ISA HK/China --- Introduction to Wood Decay Fungi (WDF)

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Dear Station Members,

An important part of Arborist's work in practising is Tree Inspection & Risk Assessment. This kind of specialist work would require all training of the Arborist & would demand all skills & knowledge of the practitioner. Not every Certified Arborist (CA) would like to take up the work of Tree Inspection & Risk Assessment due to personal competence & the legal liability involved. CA is held to a higher Duty of Care in Court & putting down the CA no. on an Arborist Report is a legal commitment. Identifying decay in tree is an important part in this kind of work.

Decay in tree is mostly caused by the micro-organisms called Fungi. There are hundreds of thousands kinds of fungi already discovered in the biological world & luckily only a handful would cause decay in living trees, whereas the others would decay dead trees (Saprophytes). In carrying out Tree Inspection in HK/China, Fungal Decay is a common occurrence in a lot of our mature trees.

This Station has taken some time to study & research into the vast topic of Fungal Decay in Trees already. Mycology is the name of the game & it is a huge & complex subject indeed. There are lots of textbooks & websites with good description in Mycology, but most would go beyond the need of an Arborist. Arborist would only need to know primarily the Wood Decay Fungi (WDF) for the work rather than antibiotic fungi, yeasts & parasitic fungi which are not of great relevance to our practice. Therefore in here, we focus on what we want & leave the rest to other experts.

Decay in living trees appeared to be mainly caused by two groups of fungi : the Basidiomycetes (BM) & Ascomycetes (AM). BM is the largest & most common of the WDF, & they are frequently known as Conks & Mushrooms. About 500 of the BM group are responsible for decaying living trees. The AM group is smaller in quantity & only a few significant species have been found to rot living tissues. Both BM & AM are capable to rot dead tissues (Saprophytes) which will lengthen their survival period in a natural environment. By doing so, they have become a threat to living trees & they would inflict wounds caused by natural processes such as sudden temperature change, or through artificial processes such as incorrect pruning.

There appears to be 3 common types of wood decay in living trees:

1. White Rot --- This is said to be the most common type of wood decay & the WDF responsible will first degrade the Lignin & then digest away the cellulose, leaving behind a white or bleached appearance in the decayed wood. White Rot is said to cause less wood strength loss then other rots in the initial stages of decay & Zone Lines would usually appear in the advanced stages of decay.

2. Brown Rot --- Brown Rot is said to be less common than White Rot & is usually found more frequently on conifers than hardwood. However, Brown Rot would decay away cellulose & would leave behind modified lignin, thus giving a brown appearance in the rotted wood. Brown Rot would bring about a significant strength loss even in the initial stage of attack & the wood would become brittle which may fracture easily under tension.

3. Soft Rot --- Soft Rot is primarily caused by AM fungi & the wood attacked would lose significant strength similar to Brown Rot. The Soft Rot & Brown Rot can sometimes be difficult to distinguish by visual means. Microscopic investigation may be necessary to carry out the taxonomy.

Decay in living trees is usually named after the position of occurrence, e.g. root rot, butt rot, sap rot, heart rot, trunk rot, etc. Healthy trees can carry out compartmentalization once the entry of WDF is detected. However, if the tree is in poor health or under stress, the WDF will advance with repeated attacks on the boundaries, sometimes with the help of other pathogens, to spread radially & longitudinally until the cambium is killed. Then the WDF will carry on their living cycles on the dead wood & seek opportunities to attack other living wood. Removal of dead wood alone does not seem to minimize infection, since the spores of the WDF are usually air-borne. Proper cultural practice appears to be more effective in controlling WDF in reality.

Among the hundreds of WDF active in our environment, the 3 most significant culprits in damaging living trees appear to be:

1. Armillaria spp --- Commonly known as Shoe-String root rot or Honey Mushroom. This group of WDF is actually edible, if you can clearly identify them (Warning: Most mushrooms are toxic especially the colourful ones. Do NOT just eat without proper identification.). Armillaria ranks among no. 1 as common in decaying root & lower part of a tree. It attacks & degrades the root system, weakening the infected tree to fall in wind. The fungal threads of Armillaria are said to be bio-luminescent. They tend to glow in darkness, giving them a name of ' foxfire' in wilderness.

2. Ganoderma lucidum --- This is the Wooden Ear that is sold in Chinese medicine shop. This WDF is easy to identify by its varnished mahogany appearance in its annual fruiting body. Nevertheless, this is the no. 2 in common to infect living trees. It occurs near the base of the tree & attack roots. It can kill cambium of roots & decline a tree to create potential failure. However, some experts reckoned the presence of the fruiting bodies alone would not usually be the only reason to suggest removal & other symptoms such as cracks, leaning, etc. would need to accompany the fungal attack. Different experts say different things all the time & the Arborist would need to rely on his/her own experience to judge the situation.

3. Ganoderma applanatum --- This WDF is also known as the Artist's Conk due to its saucer like fruiting body on base of trunk of living trees. It can also be found on dead trees & stumps as Saprophyte. This fungus is no. 3 in common as a WDF for urban trees & its presence would justify immediate removal as a sign of advanced decay. Ganoderma applanatum is a perennial fungus which can live up to 5 years or more, different from the above two WDF which are annual.

Basic understanding of WDF is essential for Tree Inspection, Risk Assessment & especially Tree Failure Analysis. WDF exist in our environment as part of ecology & we can not get rid of them unless we live in a desert. It may also be unwise to eradicate WDF or we shall have fewer agents to clean up our fallen organic matters. It may be really to promote the health & defense of a living tree to withstand WDF as wiser, rather than to constantly spray chemicals to rid them when detected. The emergence of WDF in a living tree is usually secondary to the primary cause of abiotic stresses which are likely to be caused by human intervention most of the time.

WDF should be studied together with Dr. Shigo's Tree Biology for a complete picture of Tree Pathology. Tree Pathology is necessary for Diagnosis & Treatment in writing up certain Arborist Report. Once again, it can be seen that building Arborist knowledge is like knitting a net. The different parts link together & missing knowledge would be like making a broken net ...

This Station works hard to develop Arborist knowledge for our Station Members in our territory all the time. This Station also appears to be the only entity in our territory providing such knowledge locally. Knowledge is what we need if we want to improve our Tree Care practice. This Station has to work hard.

More useful research will appear later when this Station has run over them one by one ...

best regards,

Sammy Au
Station Manager

The ISA Mission - Through research, technology, and education, promote the professional practice of arboriculture and foster a greater public awareness of the benefits of trees. (http://www.isa-arbor.com/)