Sound WoodDecay fungi at the base of a tree, and what they mean for testing
A conk or a cluster of mushrooms at the base of a tree is one of the clearest outside signs that something is going on inside. The fruiting body is the small, visible, reproductive part of a fungus. The part doing the damage is inside the wood or the roots, and it's usually far larger than what you can see.
This post isn't a field guide to identification. It's about what finding fungi at the base should mean for how you assess the tree and where you put your sensor positions.
Why fungi at the base matter most
The root flare and lower trunk carry the highest loads in the tree. Decay there can affect both the trunk and the structural roots, and decay in the roots can't be seen from above ground at all. A fruiting body at the base is a reason to look closely at both.
It's also worth remembering what a fruiting body doesn't tell you. It doesn't tell you how much wood is decayed, which side it's on, or how fast it's moving. A large decay column can produce a small conk, and a fungus can be well established for years before it fruits at all. That's the gap a sonic test helps fill.
White rot and brown rot
Wood decay fungi are usually grouped by what they do to the wood:
- White rots break down lignin (and cellulose). The decayed wood often turns pale, stringy or spongy. Wood can keep some flexibility for a while, but strength is still lost.
- Brown rots mainly break down cellulose, leaving brown, crumbly wood that cracks into cubes. Brown rot can make wood brittle, so failure can come with less warning.
- Soft rots work more slowly in the cell walls and can matter in some species and conditions.
Both white and brown rot slow sound down, so both show up on a sonic test. What the type of rot changes is how you interpret the remaining wood and how cautious to be.
Common fungi to look for at the base
Which fungi you see depends on your region and the species. A few that arborists in North America commonly find at the root flare and lower trunk:
- Ganoderma species. Shelf-like conks, often with a brown upper surface and a white pore surface. A white rot of the lower trunk and roots on many hardwoods.
- Kretzschmaria deusta. Easy to miss: a thin, grey-to-black crusty growth that looks like burnt material or asphalt at the base. A soft rot associated with brittle failure.
- Meripilus and Grifola species. Large clusters of overlapping caps at the base or on roots, often in late summer and fall. Root and butt rots.
- Armillaria species. Honey-coloured mushrooms in clusters, often with white fan-shaped mycelium under the bark of the root flare. A root rot that can kill trees as well as decay them.
- Phaeolus schweinitzii. Velvety, rust-brown to yellow brackets at the base of conifers. A brown rot of roots and butt.
- Laetiporus species. Bright orange and yellow shelves on the trunk or base. A brown rot of the heartwood.
If you're not sure what you're looking at, photograph the top, the underside and the attachment, note the host species, and get an identification. The name of the fungus can change how you weigh the result.
Note: Fruiting bodies come and go with the season. A tree with no conk today may have had one last fall. Look for old, dried remains at the base and ask the client what they've seen.
Planning a sonic test around fungi
Test low
If the fungus is at the root flare, a test at chest height may miss the decay entirely. Set your positions as low as you can get consistent contact, usually just above the flare where the trunk is close to round. If the fungus is on the trunk, test at that height.
Put a position near the fruiting body
The fruiting body often marks where the decay reaches the outer wood. Set a sensor position close to it (not on it) so you learn whether the decay runs out to the bark there. On the map, several slow paths meeting at that position with no sound paths through it is the pattern to watch for.
Consider two heights
Decay columns often taper or spread up and down the trunk. If the fungus is at the base but you also see a seam or bulge higher up, test both heights and save each as its own tree in the app so the maps stay separate.
Remember the roots
A sonic test of the trunk says nothing directly about decay in the roots. When fungi associated with root rot are present, the trunk map is one piece of evidence. Root collar excavation and a look at the structural roots may matter as much.
Reading the result
When fungi are present, read the decay range with extra care. If the map shows a confirmed pocket near the fruiting body, that fits what you'd expect. If the map comes back mostly green, check your height and coverage before you conclude the tree is sound: a clean map just above a decay column at the flare is a common trap.
Record the fungus, its position, the test height and the map together. Sound Wood's photos and notes keep them with the tree, and Read my photos can suggest visible defects from your pictures for you to confirm. If you're monitoring the tree, the same records let you compare next year.
The short version
Fungi at the base mean decay is established. They don't tell you how much or where. Test at the height of the fungus, put a position next to it, think about the roots, and read the map with the fungus in mind.