the decay fungus involved. This tree separates the two findings and routes the assessment toward the correct save-vs-remove decision.
The decision flow at a glance:
Fungal bracket or decay cavity?
|
+-- 1. Aggressive base conk? -------> BUTT ROT: HIGH
| RISK
|
+-- 2. Higher conk, mild? ----------> ASSESS THE
| COLUMN
|
+-- 3. Healed cavity, low target? --> MONITOR
|
+-- 4. Large cavity, t/R ~ 0.3? ----> RESISTOGRAPH
|
+-- 5. t/R < 0.3 on load face? -----> REMOVE
What fungal conks tell you
Conks are reproductive fruiting bodies of polypore and bracket fungi. Each species has a characteristic decay pattern and decay rate.
Ganoderma applanatum (artist's conk): brown shelf with white pore surface that turns black when scratched. Indicates white rot at the base. Slow-progressing in some trees, aggressive in others. Common in oak, maple, beech.
Ganoderma lucidum / sessile (varnish conk, reishi): shiny reddish-brown shelf, sometimes annual. Aggressive root and butt rot. High-risk finding on red and pin oak.
Inonotus dryadeus: thick brown conk at the root flare, often weeping amber droplets when fresh. Aggressive butt rot of oak. High-risk finding.
Laetiporus sulphureus (chicken-of-the-woods): bright yellow-orange shelf, sometimes mass clusters. Brown rot of heartwood, common in oak. Tree may stand for years but the heartwood is hollowing.
Armillaria mellea (honey fungus): yellow-brown mushroom in clusters at the base, white mycelium under the bark. Aggressive root rot. High-risk finding on many species.
Phellinus pini, Phellinus tremulae: conifer and aspen pathogens. Indicate established heart rot.
A fresh conk during the growing season is more diagnostic than an old weathered conk - the fresh conk confirms active fruiting, which confirms active decay metabolism in the wood. Old conks may indicate historical infection that the tree has compartmentalized.
What cavities tell you
A cavity is a void in the trunk where wood was lost (to decay, woodpecker excavation, or initial wound that never compartmentalized). Cavity size is not the whole story - what matters is residual wall thickness (the t in Mattheck's t/R ratio) and whether the cavity is currently expanding.
Healed-over cavity: trunk has formed callus tissue around the opening, the opening is small relative to the trunk diameter, the wood inside appears dry and stable. Often historic wound from a long-removed branch.
Active cavity: opening is expanding, decay wood visible inside, soft or punky wood at the cavity walls, water collecting in the cavity. Indicates ongoing decay.
The Mattheck strength loss formula: when residual wall thickness (t) divided by trunk radius (R) drops below 0.3, the trunk is at significant strength loss. A 24-inch diameter trunk (R = 12 inches) with a 3.6-inch wall thickness on any side is at the t/R = 0.3 threshold. Below that, mechanical strength drops sharply.
Sounding test with a rubber mallet: hollow trunks sound drum-like; sound trunks sound dense and short. Useful field tool but not diagnostic for partial decay or buried cavities.
Resistograph: a thin drilling probe that records resistance as it penetrates the trunk. Identifies decay fronts and residual wall thickness. Required for serious quantitative assessment.
Quick checks during the inspection
Look at the base. Conks at the root flare, ground line, or buttress roots indicate butt rot. Higher risk than mid-trunk decay because the buttress roots provide the tree's stability.
Look at the entire trunk. Conks on the lower 6 feet are more concerning than conks higher in the canopy. Decay at the trunk base affects mechanical strength leverage.
Look at the species. Some species form compartmentalized decay readily (oak, maple) and live with decay for decades. Some species do not compartmentalize well (true firs, poplars) and decay propagates rapidly.
Look at the target. Decayed tree over the house = high consequence. Decayed tree at the back fence = low consequence. The same finding produces different decisions.
Look at the lean and load. A trunk with decay on the compression face (the side under load from the lean direction) is in worse shape than decay on the tension face. Combined with wind loading patterns, decay placement determines failure direction.
Path 1: small healed cavity, sound species, low target
Best action: monitor.
Process: measure cavity opening, photo-document, record location on the trunk. Re-inspect every 1-3 years depending on species and target. The cavity may remain stable for the tree's life.
