Geographical Magazine
Dry Rot

OK, HERE'S the pitch. Big-time nasty organism escapes from its Himalayan lair to rampage through homes around the world, striking terror wherever it is spotted. Even its name sounds like something from a dodgy sci-fi blockbuster - meet Serpula lacrymans, the bad guy in Dry Rot - The Movie.

Like a building version of cancer, for many dry rot seems like a death sentence on their house. And like cancer, the treatment often seems as bad as the problem. In place of radiotherapy and unpleasant drugs, a house with dry rot usually suffers having its timbers torn out and being doused with some of the nastiest chemicals around. But as scientists have begun to investigate this alien invader further, many are realising that none of this is necessary. To understand why, we have to appreciate just what sort of organism Serpula lacrymans is.

Dry rot was first identified in 1781 in Austria, but its origins remained unknown until the first half of the 20th century, when its only wild source was discovered in the western Himalayan forests by Bagchee, an Indian forestry researcher who explored the region between 1929 and 1952. Yet even in its native land, only 13 instances of S lacrymans were found during that period, living between 3,000 and 5,000 metres up on tree stumps. Apart from two further isolated finds in 1972, no more were made until the 1990s, when a team led by British scientist Jagjit Singh rediscovered the fungus in the Himalayas and sent samples to Britain for molecular analysis. This confirmed the suspicion that the exotic import was indeed the timber- destroying scourge of our buildings.

So how did a rare Himalayan forest fungus end up in lofts from Adelaide to Aldermaston? One theory is that S lacrymans was once a global organism, but for some reason retreated to the wilds. But John Palfreyman of the Dry Rot Research Group of the University of Abertay Dundee disputes this, since the prevalence of similar climactic niches around the world (including one near his own research centre in Scotland) would surely have allowed S lacrymans to re-establish itself in the wild whatever might have hit it in any one part of the globe.

What is not disputed is that the export of large amounts of timber from the Himalayas between 1850 and 1920 allowed the fungus to hitch all around the world, aided by the fact that the logs were often floated down river and then allowed to sit in a sodden state on warm, humid docks or in holds - ideal conditions for fungal growth. While this remains a probable transmission route, the fact that dry rot was known outside India before this time - it was causing ships to sink as early as the 17th century - implies that dispersal must also have occurred some other way, almost certainly by the airborne movement of the billions of spores each fungal fruitbody produces.

All a fungal spore asks for is some damp wood (moisture content of at least 20 per cent for dry rot, 40 per cent for wet rot), the right sort of temperature (cosy, but not more than 25C) and lousy ventilation so that its supply of moisture doesn't dry out - in fact, just the sort of conditions you might find inside any poorly maintained house - since it's only where you've got things like leaky plumbing or blocked gutters that timber moisture levels get so high.

Once the invader's spores have landed on the timber surface, fine filaments known as hyphae appear. These hyphae clump together to form a white mass called mycelium. The next phase of dry rot's attack is the production of oxalic acid, which breaks down the large cellulose molecules which make up cell walls in wood. After this first assault, it sends in an enzyme called cellulase, which further breaks down the cellulose into glucose. It is this glucose that the dry rot feeds on. Without cellulose, your timber becomes crumbly and loses almost all its structural strength, prompting panicky calls to your nearest structural engineer - which, as we'll see in a minute, could be a big and costly mistake.

All the above also apply to dry rot's less nasty cousin, wet rot (of which the most common form is Coniphora puteana, known as cellar rot). But it is dry rot's unique tricks which account for its demonisation, even though wet rot actually causes up to 80 per cent of Britain's timber decay.

Firstly, there are the rhizomorphs, dry rot's tentacle-like strands which reach out from the main fruitbody - a fungal pancake up to a metre wide that would fit perfectly into our sci-fi movie. These rhizomorphs can transport nutrients and water across relatively large distances, and so enable S lacrymans to spread quickly and attack previously healthy timber some distance away from the fungal mothership. Our Himalayan foe's other unique weapon is its ability to feed on the stony parts of your house as well as timber. Experiments by John Palfreyman have shown that the fungus extracts calcium from these parts of the house's structure - plaster, lath and sandstone are rich in the stuff. And yet, despite the apparently indiscriminate hunger of the bugger, when it comes time to taming it, S lacrymans is more of a pussycat than the Himalayan tiger it is feared to be.

Dry rot's Achilles heel is very simple. It needs damp. Deprive it of moisture and you've got it licked. Get timber moisture levels below 20 per cent and keep them there, and the dry rot is doomed. It is this environmental sensitivity which explains its rarity in the wild. As John Palfreyman says, "it is a very insignificant organism in its natural environment".

Research scientist Dr Jagjit Singh's environmental work with the conservation unit of engineering consultants Oscar Faber has pioneered what he describes as "a completely natural, chemical-free means to fight dry rot" which simply involves removing the conditions it needs to thrive. The action list is straightforward: locate and eliminate sources of damp (fix those leaks); promote timber drying with improved ventilation, or with hot-air fans; determine the full extent of the outbreak using the non-destructive methods already available, from the hi-tech (fibre-optic devices) to the canine - there are now dogs known as Rothounds trained to sniff it out (a Danish initiative brought to the UK by Surrey-based Environmental Investigators Hutton+Rostron). And once you've dried out your wood all you need to do is wait a few months, then pop some champagne, because your dry rot is history.

And if you're still worried, why not have your timbers inoculated against dry rot? One of the reasons it is such a wimp in the wild is because it has so many natural predators out there on the forest floor, and scientists have been testing which ones might act as a form of bio-control mechanism, either by attacking dry rot or, preferably, preventing it from taking hold in the first place. The best bet so far is a soil fungus called Trichoderma, which has been found to inhibit or kill S lacrymans, depending on how well- established the dry rot is. In his fungal cellar, John Palfreyman has played out battle after battle between the two, with each fungus scoring victories and some fights ending in stalemate, each one holding onto what it already had but unable to advance. What did become clear, however, was that if Trichoderma is injected into rot- free timber, S lacrymans is unable even to get a foothold - in effect, a form of vaccination which is being put to the test both in labs and outdoors.

So why for so long have "experts" ripped timber out willy-nilly and pumped toxic chemicals throughout the fabric of our houses? And shouldn't we be as worried by the treatment as the problem, when the chemicals used can include PCP (implicated in brain damage, paralysis and birth defects) and Lindane (linked to convulsions and a deadly form of anaemia).

For Jagjit Singh, it's simply a combination of ignorance, panic and self- interest. "The structural engineer who doesn't have the confidence of science will panic. The first thing he will say is 'that's got to come down'." More cynically, Dr Singh also points out also that replacing all that timber is one of the most lucrative parts of the repair contract.

His approach, by comparison, is up to 80 per cent cheaper, as well as being far more environmentally and architecturally friendly, and he's won an increasing number of converts, thanks to triumphs at Windsor Castle, Brighton Pavilion and Manchester's Corn Exchange. "Our clients have been able to avoid large-scale demolition and saved a lot of time and money," says Singh simply.

So rather than acting as if some evil spirit has struck you down when rot takes up residence, perhaps guilty pangs would be a better reaction. These strange organisms aren’t strictly uninvited guests - there's a strong chance that you contributed to letting them in. If you want to reduce the risks to your home from natural invasion, don't panic. Just don't be wet.