Dry rot

This entry refers to the term 'dry rot' rather than any given species of fungi or specific process of material degradation. As noted, the term is applied to different species of fungi in different regions. For the 1956 film, see Dry Rot (film)

Wooden beam with significant fungal growth

Wood decay caused by the brown rot fungus Serpula lacrymans (true dry rot)

Dry rot refers to a type of wood decay caused by certain types of fungi, also known as True Dry Rot, that digests parts of the wood which give the wood strength and stiffness. It was previously used to describe any decay of cured wood in ships and buildings by a fungus which resulted in a darkly colored deteriorated and cracked condition.

In other fields, the term has been applied to the decay of crop plants by fungi and the deterioration of rubber.

In addition, the term can be used as a metaphor for grave underlying problems within a large organization (such as political corruption in government or low morale in the armed forces) that show no symptoms until a sudden, catastrophic failure, much as dry rot of wood in ships caused catastrophic failure.

Discussion

Dry rot is type of brown rot decay caused by certain fungi that deteriorate timber in buildings and other wooden construction without an apparent source of moisture, which is also known as true dry rot. The decayed wood takes on a dark or browner crumbly appearance, with cubical like cracking or checking, that becomes brittle and can eventually crush the wood into powder. An outbreak of certain types of a true dry rot fungus within a building is considered to be an extremely serious infestation that is hard to eradicate, and usually requires drastic remedies to correct. Eventually the decay can cause instability and collapse in structures.

The term dry rot, or true dry rot, refers to the decay of timbers from only certain types of fungi that can provide their own source of moisture and nutrients, and cause decay in otherwise relatively dry timber. It is used in reference to damage inflicted by either: Serpula lacrymans (formerly Merulius lacrymans) predominantly in the United Kingdom and northern Europe; and/or Meruliporia incrassata (which has a number of synonyms, including Poria incrassata and Serpula incrassata) in North America. Such fungi have water-conducting strands and are capable of carrying water and nutrients (usually from soil) into buildings, where they rot timber that would otherwise be dry. Both species of fungi cause brown rot decay, preferentially removing cellulose and hemicellulose from the timber leaving a brittle matrix of modified lignin.

The term dry rot is somewhat misleading, as both species of fungi Serpula lacrymans and Meruliporia incrassata do require an elevated moisture content to initiate an attack on timber (28–30%). Once established, the fungi can remain active in timber with a moisture content of more than 20%. At relative humidities below 86 percent, growth of serpula lacrymans is inhibited, but it can stay dormant at relative humidities down to 76 percent.^[1] These relative humidities correspond to equilibrium moisture contents of wood of 19 and 15 percent, respectively.

Schilling & Jellison^[2] note the potential efficiency of these 'dry rot' fungi in growing away from direct moisture sources, although there is no reference for how efficient a brown rot fungus has to be at translocating water in order to be classed as 'dry rot'. Some have suggested the importance of these fungi providing their own source of nutrients as being more significant than providing an adequate source of moisture. Schilling^[3] suggests efficient nutrient translocation and utilization, notably nitrogen and iron, may be more distinctive in these species than water translocation. Water translocated in this fashion carries nutrients to the extremities of the organism; not, as is sometimes inferred, to simply render dry timber wet enough to attack. Coggins^[4] goes into more detail about water movement in Serpula lacrymans.

The perpetual saturation of wood with water also inhibits dry rot, as does perpetual dryness.^[5]

Dry rot would appear to be a paradoxical term seemingly indicating decay of a substance by a fungus without the presence of water. However, its historical usage dates back to the distinction between decay of cured wood in construction, i.e. dry wood, versus decay of wood in living or newly felled trees, i.e. wet wood.^[6]

Historical use of the term "dry rot"

'Dry rot' is an eighteenth-century term that generally described what is now called brown rot. The term was used because the damage was present in cured or dried timber of ships and buildings and was thought to be caused by internal ‘fermentations’ rather than water.

