What specific type of decay is primarily characterized by the breakdown of cellulose in timber bridge members, leaving a brownish residue?

The specific type of decay primarily characterized by the breakdown of cellulose in timber bridge members, leaving a brownish residue, is brown rot. Brown rot fungi consume the cellulose, which is the main structural component of wood, leaving behind a modified lignin residue. This results in the wood becoming brown, dry, and brittle. The decayed wood often cracks along and across the grain, and it can crumble easily when dry. Because brown rot weakens the wood's structural integrity significantly, it is a serious concern for timber bridges and requires prompt identification and treatment to prevent further damage.