What does brown rot fungi primarily decay in trees?

Brown rot fungi primarily decay cellulose in trees. These fungi have a unique ability to break down the cellulose in wood while leaving much of the lignin intact. This selective decay process means that the integrity of the wood's structure is compromised, leading to a progressive deterioration.

Cellulose, a significant component of the cell wall in plants, is comprised of long chains of glucose molecules. When brown rot fungi attack wood, they secrete enzymes that specifically target and degrade cellulose. This results in a dry, crumbly texture in the affected wood, which is characteristic of brown rot.

In contrast, lignin is more resistant to decomposition and is largely unaffected by brown rot fungi, making the ongoing decay process less visual in terms of wood color change compared to white rot fungi, which break down both cellulose and lignin. The other options like starch and sugars are not primary targets for brown rot fungi, as they are more relevant to living tissues or different kinds of fungi that focus on exploiting soluble carbohydrates rather than the structural components of wood. Understanding this mechanism is crucial for arborists when assessing tree health and managing wood decay problems effectively.