5.2 Biotic Tree Pathogens: Wood Decay Fungi, Vascular Wilts & Cankers

Biotic pathogens represent significant biological agents of structural failure, physiological decline, and mortality in urban and forest trees. Understanding how pathogens degrade secondary xylem, disrupt vascular transport, or destroy meristematic tissues is vital for tree risk assessment, Plant Health Care (PHC), and structural stability diagnosis.

---

Wood Decay Fungal Biology and Structural Failure Mechanics

Secondary xylem (wood) is constructed of three primary structural polymers:

1. Cellulose: Linear chains of β-(1,4)-D-glucan assembled into crystalline microfibrils providing immense tensile strength parallel to the grain.
2. Hemicellulose: Heterogeneous, branched polysaccharides (e.g., xylans, glucomannans) that cross-link cellulose microfibrils to the lignin matrix.
3. Lignin: A complex, three-dimensional, amorphous aromatic polymer constructed of phenylpropane units (coniferyl, sinapyl, and p-coumaryl alcohols) that encases the carbohydrate framework, providing compressive stiffness, shear strength, and chemical recalcitrance against enzymatic digestion.

Wood-decaying Basidiomycota and Ascomycota employ fundamentally different enzymatic and physiological strategies that dictate macroscopic wood appearance and biomechanical failure modes.

WOOD CELL WALL POLYMERS & DECAY PATHWAYS

     ┌────────────────────────────────────────────────────────┐
     │                  Intact Wood Matrix                    │
     │  [Cellulose (Tensile)] + [Lignin (Compressive/Shear)]  │
     └───────────────────────────┬────────────────────────────┘
                                 │
         ┌───────────────────────┴───────────────────────┐
         ▼                                               ▼
   WHITE ROT FUNGI                                 BROWN ROT FUNGI
 (e.g., Ganoderma, Trametes)                    (e.g., Laetiporus, Phaeolus)
         │                                               │
   Degrades BOTH Lignin & Cellulose                Hydrolyzes Cellulose & Hemicellulose
   (Enzymatic: Laccase, Peroxidases)               (Non-enzymatic Fenton Reaction: OH•)
         │                                               │
         ▼                                               ▼
   Wood: Bleached, Spongy, Fibrous                 Wood: Dark Brown, Cubical Checking
         │                                               │
         ▼                                               ▼
   BIOMECHANICAL FAILURE MODE:                     BIOMECHANICAL FAILURE MODE:
   Ductile Bending Warning                         Catastrophic Brittle Snap
   (Splintering, groaning, shear)                  (Instant snap-buckling, no warning)

White Rot Fungi: Simultaneous vs. Selective Delignification

White rot fungi possess specialized oxidative enzymes—including laccases, lignin peroxidases (LiP), and manganese peroxidases (MnP)—alongside cellulolytic endoglucanases, cellobiohydrolases, and β-glucosidases.

• Simultaneous White Rot (e.g., Ganoderma applanatum, Trametes versicolor, Fomes fomentarius): Fungi concurrently metabolize lignin, hemicellulose, and cellulose at equal rates. Wood progressively bleaches, loses density, and transforms into a soft, spongy, fibrous, or laminated mass.
• Selective Delignification (e.g., Phellinus igniarius, Inonotus dryadeus): Lignin and hemicellulose are enzymatically stripped away before cellulose is substantially attacked, leaving behind pockets or sheets of pure white, cellulose-rich fibers.
• Biomechanical Failure Characteristics: Because cellulose microfibrils are preserved or degraded gradually in parallel with lignin, wood decayed by white rot fungi retains ductile bending strength and plasticity. Under wind loading, white-rotted stems undergo substantial elastic and plastic deformation, exhibiting audible fiber groaning, longitudinal shear, and fibrous splintering before ultimate failure. Trees with white rot provide structural warnings prior to collapse.

Brown Rot Fungi: Destructive Cellulose Hydrolysis and Brittle Failure

Brown rot fungi (e.g., Laetiporus sulphureus [chicken-of-the-woods], Phaeolus schweinitzii [velvet-top fungus], Fomitopsis pinicola [red-belted conk], Fomitopsis betulina [birch polypore]) selectively target the carbohydrate fraction (cellulose and hemicellulose), leaving the aromatic lignin structure largely intact, though chemically modified (demethylated).

• Mechanism of Depolymerization: Because large enzyme complexes cannot diffuse through the tight pores of intact lignified cell walls, brown rot fungi deploy low-molecular-weight reactive oxygen species via non-enzymatic Fenton reactions:

$\text{Fe}^{2+} + \text{H}_2\text{O}_2 + \text{H}^+ \rightarrow \text{Fe}^{3+} + \text{OH}^\bullet + \text{H}_2\text{O}$

Hydroxyl free radicals (OH•) diffuse rapidly through cell walls, cleaving crystalline cellulose microfibrils into fragments.

