Mushrooms, conks and other plate- or shelf-like structures are rarely good signs when seen growing on live trees or popping up from tree roots.
The fungi that produce these types of fruiting structures feed on tissue inside of the tree, weakening it and increasing the potential for tree failure.
This summer has brought me many pictures and samples of one internal decay fungi in particular: Ganoderma. I think the increase in samples is only indicative of a growing awareness by tree owners rather than an increase in actual occurrence of the disease.
Ganoderma exists in the soil and enters tree roots through wounds in the root tissue. Wounds are most likely to occur with digging or construction in the root zone — installation of sidewalks, patios, driveways and retaining walls can cause substantial damage to tree roots.
Lawn mowers and weed trimmers that contact exposed tree roots or the base of the tree also create wounds that allow Ganoderma and other similar fungi to move into a tree.
The fruiting structures that indicate the presence of Ganoderma are typically brown to reddish brown with creamy margins. They look more like a dried pile of goop, than a standard mushroom and sometimes even look like they have been varnished. Sometimes they are yellowish or purplish.
They appear at the base of the tree near the soil line or on exposed roots but never in the high branches of a tree. One species of Ganoderma produces a shelf-fungi known as the artists’ conk because the underside can be drawn upon.
If you observe what look like Ganoderma fruiting structures on a tree in your yard, there are a few things to consider. Most important is the presence of a target — meaning something that could be damaged if the tree fell on it.
Tree size, species, age and extent of decay are also important. For example, a redbud will typically cause less damage than an 80-year-old oak.
There are too many factors that play a role in tree health for anyone to accurately predict how long a tree will stay standing. A certified arborist may be able to provide a recommendation to leave or remove the tree after careful evaluation of the tree and site.
To prevent Ganoderma infection, avoid wounding tree roots and follow good maintenance practices.
Trees should be watered deeply and infrequently over extended dry periods, fertilized only when a soil test indicates a nutrient deficiency and pruned occasionally to maintain health, vigor and structure.
Mulch applied over the root zone of a tree minimizes fluctuations in soil moisture and temperature, thus reducing stress for the tree. Use caution with mulch, however, because too much can hold moisture against the base of the tree and interfere with air and water exchange.
For best results, apply mulch at a maximum depth of 3 inches to 4 inches and keep it pulled away from the trunk of the tree. Make mulch doughnuts rather than volcanoes.
Fungicides are ineffective on Ganoderma because the fungus grows in the interior tissues of the trees it infects.
Trees infected with Ganoderma may exhibit smaller than normal and/or yellowed leaves, numerous dead branches and a slowed growth rate. It may feed on a tree for several years, hollowing it out a little at a time, before tree failure occurs.
There are many species of Ganoderma fungi, with different tastes for the species of trees they prefer. Honeylocust, oak, ash, maple and elm are common hosts in our area.
The disease is more prevalent in the stressful environment of an urban yard than in the forest.
Some species of Ganoderma are considered medicinal in some parts of Asia and are cultivated. Ganoderma fungi are also being researched for therapeutic and medicinal benefits.
Several other fungi produce mushrooms and conks on trees, and all are indicative of internal decay. A few — shiitake mushrooms, oyster mushrooms, and a fungus called hen-of-the-woods — are delicious to eat. Others are just pretty to look at. Regardless of the type of fungi, the potential for tree failure and presence of a target should be considered carefully by the tree owner and/or a tree care professional.
Extension Master Gardeners can help identify your tree root or trunk fungi with pictures and/or samples through the Horticulture Hotline at 843-7058, Monday through Friday, 1 p.m. to 4 p.m., and at 2110 Harper St. You can also stop by the Fairgrounds Demonstration Gardens at the same location to see healthy trees that grow well in northeast Kansas.