Root and trunk decay

Cause

Wood-rotting fungi entering through wounds, stubs and dead branches

Outside signs

Conks on trunk or limbs, mushrooms at the base, cavities, cracks, lean

Root clues

Soil fissures near the base, lowered grade, exposed roots, construction damage

A conk means

Rot generally extends several feet above and below it

Local example

A root disease complex damaged 164 acres of subalpine and white fir in Nevada in 2023

A conk generally means rot several feet above and below it. A tree can look green and still be hollow. Decay fungi enter through wounds, old pruning cuts and dead branch stubs, and then spread through the heartwood or the roots. This page explains how to spot decay, what an arborist checks and how the decision between keeping, supporting, pruning and removing is made. Battle Born Tree Service does not spray, inject or apply products to trees. We identify the problem, prune out dead or hazardous wood and remove trees that cannot be saved.

What are the signs of decay?

UC Statewide IPM lists the warning signs of a hazardous tree as brackets, conks, mushrooms or other fungal fruiting bodies on the bark or around the base, along with cankers, cavities in the trunk or at the base, cracks in the trunk or at a fork, dead or dying limbs and a lean. Washington State University Extension explains that conks are hard, woody and often hoof-shaped, that a conk generally means rot several feet below and above it, and that more or larger conks mean more decay.

No conk does not mean no rot. WSU notes that some fungi fruit only after extensive decay has developed, and that others fruit rarely, so a tree with no visible fruiting bodies can still be decayed. Root decay is the hardest to see, since roots are less accessible and few decay fungi fruit directly from roots.

What do the roots show?

UC IPM lists fissures in the soil near the base of the tree as a warning sign, and its examples include exposed roots from a lowered soil grade. Construction damage can cause decline, hazard or death, and lightning can damage roots or internal wood without much outside sign. Roots cut by trenching, covered by fill or compacted by vehicles are common causes on residential lots. See tree roots, sidewalks and sewer lines and calling 811 before digging.

Which trees and where?

Decay can occur in any tree, but WSU notes that resinous species such as pines and spruces are considered more resistant than non-resinous species, including true firs and hardwoods. In Nevada the Forest Service's 2023 forest health report describes subalpine fir decline as a root disease complex, caused mainly by the root fungus Heterobasidion annosum together with the western balsam bark beetle, that damages subalpine fir and white fir. It mapped 164 acres in 2023, down from 414 in 2022. For yard trees on the valley floors, decay is most often found in older trees with old pruning wounds, broken limbs and dead stubs. See Fremont cottonwood and white fir.

What does an arborist check?

An arborist checks fruiting bodies, cavities and cracks, the root collar and soil, crown condition and what the tree could hit. The list below says what each check tells them.

• Fruiting bodies, their size and position, and what species of fungus they suggest
• Cavities, cracks, included bark and old wounds, and how much sound wood surrounds them
• The root collar, the soil around it, and any lean or heaving
• Crown condition: thinning, dieback or early fall color can point to root trouble
• The target: what the tree could hit, and how much the use of the space matters

When is pruning, support or removal the answer?

WSU is plain that there is no cure for wood decay. The choice depends on how much of the trunk or root system is affected and what is underneath. A tree with a small decayed wound and a sound ring of wood may be kept, with dead limbs pruned out and, for a weak fork, cabling to hold it. See dead wooding and cabling and bracing. Support cannot make a rotten trunk sound, so it is used on structure that is otherwise good.

Removal is the answer when decay is extensive in the trunk or the structural roots, when the tree leans with soil heaving at its base, or when a failure would hit a house, a car or people. UC IPM recommends that a tree whose failure could injure people or damage property be inspected by a competent expert, such as an arborist certified by the International Society of Arboriculture. Battle Born Tree Service has two ISA Certified Arborists on staff; request a free estimate or call 775.232.8243. See arborist consultation, tree removal and warning signs before a tree fails.