A minute spore of dry rot lands on wet wood and germinates. The first growth that emerges from the spore is known as the germ tube. This grows and divides to produce fine filaments, hyphae, which invade the timber and secrete enzymes to break down the wood.
As the enzymes break down the wood it becomes even more porous so allowing further water to penetrate into the wood. Furthermore, the byproduct of the decay process is water, which can also contribute to the moisture within the wood. In other words, once you have rot, it begins to generate its own water as a byproduct of metabolism and the process is like an unstoppable chain reaction.
The fine filaments of fungal growth, the hyphae, develop into a larger mass, the mycelium, which grows into and across the damp wood. Under humid conditions the mycelium is white and cotton-like. In a very humid and stagnant environment droplets of water will form on the mycelium like teardrops. The fungus removing excess water from the wood probably causes these droplets.
Under less humid conditions the mycelium forms a silky gray colored skin which is often tinged with yellow and lilac patches. This form of the mycelium can be peeled rather like the skin on the cap of a mushroom.
Strands: Within the mycelium special thick walled hyphae develop-these are known as strands. They are resistant to desiccation and assume their real importance when the fungus spreads over and into ‘inert’ materials such as mortar and brick.
In these situations they conduct water and nutrients to the growing hyphal tips so allowing the fungus to continue to spread over non-nutrient substrates. It is this ability to travel away from the food source, over and through inert materials allowing the fungus to reach more wood. This is what makes dry rot so potentially destructive.
FRUITING BODY (SPOROPHORE)
When growth is usually advanced, a fruiting body-sporophore may develop. This can occur as the result of two different mycelia meeting. The onset of stress conditions such as drying out the wood or environment as well. Light is also thought to be the cause of fruiting body formation in some situations.
The fruiting body takes the form of a fleshy pancake or a bracket, the surface of which is covered with wide pores of corrugations. The surface is orange/ochre colored. The corrugations form the spore bearing surface. The spores themselves are very small, ovoid in shape and orange in color. They develop on a structure known as the basidium, four spores to a basidium. When fully developed a small droplet of fluid forms at the junction between the spore and the fine stalk on which it developed. The pressure exerted by the droplet of fluid trying to form a true sphere is sufficient to eject the spore an inch or more away from the fruiting body, dispersing into the air.
Large numbers of spores frequently collect around the fruiting body under still conditions and form the red ‘dust’ often visible where there is a significant attack of dry rot.
GROWTH AND SURVIVAL
It is essential to understand that water is absolutely fundamental to the growth and survival of not only dry rot, but all wood destroying fungi; wood decay cannot occur, exist or survive without it!!
Spore germination: To initiate growth from a spore the wood must be physically wet. In other words it must be subject to a source of water ingress, e.g. leaking gutters, wet masonry, condensation on cool wall voids etc. In practical terms the wood must have a moisture content in excess of 28-30%. Spores will not germinate on dry surfaces or surfaces which are not suitably wet. In other words, unless the wood is wet, dry rot cannot become initiated.
Growth: While wood needs to be wet for growth to be initiated, a moisture content of around 20% existing mycelial growth ceases and the fungus will eventually die. Decay in wood below 20% moisture content is likely to be very minimal. However, for practical purposes when dealing with fungal decay as a whole, moisture contents of 20-22% should be taken as the threshold figure and assume moisture contents in excess of this level will put the timber at risk.
The fungus flourishes under humid, stagnant conditions; hence growth tends to be secretive and hidden and is therefore often extensive before it becomes evident. Unlike other wood destroying fungi, dry rot can grow significantly on and through damp masonry. Under special conditions, very limited growth might occur over and through dry materials. Distances in excess of 20 feet away from its food source have been recorded. This ability to grow over and through inert material can lead to significant problems of spread. Like all wood destroying fungi, dry rot flourishes in the slightly acidic conditions found in wood. Unlike the others it also flourishes under slightly alkaline conditions, which explains the frequently encountered rapid growth behind and through old mortars and concrete. Growth rates of up to 12 feet per year have been recorded. In other cases the organism may only have spread a few inches in the same period of time. Because there are large variations in growth rates, the age of an outbreak cannot be positively determined. The problem is further complicated since it is not always possible to tell if an outbreak is the result of a single outbreak or the coalescing of numerous outbreaks.
Without a source of food (wood) growth will quickly cease and the fungus eventually die. But research has shown that in the laboratory the food reserves in the mycelium may allow up to 20% growth before spread ceases. This might have important implications in control measures since it could theoretically allow the infection to pass to immediately adjacent non-infected wood even though the original food source had been removed but leaving the mycelium on, say, damp brickwork. For these reasons, the Borates are by far the best means to control the rotting fungus since they will poison the wood as a food source, regardless of the moisture content, temperature or oxygen supply.
Survival: The spores are reported to remain viable for up to 3 years. They could therefore lay dormant until such times when conditions become suitable for their germination, that is, when any exposed wood surface on which they have landed becomes wet. The mycelium can remain viable in damp masonry at around 50 degrees without a food source for up to 10-12 months. But under the damp, humid conditions such as found in a crawl space with temperatures of above 40 degrees, the mycelium may remain viable for up to 9-10 years! If untreated wood is put in contact with damp infected masonry there is always the potential for the new wood to become infected.