Entering the rainy spring season, it is not uncommon to begin noticing fleshy mushrooms and other fungi throughout the landscape. The warmer, wet conditions are very favorable for the growth of these structures, and they start to pop up everywhere. Mushrooms and other forms of fungal structures are the fruiting bodies formed by different types of fungi. By the time the fruiting body has appeared, the fungus has already been spreading in the tissue on which it is growing. Sometimes they grow out of dead tissue, such as a root that remains in the soil from a tree that is long gone or a standing tree that has recently died… but not always. Have you ever thought about why these structures grow on living trees, and what that could mean for their structural condition?

According to Christopher Luley, Ph.D., a researcher who published the visual guide called Wood Decay Fungi Common to Urban Living Trees in the Northeast and Central United States, “all decay of any consequence in urban trees is caused by fungi”. In his manual, Luley states that there are two types of fungi which cause wood decay: basidiomycetes and ascomycetes.

Basidiomycetes represent conks, or shelf fungi, and mushrooms that we see growing on living and dead trees. Ascomycetes cause leaf diseases and stem cankers. Luley explains that “fungi decay wood by secreting an array of enzymes with the unique ability to disassemble the complex molecules that comprise wood.” Different species of fungi do this in different ways, and we see this through different types of decay of varying degrees, including white rot, brown rot, and soft rot.

Decay can occur in all woody parts of a tree, and the location and extent of decay impacts the tree’s risk for failure through a range of severity. Root rot refers to decay that occurs just in the roots, and butt rot is decay in the tree’s trunk from the ground level and up a few feet. Heart rot occurs in the center of a tree’s trunk, while sap rot affects the outer part of the trunk. Different species of fungi are typically specific in the part of a tree they will impact.

The presence of a fungal fruiting body on a living tree is always a sign that decay is present in that tree. Decay compromises the structural integrity of wood tissue and should be examined by an ISA Certified Arborist to determine what mitigation is needed. (Find a Certified Arborist in your area ) Trees that are impacted by significant decay pose a greater risk for failure and may be considered hazardous to persons and property when positioned in a location with a known target such as a street or frequently used area.

It is important not to assume that the presence of fungi on a tree creates an immediate death sentence, however. Trees can survive and remain standing with some amount of decay in them for a long time. It’s just important to have it assessed by a professional arborist in order to make that determination. Likewise, trees that have decay in them may not always exhibit fungal fruiting bodies. Look for other indicators including cracks, carpenter ants, cavities, and swollen areas as well. With a keen eye and an arborist in your Rolodex (or, well… your smart phone contacts), you can stay on top of hazardous defects that may arise in your home landscape.