Fungi with an appetite for wood

Research in focus August 2015

This house-invader creeps in silently, making itself comfortable in your home for months or even years before you notice its presence. If left undisturbed, it will spread and grow relentlessly, ultimately unleashing its full destructive power. Its scientific name, Serpula lacrymans, which can be freely translated as "weeping serpent", might sound innocent, but the damage caused by this fungus is a veritable nightmare for home-owners and building specialists. Commonly known as dry rot, it is the most-feared species of wood decay fungus in the temperate regions of Central and Northern Europe. Unlike mold fungi, which colonize exposed surfaces and are therefore easily detected, wood decay fungi start their life hidden in the deeper regions of timber structures. Once established, they develop tendrils known as hyphae that burrow through the wood and, given the right climatic conditions, continue to grow out of sight. In this way, they can wreak devastating harm without being noticed and cause such extensive structural damage that, in the worst case, the whole building might collapse. Dry rot has even been known to eat its way through brickwork and concrete. It is also remarkably tenacious, because it can remain dormant for years in a dry state. As soon as conditions allow it to take up moisture again, it continues its destructive work as before. Fortunately, other species of wood decay fungi don’t possess this particular ability, or at least not to the same degree, but that doesn’t make them any more welcome in your home.

The decisive factor applicable to all species of wood decay fungi is "the matching climatic conditions". Like any other living organism, great or small, these fungi require an environment in which they feel “at home” in order to grow. Group manager Dr. Wolfgang Hofbauer and his team of biologists at the Fraunhofer Institute for Building Physics IBP are currently engaged in intensive research to determine the exact nature of these conditions.
"We are not focusing exclusively on dry rot, which is only one of approximately 200 species of wood decay fungi responsible for damage to buildings. But since we can’t cover them all, we have selected the most important representatives of the different groups, which in Germany include the wet rot fungus Coniophora puteana, the white rot fungus Donkioporia expansa, and the brown rot fungus Antrodia sinuosa, as well as the dry rot fungus Serpula lacrymans," the scientist explains. The aim of this research is to gather fundamental data for input into a software tool that will make it possible to predict the probability of infestation by various species of wood decay fungi. The results of this work will also be a basis to better specify the border conditions given in the German standard on wood protection, DIN 68800. The precise data obtained in this way could make it possible to further reduce the unnecessary use of wood preservatives.
Both old and new buildings are susceptible to attack by dry rot and other wood decay fungi. As Hofbauer points out: "Fungi will grow anywhere they find enough moisture." Favorable habitats include the cellars of old houses that were built without a damp proof course to prevent water infiltration from the ground. But even in modern buildings, certain timber elements might be exposed to high levels of humidity, especially the ends of roof beams, where driving rain or thermal bridges allow the passage of moisture. Other environmental factors that influence the growth of wood decay fungi include temperature, the species of timber and its age, ventilation, and elapsed time. "The existing literature provides contradictory or insufficiently founded information about these aspects. And even DIN 68800 largely ignores other factors that contribute to fungal growth, other than moisture levels," says the biologist. "We want to find out more about the minimum growth conditions for wood decay fungi. Among other things, this information will enable us to enhance the hygrothermal models used in the WUFI® simulation software developed by Fraunhofer IBP, thereby improving the support provided to architects responsible for planning new buildings and the rehabilitation or renovation of existing buildings and historic monuments," says Hofbauer, who has been working in close collaboration with his colleagues in the institute’s hygrothermics department on these and similar questions.
To define these minimum growth conditions, the Fraunhofer IBP scientists are developing an add-on module that enhances the method used to assess building materials for their resistance to attack by wood decay fungi. Using a technique known as isopleth range testing, the team is working on measurements to determine the fungal resistance of the timber species most commonly used to construct buildings in Germany and neighboring countries, namely spruce, pine and larch. For Wolfgang Hofbauer and his team, the first challenge was to culture fungi in sufficient quantities for their experiments. They were supported in this effort by a one-of-a-kind living collection of building relevant microorganisms. They inoculated blocks of wood measuring 5 x 5 x 2 cubic centimeters with fungal spores, and tested them in the isopleth range testing unit in their laboratory. "By varying the relevant boundary conditions, we can ultimately define the minimum conditions under which the fungi thrive," says Hofbauer, describing the next step of the process. "Similarly, we can also determine the circumstances under which fungal attack is ruled out."
This is no easy task because, unlike mold fungi, wood decay fungi start to grow beneath the surface. For the researchers – and affected home-owners – this means that the presence of these fungi is not normally detectable until they appear above the surface, by which time most of the wood has already been destroyed. However, the scientists have developed special test methods to reveal these "undercover operations". For instance, they have integrated CO 2 sensors in the isopleth range testing unit. When fungi break down wood, they consume oxygen in a process of respiration that releases carbon dioxide, which can be detected in the surrounding air. Another technique involves microscopy in combination with staining methods. The wood specimens are split open and a thin sliver of material is removed from their core. The researchers then stain the sample with a dye and examine it under a microscope. Depending on the color or intensity of the staining, they can distinguish between wood and fungus. Methods from the field of molecular biology are also used to detect fungal growth.
At the moment, Wolfgang Hofbauer and his colleagues are still busy setting up and conducting tests. The findings of a single series of tests are not conclusive enough to be regarded as solid scientific evidence. The biologists have to inoculate a large number of test specimens with the fungi and analyze them to obtain definitive results. "Sometimes a technique doesn’t work as well as we imagined," remarks Hofbauer. Especially in the initial phase, a lot of time and effort is spent trying to find the most promising test setups using heuristic methods. The results of this fundamental research will be of benefit to all concerned – not least construction planners and home-owners.

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