brown rot is caused by certain saprotrophic fungi belonging to the Basidiomycota division. This type of rot is characterized, unlike white rot), by being carried out by the secretion of hydrogen peroxide by the hyphae. This hydrogen peroxide, through a process known as the Fenton reaction, releases free radicals "Radical (chemistry)") that are responsible for degrading lignin through a chain reaction that will destroy the lignin polymer.[1]​.

This rot focuses more on degrading plant cell wall compounds, such as cellulose and hemicellulose. Although lignin is partially modified, it is not completely degraded. This is why we can recognize brown rot in nature by the formation of cubic patterns on a decaying log.[2]​.

Brown rot is considered an evolutionary novelty because "white rot" implies that there are several copies of genes that code for class II peroxidase enzymes, however, in brown rot this number of genetic copies is extremely reduced; this implies that there will be no energy expenditure by the cell to synthesize peroxidase enzymes, hydrogen peroxide will simply be released into the environment.[2]​.

This evolutionary novelty is repeated in five groups of fungi: Boletales, Polyporales, Dacrymycetes, Gloeophyllales and Lyophyllaceae. This can be interpreted in such a way that brown rot forms a polyphyletic group of fungi.[3]​.

Although brown rot is almost exclusive to saprobic fungi, it also exists within the fungi that form ectomycorrhizas: in the Boletales group. This phenomenon is associated because ectomycorrhizal fungi, by living in the roots of plants and establishing a mutualistic symbiotic relationship "Mutualism (biology)"), do not need the degradative power that fungi that live in harder parts of the tree, such as the trunk, need; By living in the roots "Root (botany)") they carry out intracellular penetration without there being a degradation of the cell wall.[1]​.

Brown rot is exclusive to soft woods, such as coniferous woods. This aspect is relevant because the members of Pinales produce resins that have antimicrobial activity: these resins prevent the colonization of microorganisms in different parts of the plant; It is a defense system. Taking this into account, the fungi that carry out brown rot are capable of resisting these resins through the production of secondary metabolites, these will be secreted extracellularly as a protective layer for the mycelium.[2]​.

On the other hand, white rot fungi") are adapted to grow on hardwoods; this type of wood is found in plants belonging to the Angiosperm group.[2]​.