Fungi have been found to be the most important cause of wood decay.  The three groups most commonly identified to classify the degradative processes of wood decay by fungi are white, brown and soft rot.  Brown rot wood decay is caused exclusively by fungi in the Basidiomycetes.  Basidiomycetes include the polypores (Polyporales) and gill fungi (Agaricales) which encompass the overwhelming majority of wood-decay fungi which impair the stability and fracture-safety of urban trees.  Brown rot is caused by only 6% of all the known wood-decay fungi and breaks down cellulose and hemicelluloses in the wood substrate while leaving lignin preserved in a modified form, giving the characteristic color and consistency of the decayed wood.   White rot fungi are found in the Basidiomycetes and Ascomycetes.  Befitting of its name the white rot fungi are known for the bleached appearance of the decayed wood where lignin, cellulose and hemi cellulose are broken down.

Wood decay fungi are divided according to their region of occurrence in the tree into stem decay pathogens and root decay pathogens including root and butt decay.