Hazardous Tree Diseases: Recognizing Structural Risks from Fungal Decay

The Hazard Potential

Most plant diseases reduce yield or aesthetics, but certain tree diseases weaken structure, creating hazards where falling limbs or toppling trees can cause property damage or serious injury. Arborists, landscapers, and grounds personnel must recognize signs of hazardous trees.

Typical Disease Progression

1. A wound occurs (pruning, wind, mower injury, excavation, trenching).
2. Fungus enters the wound (or directly infects roots) and discolors wood.
3. Fungal enzymes decay wood, weakening fibers.
4. Internal trunk/limb wood loses strength, or root support is reduced. Limbs die, crown becomes asymmetrical; small twigs/branches litter ground.
5. Fungus produces fruiting structures (mushrooms, conks, shelves) on limbs, trunk, butt, root flares, or roots.
6. Large limbs fall; in root rot, the entire tree may topple in a windstorm.

Symptoms and Management

Early symptoms: slowed growth, branch dieback, smaller leaves/needles, excessive cone/seed set, premature fall color, severe winter twig kill. Nothing can cure an infected tree; the fungus cannot be fully eliminated from the site after tree removal.

Action: Remove promptly any tree with fungal fruiting structures on multiple limbs, trunk, butt, or roots if it poses a risk to people or property. If most of the tree appears healthy, remove individual branches with fruiting structures immediately, regardless of the fungus identity.

Key Fungi Involved

Armillaria (Armillaria Root Rot / Shoestring Root Rot)

Wide host range (conifers and hardwoods). Annual honey-colored mushrooms (1.5–6" cap, 0.5–1" thick stem) in autumn clusters. White fungal fan under bark at base; dark brown rhizomorphs (shoestring threads) under bark or on roots of hardwoods. Most susceptible: stressed trees (insect defoliation, drought, mechanical injury, improper planting). Mushroom edible but only experts should collect/consume.

Fomes fomentarius

Hard, gray-topped, hoof-shaped conk (6–8" across), enlarging perennially. Underside white with tiny pores. Hosts: cherry, hickory, apple, maple, beech, birch (most frequent).

Ganoderma lucidum

Annual shelf-like structures, single or overlapping clusters, brown to reddish-brown with cream/white margin, up to 14" across; upper surface varnished appearance. Hosts: apple, ash, beech, birch, cherry, elm, sweetgum, hickory, locust, maple, oak, redbud, sassafras, willow, others.

Ganoderma applanatum (Artist's Conk)

Perennial shelf at/near soil line, brown to reddish-brown with cream margin, dry (not varnished), 8–12" across after 5–10 years. Underside light with pores; scratches turn brown (drawing surface). Hosts: apple, basswood, beech, birch, cherry, elm, sweetgum, horsechestnut, locust, maple, oak, poplar, spruce, hemlock, sycamore, willow. Tree slows growth, branches die, leaves small/yellow. Hazardous even before death.

Inonotus dryadeus

Root/butt rot on oaks. Trees may topple before obvious symptoms. Branch dieback, fewer yellowed leaves. Large, tough, irregular shelves at/just above soil line; light to dark brown, roughening to black with age. Cut shelf reveals reddish-brown center. Underside brown with pores. Fruiting structures = sure sign of severe damage. Remove immediately.

Laetiporus sulfureus (Chicken of the Woods)

Massive clusters of bright sulfur-yellow to salmon/orange shelves turning white with age. Form summer/autumn, fall off winter; new ones form next season. Underside has pores. Bark at fruiting site depressed/cracked. Fruiting occurs long after major damage. Trees prone to wind breakage before fruiting. Remove at first sign. Hosts: ash, beech, cherry, butternut, walnut, elm, pine, spruce, maple, oak, locust.

Note: Many other fungi colonize dead/dying trees. Fruiting structure formation, regardless of species, is the final step in structural weakening.