For many years the True dry rot fungus (Serpula lacrymans (Wulf.: Fr.)Schroet.) has exclusively been found in buildings. That is why it is called the True dry rot fungus. The origin of the fungus has always been a mystery, but a wild ancestor must have occured. In the literature there is some information about finds of Serpula lacrymans in nature, however it is difficult to distinguish it from the closely related Serpula himantioides, so the identity of these finds are somewhat dubious. For example a specimen from 1896 kept in alcohol at the Royal Veterinary and Agricultural University in Copenhagen and identified as Serpula lacrymans by the famous Danish mycologist Emil Rostrup proved to be Serpula himantioides on closer examination. A paper by Bagchee from 1954 reported finds of Serpula lacrymans in nature from the Himalayas in Northern India, and Cooke (1955) showed its occurrence on Mount Shasta in California, USA. Later Kotlaba (1992) revised 12 finds from nature of Serpula lacrymans from Bohemia and Moravia in the Czech Republic. We have visited all of these areas and either found Serpula lacrymans ourselves in nature or examined and confirmed dried herbarium specimens. During the last fifty years less than 20 collections have been made from these areas. The natural occurrence is believed to be limited by competeting species of fungi. Soil samples from the investigated areas have been analysed for their mineral content. Calcium was found in varying amounts but much less than in mortar infected by dry rot mycelium. Our theory concerning introduction of Serpula lacrymans into houses is that it did not occur until the 16th century. The explanation for this is that Danish and European houses prior to that period had been built entirely of wood, the so-called 'stavhus' and 'bulhus' (engl. transl. 'stave house' and 'bole house') and on stone foundations without mortar. Due to lack of timber a royal order prohibited the building of wooden houses. Instead studwork, bricks and mortar were introduced. About this time reports of severe dry rot attacks in houses began to occur in literature. So this is expected to be the period when the dry rot fungus first invaded the houses. It soon propagated strongly in the houses and spread from one house to another. Since then it has caused problems in houses whenever wood, mortar and moisture were combined.

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