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.

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

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