Mushrooms at the base of a tree: what the sources say they may indicate
The International Society of Arboriculture, the University of Minnesota and Oregon State University each describe fungus growing at the foot of a tree. This page reports what they say it may indicate and who the sources say can look at the tree.
This page is for tree owners. It reports what the named sources say and is not advice about your tree: for that, ask a qualified arborist or one of the offices under Who to call.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page reports what the named sources say. It does not say whether any tree has decay, whether it is safe or what should be done with it. ISA and Oregon State each point to an arborist, and Oregon State says a photo of the fungus cannot show how much rot there is.
What ISA lists
The International Society of Arboriculture (ISA) brochure Recognizing Tree Risk includes, among the questions in its tree risk checklist, whether mushrooms are present at the base of or under the tree. It says the more obvious signs of decay include fungal fruiting bodies and cavities, and that an answer of yes to any checklist question is a reason for the tree to be examined by a qualified arborist.
What a university extension service describes
The University of Minnesota Extension page on fungi growing on the trunk or branches of birch sets out four kinds of fungal growth, each with the signs it lists.
| Name on the page | Signs it lists |
|---|---|
| Armillaria root rot | Honey-coloured mushroom clusters at the base in autumn; soft, white, spongy rotted wood that can extend high up the trunk; black strands like shoelaces in the soil or beneath the bark; white fungal sheets between bark and sapwood near the base; weak growth and dead branches in the upper crown |
| Heart rot | Fruiting bodies of varied colour, shape and size along the stem, often by a pruning wound or crack; wood at the centre that is stained, soft or crumbly; a crown that can look normal |
| Canker rot | Black, rough, cracked knobs or cones growing out of the trunk, often by an old wound or branch stub; trees often snap where the growth is |
| Ganoderma root and butt rot | Shelf-shaped conks from the base up to 3 feet (0.9 m) up the trunk, shiny reddish-brown on top and porous white below; trees often snap or topple in storms; small leaves that yellow early and a thinning crown |
The page is written for birch, and the extension service's own list covers a few fungi only. The site's guide on decay fungi and its page on Ganoderma describe how arborists name the fungi that cause these signs.
A question answered by Oregon State
An Oregon State University Extension answer, what is this fungus and could it indicate internal decay in my Douglas-fir, dated January 2024, was written for an owner who found a large fungus at the base of a Douglas-fir. It names the fungus as Phaeolus schweinitzii, called cow pie fungus or Schweinitzii root and butt rot. It says the fungus could indicate internal decay of the root and butt, which it describes as the lower trunk up to 8 feet (2.4 m), but that it is difficult to know how much rot is there from a photo of the spore-producing structure. It says that if the tree is near a house, this is an issue warranting further inspection and possible tree removal, and that an arborist should be able to detect internal decay in a Douglas-fir.
Where the sources agree
All three sources link fungus at the foot of a tree to possible decay below or inside, and Oregon State says a photo of the fungus alone cannot show how much rot there is. The guide on whether a tree is safe reports who the sources say assesses trees, and the guide on decay and cavities describes the words arborists use.
Who to call
- ISA Find an Arborist: the ISA directory the brochure points owners to for an arborist who can examine a tree
- Consulting arborists by country: the credentials and registers consulting arborists hold in each country
- Oregon State Ask Extension: the university extension service that published the Douglas-fir answer; other states have their own extension offices
Sources
- International Society of Arboriculture, Recognizing Tree Risk brochure, read October 9, 2026
- University of Minnesota Extension, fungi growing on trunk or branches of birch, read October 9, 2026
- Oregon State University Extension Service, fungus and internal decay in Douglas-fir, January 2024, read October 9, 2026