• Physical Signature: Wood turns dark reddish-brown, dries out, and undergoes severe shrinkage along radial and tangential planes, cracking into characteristic rectangular and cubical blocks—a condition termed cubical brown rot. The remaining wood has zero fiber cohesion and can be pulverized into powder between fingers.
• Biomechanical Failure Characteristics: Brown rot is exceptionally hazardous in tree risk assessment. Cleavage of cellulose chains causes rapid, catastrophic loss of tensile and bending strength at the earliest stages of infection. Laboratory testing demonstrates that less than 10% wood weight loss from brown rot causes a 50% to 70% reduction in bending strength. Stems and roots colonized by brown rot fungi fail through sudden, brittle snap-buckling without prior ductile deformation or external signs of distress.

Soft Rot Fungi: Micro-Cavities in Secondary Cell Walls

Soft rot fungi (predominantly Ascomycota and Deuteromycota, such as Chaetomium, Ceratocystis, and Phialophora) tolerate high moisture levels, low oxygen, and high extractives where Basidiomycetes cannot thrive. Hyphae penetrate the secondary cell wall (S₂ layer), excavating microscopic helical cavities parallel to the microfibril angle. Soft rot causes superficial softening in wet environments or, in specialized cases, deep structural brittleness.

Major Butt and Root Rot Indicators & High-Risk Fungi

1. Armillaria mellea complex (Honey Fungus):
   • Pathogenic root and collar rot attacking conifers and broadleaf trees.
   • Signs: Sub-cortical white, fan-shaped mycelial plaques beneath bark smelling of fresh mushrooms; dark brown-to-black melanized rhizomorphs ("shoestrings") traversing soil and root surfaces; clusters of honey-colored gilled mushrooms at trunk bases in autumn.
   • Failure Mode: Decays anchoring buttress roots and girdles cambium, leading to tree uprooting in moderate winds.
2. Kretzschmaria deusta (formerly Hypoxylon deustum, Brittle Cinder):
   • Severe pathogen of mature hardwoods (Fagus, Acer, Tilia, Fraxinus).
   • Signs: Early stage forms wavy grey-white crusts with white margins in spring; matures into flat, dull black, carbonaceous crusts resembling burnt charcoal adhered tightly to bark fissures at the root collar.
   • Mechanism: Degrades cell walls via a unique combination of soft-rot cavity formation and white-rot delignification, completely destroying the lignin-cellulose matrix.
   • Failure Mode: Causes brittle ceramic fracture (porcelain fracture). Wood snaps cleanly across the grain with zero splintering or fiber pull-out. Because trees maintain full, vigorous green crowns while root flares are severely compromised, Kretzschmaria deusta is among the most deceptive and lethal structural hazards in arboriculture.
3. Inonotus dryadeus (Warted Oak Polypore):
   • Parasitizes mature oaks (Quercus spp.) and occasionally sweetgum.
   • Signs: Large, thick, cushion-like bracket conks at the root flare that weep amber liquid droplets when actively growing; turning dark brown/black with age.
   • Failure Mode: Progressive white rot of the underside of structural buttress roots, causing whole-tree overturn while leaving the upper root collar visually sound.
4. Ganoderma applanatum (Artist's Conk) vs. Ganoderma sessile / G. curtisii:
   • G. applanatum: Perennial woody bracket with white pore surface that bruises brown; causes extensive white mottled heart rot and butt rot.
   • G. sessile: Annual reddish-brown lacquered shelf; rapidly degrades sapwood and buttress roots in urban hardwoods.

---

Vascular Wilt Diseases

Vascular wilts represent systemic infections where fungal hyphae and microconidia colonize xylem vessel elements. The host tree responds by producing tyloses (balloon-like outgrowths of xylem parenchyma into vessel lumens) and polyphenolic gums to seal off the invader. However, widespread occlusion of xylem vessels cuts off transpirational water flow, causing irreversible vascular wilt.

Bretziella fagacearum (Oak Wilt)

• Taxonomy: Formerly Ceratocystis fagacearum.
• Host Differential:
  • Red Oak Group (Section Lobatae: Q. rubra, Q. velutina, Q. palustris): Highly susceptible. Infection spreads systemically within weeks. Foliage wilts, turns dull bronze-green, and abscises from the crown down while leaves are still partially green. Sub-cortical fungal mats (endoconidiophores) form under bark, producing pressure cushions that crack the bark and emit a sweet, fruity odor attracting vectors. Mortality occurs in 4–8 weeks.
  • White Oak Group (Section Quercus: Q. alba, Q. macrocarpa): Moderately resistant. Vessel anatomy allows rapid, localized tylosis formation that compartmentalizes the pathogen into individual branches. Infection causes chronic "flagging" and branch dieback over multiple seasons; trees often survive.
• Transmission Pathways:
  • Root Grafts: Accounts for 90% of localized mortality in contiguous stands. Functional vascular connections between adjacent oaks of the same group transport fungal spores up to 15–20 meters. Control mandates trenching to 1.2–1.5 m (4–5 ft) deep or vibratory plowing outside the buffer zone prior to tree removal.
  • Insect Vectors: Nitidulid sap beetles (Colopterus, Carpophilus) attracted to sweet fungal mats carry conidia to fresh spring pruning wounds. ANSI A300 Part 1 explicitly prohibits non-emergency oak pruning during peak beetle flight periods (spring/early summer).

