Brown rot describes two completely unrelated forms of fungal damage: a destructive agricultural disease affecting living plants and a serious form of decay that compromises the structural integrity of timber in buildings. While both are caused by fungi, they are distinctly different organisms that target different materials and require specific environmental conditions to thrive. The primary difference is that one attacks the living tissues of fruit trees, while the other consumes the non-living cellular components of wood.
Brown Rot Disease in Fruit and Ornamentals
The brown rot that devastates orchards and gardens is a fungal disease caused primarily by species in the genus Monilinia. Monilinia fructicola is the most common pathogen, specializing in stone fruits like peaches, plums, cherries, and apricots, causing significant crop loss. The disease cycle begins in early spring as a blossom blight when spores are dispersed to newly opened flowers by wind, rain, or insects.
Infected blossoms wilt, shrivel, and turn brown, sometimes developing dusty gray fungal growth. The fungus can grow into the supporting twig, resulting in an elliptical canker that may exude gum. These cankers can girdle and kill the shoot, remaining attached to the branch and providing a source of spores.
The most recognizable phase occurs as the fruit ripens. A light brown spot appears, rapidly enlarging into a soft rot covered in powdery, ash-gray spore masses. If not removed, the infected fruit shrivels into a hard, blackened structure known as a “mummy.” These mummies and twig cankers allow the Monilinia fungus to survive the winter, perpetuating the disease cycle.
Brown Rot Decay in Structural Timber
The second type of brown rot is wood decay that damages structural timber in homes, basements, and crawl spaces. It is caused by Basidiomycete fungi such as Serpula lacrymans. Brown rot selectively breaks down the cellulose and hemicellulose components of the wood cell walls, leaving behind the weaker lignin. This process gives the decayed wood a characteristic dark brown color.
The wood becomes brittle and loses structural strength quickly since cellulose provides the main tensile strength. As the fungus digests the cell components, the wood shrinks dramatically upon drying, leading to a distinctive pattern of cracking. This cracking forms roughly square chunks, known as cubical fracture.
This decay is highly damaging because it causes significant strength loss even in early stages of infection. Brown rot fungi require elevated moisture content to colonize and grow. Decay is common in areas with persistent moisture issues and poor ventilation, such as subfloors or around plumbing leaks.
Control and Mitigation Strategies
Controlling Fruit Brown Rot
Management of agricultural brown rot requires sanitation and chemical protection. The most effective cultural practice is sanitation, which involves removing all mummified fruit from the tree and the ground, along with pruning out infected cankers during the dormant season. This eliminates the primary source of fungal spores that initiate new infections.
Pruning trees to ensure good air circulation and sunlight penetration helps the foliage and fruit dry quickly, discouraging fungal growth. Chemical control is often necessary in areas with high disease pressure. Fungicides are applied at specific times, primarily to protect the highly susceptible blossoms and again in the weeks leading up to harvest when ripening fruit is most vulnerable.
Mitigating Timber Brown Rot
Controlling structural brown rot requires eliminating the excessive moisture the fungi need to survive. The first step is identifying and correcting the source of water, which may involve improving drainage, fixing leaks, or increasing ventilation in confined spaces. Simply removing the water source will halt the decay, as the fungi cannot degrade wood below a certain moisture threshold.
Wood exhibiting cubical cracking should be removed and replaced with new, treated lumber to prevent the decay from spreading. For prevention, wood can be treated with approved preservatives designed to resist fungal colonization. Maintaining low relative humidity and ensuring continuous airflow are long-term strategies for protection.