When a spore lands in the right environment and starts to grow –the
fungusthread, named hypha, grows to becomes a network, a mycelia. These
mycelia can vary in size and colour but is most often white, yellow
or grey. Depending on fungal species this can grow both on the surface
and inside the material.
All fungi multiply by very small spores that spread by air. These spores are produced in special carpophores –fruit bodies- that differ form species to species. A fungus can produce a large amount of spores.
The amount of fungus varies with the seasons and is largest during August and September.
Not all microscopic organisms can grow on walls –the environment is very specific due to the changing temperature, insulation and humidity. The growth varies a lot with the local climate –most important for the growth is the micro climate i.e. temperature, light and damp at the wall.
For accretion to happen, nutrition, warmth, water and light or darkness is necessary. The nutrients usually comes from dirt but can also be provided by old paint or when it is chalking.
A limiting factor is the surface in question –how smooth the surface is. On a very smooth surface –a small active surface- gathers less dirt than on a coarse, large active surface. The result is that dirt and nutrition increases with the active surface and thereby increases the risk for accretion.
The most visible sign of dry rot affection is brown dust –it is fungus spores looks similar to cinnamon. The dust can be found on skirting boards and horizontal surfaces indoors.