Ophiostoma novo-ulmi (Dutch Elm Disease - DED)

• Host: Ulmus species (U. americana highly susceptible; Asian species U. pumila, U. parvifolia resistant).
• Vectors: European elm bark beetle (Scolytus multistriatus) and native elm bark beetle (Hylurgopinus rufipes). Beetles emerging from brood galleries beneath diseased bark carry sticky conidia and feed in the 2- to 4-year-old twig crotches of healthy elms.
• Symptomology: Early "flagging" of a single upper branch (yellowing, curling, browning of leaves), progressing to vascular collapse of the entire crown. Diagnostic sign: distinct dark brown streaking in current-season sapwood rings observed upon peeling bark.
• Management: Macro-infusion of systemic triazole fungicides (e.g., propiconazole) into root flare sapwood; sanitation removals; barrier trenching.

Verticillium dahliae / V. albo-atrum (Verticillium Wilt)

• Etiology: Soil-borne fungal pathogen surviving for decades as durable microsclerotia. Enters roots through natural openings or wounds; colonizes xylem vessels.
• Host Range: Broad host range affecting >300 dicotyledonous species (Acer, Fraxinus, Cotinus, Catalpa, Liriodendron, Tilia). Notably immune: all gymnosperms (conifers), monocots, and certain hardwoods (Betula, Quercus, Juglans, Platanus, Cornus).
• Diagnostic Signs: Subacute to chronic crown thinning; rapid wilting of isolated branch sections; pathognomonic olive-green to greenish-brown vascular discoloration in sapwood of maples (brown in other species).

---

Canker Pathogens and Bacterial Infections

Canker Dynamics: Diffuse vs. Perennial Target Cankers

A canker is a localized necrotic lesion of the bark, phloem, and vascular cambium:

• Perennial / Target Cankers (Neonectria ditissima, Eutypella parasitica): A multi-year host-pathogen conflict. In the dormant season, the fungus expands through cambial tissue. In the growing season, the tree produces defensive callus rolls to wall off the pathogen. This seasonal alternation produces concentric, raised callus rings resembling a target or bullseye.
• Diffuse Cankers (Cryphonectria parasitica [Chestnut Blight], Geosmithia morbida [Thousand Cankers Disease of Walnut]): Virulent pathogens or severely stressed hosts where fungal growth outpaces host compartmentalization. Necrosis expands rapidly without callus formation, girdling the branch or trunk in a single season.
• Facultative Stress Cankers (Botryosphaeria dothidea, Biscogniauxia atropunctata): Endophytic or weak parasites that switch to aggressive pathogenesis when host xylem water potential drops below -1.5 to -2.0 MPa during severe drought.

Bacterial Pathogens

• Erwinia amylovora (Fire Blight):
  • Affects Rosaceae (Malus, Pyrus, Crataegus, Sorbus, Amelanchier).
  • Bacteria overwinter in canker margins. In spring, warm, humid weather stimulates bacterial exudation ("bacterial ooze"). Pollinating insects transfer bacteria to floral nectaries.
  • Rapid downward migration through vascular traces causes scorched, blackened leaves that remain attached to twigs, with terminal shoots wilting into a classic hooked shepherd's crook.
  • Sanitation pruning requires cutting at least 20–30 cm (8–12 in) into healthy wood below visible necrosis, sterilizing tools between cuts.
• Xylella fastidiosa (Bacterial Leaf Scorch - BLS):
  • Fastidious, xylem-limited bacterium vectored by sharpshooter leafhoppers (Cicadellidae) and spittlebugs (Cercopoidea).
  • Causes marginal foliar necrosis on mature urban trees (Quercus palustris, Quercus rubra, Platanus occidentalis, Acer rubrum).
  • Characteristic Sign: Marginal scorch is demarcated from healthy green tissue by a distinct, vibrant yellow or reddish-brown chlorotic halo. Symptoms emerge in late summer and intensify annually, moving basipetally.
• Phytoplasmas (Lethal Yellowing):
  • Specialized, wall-less bacteria inhabiting phloem sieve tubes. Candidatus Phytoplasma palmae affects Arecaceae (coconuts, date palms), vectored by the planthopper Haplaxius crudus.
  • Induces premature fruit drop, blackened flower inflorescence necrosis, progressive foliar yellowing from lower to upper fronds, and fatal collapse of the central spear leaf.

---

Comparative Wood Decay Fungal Matrix