stem decay of black walnut (Phellinus weirianus)

Stem decay of black walnut (Phellinus weirianus)
Photo: Whitney Cranshaw, Colorado State University,

Updated: February 28, 2023

Key points

  • Various fungi such as Phellinus spp., Trametes, Perenniporia, Stereum, Oxyporus, Climacodon, Hericium, and Polyporus may cause stem, trunk, and branch rots.
  • These fungi cause the heartwood of the tree or shrub to decay. The outward symptoms of stem rots are basidiocarps (fruiting body of the decay-causing fungi) emerging from the main stem or branches. They can be fleshy mushroom-like structures that appear annually or hard, flattened, shelf-like, triangular, or shell-shaped perennial structures that grow a new layer each year as the rot progresses. Colors range from white, yellow, or orange to brown, or pale gray.
  • Laetiporus sulphureus produces sulfur yellow to bright orange basidiocarps on ash, beech, cherry, maple, oak, and tulip tree.

Wood rot problems

  • Wood rots cause wood to decay in a number of chemical and physical reactions.
  • Infection usually occurs when the fungus gains entry through pruning wounds, frost cracks, or broken branches.
  • Wood rot fungi can infect healthy or dead branches.
  • As the wood decays, the wood lignin dissolves and cellulose predominates.
  • Lignin is the major chemical component of wood and gives wood its strength. The end result is brittle, stringy, or crumbly heartwood, which is weak and may fracture across the grain, or into cubical chunks, depending on the type of wood rot.

Wood rot of trees and prevention

  • Basidiocarps of trunk and limb rots are found anywhere on the trunk or large branches.
  • Basidiocarps located on the trunk near ground level may be the symptom of root rot or trunk rot. To determine whether it is a trunk or root rot look for other symptoms. Because the heartwood of a tree is not involved in translocation or storage of sap, there may be no other exterior symptoms of trunk rots. Root rots often cause dieback and other symptoms because they disrupt translocation of sap.
  • When basidiocarps are noticed, consult a certified arborist to determine the extent of the rot. Although there are no chemical cures for trunk and limb rots, they can be prevented by proper maintenance practices.
  • To prevent infection, prune during the dormant period, as these fungi produce infection-causing spores that are released throughout the growing season, especially in fall.

Wood rots of shrubs and prevention 

  • Many fungi can be found on dead and dying shrubs. There are no chemical cures once the shrub is infected and fungi cannot be eliminated from the surrounding soil once the shrub is removed.
  • They can be prevented by proper planting and maintenance practices.
  • Avoid damage from improper pruning, lawnmowers, and excavation within the root zone. Avoid planting shrubs in poorly drained compacted soils or in low areas where water collects. Also, avoid placing shrubs near downspouts.
  • Construction of raised beds or grade changes may be needed to ensure proper drainage. Root diseases on older established plants can result from changes in water drainage patterns.

Related Information

Additional resource