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.