Wood decay fungi guide

Wood decay guide: a resource for non-specialists.

A wide variety of fungi occur on, and in trees, and interact with trees in various ways.  These range from negative interactions, where pathogenic fungi cause disease (on leaves, twigs, roots, flowers, cambium, and other tissues) to essential positive interactions such as mycorrhizae where fungi form an extension of tree roots.

A notable type of interaction is wood decay, where fungi decay the woody components of tree tissues.  This is a negative interaction from the point of view of arborists, and - presumably - from the point of view of the trees themselves :), because by reducing the material strength of wood, it reduces the tree's ability to bear loads (forces) such as gravity, wind, and added weight from growth. 

However, wood decay on standing trees is both
(a) very common especially on older trees that are often extensively decayed,
(b) very  complicated in that is only sometimes "pathogenic" (i.e., involves the damage to living tissues) - those old decayed trees may be physiologically healthy (i.e., continue to photosynthesize, grow, flower/fruit and reproduce), and
(c) very challenging to evaluate in regards to tree stability and tree risk. 

That last point is very important, and is a result of several factors.  For one, research-based information on the exact relationship between the presence of wood-decay fungi and the increase in tree failure is largely unavailable.  Furthermore, the wood decay effects vary across fungal species, but also across tree species, i.e., the same fungus seems to produce somewhat different effects in different tree species.  They even seem to vary geographically (the same fungal species can seem to have different effect in different climates or geographic areas), and they vary over time - wood decay can progress, either rapidly or slowly, or it can be contained and arrested. I emphasize again that research on these issues is scarce.

This guide is an attempt to compile the best available information, as well as professional opinion from contributors, to produce a very basic resource for those who do not specialize in tree assessments.

The Guide is intended to help with identification of the most common wood-decaying fungi seen on standing trees in California, and provides basic information about the decay they cause.

Please read the important notes on page 1. 

The publication is NOT a tree risk assessment guide, and is NOT a substitute for having a tree evaluated by a qualified arborist.  Resources for obtaining professional help are listed in the back of the guide.

The Guide has not been peer reviewed. 

The version posted here was developed in April 2023.  The Guide may be changed in the future; please, check this webpage for any updates.

Wood Decay Fungi Guide April 2023