If you discover a fungal infestation in your house, you quickly wonder whether it's dry rot (Serpula lacrymans). You've either heard of this fungus before, or you'll quickly come across it when searching online for "fungal infestation in the house." And rightly so, because dry rot is a specialist that thrives best in the changing environmental conditions found in a house. A fruiting body of dry rot can be seen at the top of this page.

Studies from Germany and other European countries show that dry rot fungi are the causal agents in 20-70% of the investigated cases of fungal damage in buildings. Other fungi that are less frequently reported are the cellar fungus and the white pore fungus.

In cases of fungal wood damage, the focus, is clearly on the "true dry rot fungus". It can cause extensive damage, and remediation must be carried out meticulously to reliably prevent recurrence of the infestation.

How can a fungal infestation with wood-destroying fungi occur in the house?

To better understand this, let's consider how fungi spread and reproduce: Fungi reproduce through spores, which are released in large quantities by their fruiting bodies and dispersed by wind, humans, and animals. On average, 1,000 to 10,000 fungal spores are found in one cubic meter of air. Therefore, in a year, approximately 1,000 spores land on every square millimeter of the Earth's surface. This is why spores are often described as omnipresent. It's safe to assume that every house contains spores of wood-destroying fungi.

However, this does not mean that your house is constantly at risk of being infested by fungi, because the spores only germinate on a suitable substrate and under favorable environmental conditions:

Either the wood on which the spore rests must be damp (so that the spore is moistened and absorbs water), or there must be a consistently high relative humidity level (> 95%) so that the spore is moistened by absorbing moisture from the air, and the wood also reaches a moisture content of approximately 30%. Only then can the delicate hypha, which grows from the spore during germination, survive. Like all living things, fungi cannot survive without water – including the dry rot fungus.

The chance for successful germination and mycelium formation (mycelium being the network of fungal hyphae) increases when there is no draft - as drafts could cause the initially very sensitive mycelium to dry out. Therefore, successful fungal infections through spore germination are most likely to begin in rooms with high relative humidity and little air exchange, such as in infrequently used damp cellars, crawl spaces, gaps in masonry, between brick wall and wood panelling, or in cavities under wooden floors. When temperatures are around 20-23°C and the wood is optimally moistened, dry rot can thrive and, under optimal conditions, grow approximately 5 mm per day (15 cm per month).

We have now discussed the most important prerequisites for infection through spore germination. However, there are also known cases where dry rot, with its already established mycelium, has penetrated a house, for example, from an infested neighboring house, or, less frequently, from infected wood outdoors. This can only happen if the building envelope has weaknesses, such as damaged seals or leaks like faulty downpipes, and the house and the wood within it are therefore no longer completely dry.

How could dry rot spread in a house after successful germination?

Once the spore has germinated, the hyphae grow and colonize the surface of the wood in search of nutrients. This results in the formation of a white, loose surface mycelium. At the growth front, the hyphae release an organic acid and other substances that penetrate the wood and destroy its structure. These wood fragments are further broken down by released enzymes and then absorbed by the hyphae. New hyphae grow deeper into the wood, destroying it. The fungus precisely monitors the balance between nutrient availability and environmental conditions.

Sufficient water is particularly important at the growth front, because the substances released by the fungus and the wood debris can only be absorbed by the fungus from an aqueous solution.

As the mycelial front grows and moves further away from the moisture source, specialized hyphae differentiate in the surface mycelium: vascular hyphae for water and nutrient supply and fibrous hyphae for strengthening the developing mycelial strands. These mycelial strands, which can reach up to 8 mm in thickness, are crucial for the dry rot fungus, as they allow it to spread across larger areas and through unfavorable nutrient zones. This enables it to grow on walls and even penetrate them. With its extensive network of strands, the dry rot fungus can spread over a large area of the house and keep the hyphae at the growth front alive and supplied with water and nutrients, even under changing environmental conditions.

The water transport through the strands is so efficient that the fungus releases excess water back into the environment, thereby also moistening the wood. Water droplets can form on the mycelium and fruiting body, making it look as if the fungus is crying. This observation is reflected in its Latin name, "Serpula lacrymans," which literally means "shedding tears."

During the life cycle of dry rot, the initially white surface mycelium develops into a gray, lobed surface mycelium with embedded strands. This mycelium is highly hydrophobic and can reduce water evaporation from the wood (see image below). This mycelial structure allows the fungus to retain moisture in the wood. Furthermore, as described above, wood moisture retention is promoted in narrow, poorly ventilated spaces.

The formation of a widely branched mycelial network with strands that allow the fungus to overcome barriers and hydrate and supply nutrients to the growth front even under unfavorable growing conditions is one of the main reasons for the dominance of the true dry rot fungus in buildings.

For what other reasons is it necessary to be particularly careful in the case of a dry rot infestation?

Firstly, because the dry rot fungus can produce enormous quantities of spores, and secondly, because the spores can remain viable for up to 20 years. During its development, the dry rot fungus forms fruiting bodies that can produce billions of spores. It has been calculated that a single fruiting body measuring 1 square meter can release 3 billion spores per hour. This release continues for weeks and months. Surfaces and objects in a room with dry rot fruiting bodies can indeed be covered with an orange dust layer of dry rot spores. Additionally, "resting spores" form in the mycelium of the dry rot fungus, which can also survive periods of drought for several years.

Therefore, thorough removal of the mycelium is crucial during remediation. Depending on the specific situation, it may be advisable to flame the walls to destroy any adhering mycelium and spores.

The most important remediation measure, however, is always the permanent elimination of the moisture source! Because if the wood used in construction remains dry, any remaining spores cannot germinate and mycelium remnants cannot grow again.

It is also important to ensure that all soaked areas are completely dried.

In general, the control of wood-destroying fungi (and insects) in buildings should be carried out by specialist companies in accordance with the specifications of the wood protection standard DIN 68800-4 Wood protection - Part 4: Control measures against wood-destroying fungi and insects and remediation measures.