Brown rot fungi are a group of wood-decaying fungi that primarily affect hardwood trees. They break down cellulose and hemicellulose in the wood, leaving behind a brown, crumbly residue. This process can weaken the structural integrity of the wood, making it more susceptible to further decay and damage. These fungi thrive in warm, moist environments and can spread rapidly through spores. Common species include Serpula lacrymans and Postia placenta. Brown rot is a significant concern in forestry and construction, as it can lead to economic losses and safety hazards in wooden structures.