The (London) Times on Tuesday 12 March 1793 carried an advertisement that informs the reader that the British Colour Company, No. 32, Walbrook, London continues to use, manufacture and sell paints prepared with the Oil of Coal, which is of a very penetrating nature, and hardens wood in an uncommon degree protecting it from weather, dry rot and ice.^[7]

In the early nineteenth century the rapid increase in instances of timber decay attributed to dry rot (brown rot) in the British naval fleet brought the term into wider usage. Thomas Wade's 'A Treatise on the Dry Rot in Timber' was published posthumously by the Navy Office in 1815 following his investigation of the matter in ships from various countries.^[8] The second HMS Queen Charlotte was launched in 1810 and, when inspected, the timbers of the upper decks were found to be infected with 'the dry rot'. By 1816 the cost of repairs for this vessel had exceeded the ship's original construction cost.^[9]

Texts published in the nineteenth century and early twentieth centuries also used the term to describe fungi which produced substantial (white colored) mycelium including; Antrodia (Fibroporia) vaillantii.

Eventually, the term dry rot came to apply to only one or two fungi: the main one being 'Serpula lacrymans', which have water-conducting strands capable of carrying water and nutrients to decay timber that would otherwise be relatively dry. This type of fungal decay subsequently also became known as 'true dry rot'.

Forest Products Laboratory suggests that a more correct term for dry rot would be water-conducting fungi in the category of brown rot as it better describes the true situation as these fungi, like the others, must have moisture.^[10]

Treatment of dry rot timber

See also: Dry rot treatment

There are epoxy treatments available that kill rot by filling in the channels of the damaged wood, killing the rot and restoring structural integrity. Commercial antifreeze (ethylene glycol) is also very effective at preventing dry rot formation as well as killing the fungus. Certain copper compounds, such as copper naphthenate, are available as a brushable solution and are frequently used when dry-rot damage is repaired by splicing in new wood; after removal of bulk rotten wood the remaining original surface is saturated with such a compound (typically green in color) before application of the new wood.

In certain buildings, particularly those with solid 9 inch (or greater) brickwork and those built using lime mortar and flintstone, dry rot has been known to travel through and along the wall surface behind plaster and render. It is therefore recommended that where dry rot is found, plaster and wall coverings should be stripped back to a metre past the infestation in all directions, and the whole area treated. However, given that dry rot attacks only wet timber, common sense should dictate that plaster need not be removed where there is no timber or any timber is dry (outside the zone of wetting that caused the outbreak). Identifying the source of water and allowing the affected timbers to dry will kill dry rot, as it is a fungus and requires water as all fungi do.

Damaged wall with fungal growth

References

1. ^ J.W. Palfreyman, The Domestic Dry Rot Fungus, Serpula lacrymans, its natural origins and biological control. Ariadne workshop 2001.
2. ^ Schilling, J.S and Jellison, J. (2007). International Biodeterioration & Biodegradation. Elsevier. 
3. ^ Schilling, J. (2009). Holzforschung. Elsevier. 
4. ^ Coggins, C.R. (1977). Aspects of the growth of Serpula lacrymans the dry rot fungus. PhD.Thesis. University of Liverpool. 
5. ^ Forest Products Laboratory, Wood Handbook - Wood as an Engineering Material. U.S. Department of Agriculture, 2010.
6. ^ Ramsbottom J (1923). A Handbook of the larger British Fungi. British Museum. London. 
7. ^ The Times (London), Tuesday, 12 March 1793
8. ^ A treatise on the dry rot in timber. http://books.google.com/books?id=uEkVAAAAQAAJ&printsec=frontcover&dq=A+Treatise+on+the+Dry-Rot#v=onepage&q=&f=false. Retrieved 2009-09-12. 
9. ^ "Wooden Ships and Fungi". http://www.uoguelph.ca/~gbarron/MISC2007/jun2007.htm. Retrieved 2009-09-12. 
10. ^ Forest Products Laboratory, Wood Handbook - Wood as an Engineering Material. U.S. Department of Agriculture, 2010.

See also

• Mold health issues
• Wood-decay fungus