The True Dry rot fungus was found in the Himalayas several times in this century. The finds were able to cross breed with Serpula lacrymans from Denmark and therefore all belong to this species (Harmsen 1960). In order to understand its successful colonisation in buildings we were interested in studying the natural biotope, i.e. the moisture, temperature and soil relationships. All together 15 fruiting bodies of the True Dry rot fungus have been found in the wild during the last 50 years. This suggests that competition from other species is limiting its occurrence in the wild. For comparison every second pre-1920 house in Copenhagen is infected. Dry rot has a temperature optimum of 20°C, which is much lower than in most other fungi. It is able to grow down to -2°C and to endure cooling to -5°C with repeated thawing, but does not survive temperatures between -5°C and -10°C even for a few hours. Other wood decaying fungi such as Tinder fungus, Manyzoned Polypore and Maze-gill can withstand the latter conditions. The occurrence of Dry rot in nature is strongly correlated with thick and persistent snow cover in winter. The upper limit for growth is 25°C and an hour at 37°C is lethal to the fungus, which is why it is not found under tropical and subtropical conditions. The summer period is rather hot both in the Himalayas and on Mt. Shasta, and a rise in temperature is found even within very large stumps and logs. In part of a log temperatures may rise to above 25°C, but the lethal temperature of 37°C is not reached deep in the wood. Similar growing conditions are found in European houses, where fruiting bodies are formed after about 4-5000 degree-days. Dry rot is oxygen demanding and will drown at about 60% wood moisture content of fresh wood, e.g. of pine which has a specific weight of 0.5 and a pore volume of about 70%. During breakdown the specific weight falls to 0.3 and 0.2 with a simultaneous rise in pore volume to 80% and 87% respectively. At the same time the wood moisture content rises with no oxygen deficit problems. It is suggested that wood moisture content should always be compared with specific weight to take into account the degree of breakdown. This can be expressed as an adjusted optimal wood moisture content which is approximately equal to the optimal wood moisture content of fresh wood divided by the specific weight at the actual state of decay. The application of the adjusted optimal wood moisture content gives a better understanding of why dry rot can be found in the Himalayas in stumps with a wood moisture content of 125%, about 5 times the optimal level usually stated, and more than twice the lethal Ievel of 55%. Serpula lacrymans is called dry rot in English, which reflects the fact that in old attacks the wood is actually dry. This is caused by its effective water transport system, where the wood is emptied of water and nutrients, which are transported to areas with remaining cellulose supply. The equilibrium moisture content will fall with falling specific weight during breakdown. Danish houses contain large amounts of calcium and iron in their building materials, much more than can be found in the soil of the Himalayas and on Mt. Shasta. These are compounds which are used for neutralisation of oxalic acid and in cellulose degrading enzyme systems respectively. The sensitivity of dry rot to high moisture content and frost make our houses a perfect habitat, where also rich supply of wood, calcium and iron is present. In houses there are also fewer species to compete with for space and resources than in nature, and those that are there are ousted, due among other things to its ability to effectively transport water from moist areas.

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