Path 2: large cavity, t/R approaching 0.3, sound species, high target
Best action: detailed assessment with Resistograph and Mattheck calculation. Decision depends on quantitative results.
If t/R is above 0.3 on all faces and species is decay-resistant (oak), consider canopy reduction to lower wind load on the trunk plus monitoring. Bring the load down to match the residual strength.
If t/R is below 0.3 on the compression face or load-bearing direction, or decay is progressing, remove. The next storm event will exceed residual strength.
Path 3: conks at the base, aggressive species (Ganoderma lucidum, Inonotus dryadeus, Armillaria)
Best action: remove in most cases.
Aggressive butt rot fungi cause rapid loss of buttress root strength. Tree can fail at the root plate with little warning. Even with no other visible defect, the conk diagnosis is itself a high-probability failure indicator.
Exception: very low target, customer accepts risk, willing to accept that the tree may fall. Document the risk acceptance.
Path 4: conks higher on the trunk, less aggressive species (older Ganoderma applanatum, well-compartmentalized infection)
Best action: detailed assessment, then monitor or cable depending on findings.
A tree with established heart rot but a thick residual wall can stand for decades. The decision is canopy reduction to lighten the load, monitoring to track the decay progression, and acceptance that the tree's life span has been reduced.
Conks high in the canopy indicate localized branch or trunk decay, not necessarily systemic. Address the affected limb (pruning) and monitor.
Path 5: cavity with active decay, water collecting, soft punky walls
Best action: advanced assessment now, then load reduction or removal. This is the one finding on the list that is telling you the decay is winning in real time.
Punky walls mean the cavity is not a healed historic wound, it is the current working face of a decay column. Standing water means the void is holding moisture against that face year round. Together they say the tree is losing wall thickness while you stand there, so an inspection interval measured in years is the wrong answer.
What to do:
- Measure residual wall thickness at several points around the cavity, not just at the opening. Decay is rarely symmetrical, and the thin spot is what fails. Where the geometry is not readable from the opening, this is the case that justifies resistance drilling or sonic tomography rather than a mallet and a guess.
- Compare the thin wall against the load direction. A thin wall on the compression side of a lean, or on the side facing the prevailing storm wind, is worse than the same measurement on the opposite face.
- If the residual wall is at or below the strength-loss threshold on any face, or the opening is a large arc of the circumference, the assessment is done. Recommend removal, and restrict use of the target area until the crew arrives.
- If the wall is still adequate, the intervention is crown reduction to lower the load the compromised section has to carry, plus a short reinspection interval, typically annual or after any major storm. Mark a reference point and photograph from the same spot every visit so wall loss and opening growth are measurable, not remembered.
What not to do, because customers and older crews will ask for all three:
- Do not drill a drain hole at the bottom of the cavity. It breaches the tree's barrier zone below the existing decay and opens fresh wood to the same fungus. The tree is not drowning. The water is a symptom.
- Do not fill the cavity with concrete, foam, or gravel. Fill traps moisture against the decay face, abrades the callus with every wind cycle, hides the progression you need to monitor, and turns a removal into a saw-destroying job later.
- Do not excavate or scrape the cavity clean. Aggressive cleaning cuts through the boundary tissue the tree built to wall the decay in, and spreads it.
- Do not seal or paint the opening. Same research, same answer, as with pruning wounds.
What is legitimate: remove loose debris by hand if it is holding water, prune deadwood associated with the decay column, reduce the load, document, and set the return date. Then tell the customer plainly that an active cavity is a progressive condition and the question is not whether the tree will need removal but when, so the annual look is what buys the remaining safe years.
References
- ANSI A300 Part 1 Pruning standard, current edition
- ANSI Z133 Arboricultural Operations - Safety Requirements
- ISA Best Management Practices: Tree Risk Assessment, second edition
- ISA Tree Risk Assessment Qualification (TRAQ) manual
- Mattheck, C. - body language and trunk strength loss formulas
- Shigo, A.L. - CODIT (Compartmentalization of Decay in Trees) model
- USDA Forest Service Forest Insect and Disease Leaflets - common wood decay fungi identification
- IML and Frank Rinn - Resistograph methodology and interpretation