Spruce (Picea abies) and beech (Fagus syivatica) are the two most important tree species in Slovenian forests, together representing the majority of wood biomass used in Slovenia. However, they are also considered as less durable wood species, and their durability can be further affected in case of inadequate construction and installation. To avoid this, wood is chemically treated, but this is problematic because of its adverse effect on human and the environment. Nowadays, it is preferred to preserve wood by ensuring proper construction and installation of timber products, while the wood itself is treated with natural wood preservatives, such as beeswax and different natural oils. This graduation thesis examined the fungicidal properties of grey clay, the material used by local fruit growers in pruning fruit trees. The samples were treated with three different solutions of clay and distilled water and then placed in a vacuum chamber. Both, the treated samples and the control samples were exposed to wood fungi. The spruce samples were exposed to Gloeophyllum trabeum, the beech samples were exposed to Trametes versicolor and Schizophyllum commune, and the oak samples were exposed to Daedalea quercina, according to the standard EN 113:2004. After 16 weeks of decay, the comparison of weight loss in both, the treated samples and the control samples showed that the grey clay solution did not slow down the degradation of wood exposed to mycelia of different wood fungi, since in all treated samples the weight loss was very similar to or even greater than in the control samples. There was no significant deviation in any of the samples. Consequently, it was determined that different solutions of grey clay do not affect the fungicidal properties of wood.