Wetwood is wood in a living tree that appears watersoaked, darker than normal wood, has a fetid odor, and is colonized by bacteria. It is common, even the normal condition, in the inner wood of many tree species. It benefits the tree by inhibiting wood decay, but in some cases, it can develop into the disease, slime flux.

A young Abies concolor (white fir) with wetwood occupying the heartwood.

Wetwood can be defined as wood in living trees that:

  • is non-conducting but has a high moisture content and appears watersoaked
  • is somewhat darker in color than surrounding wood
  • has a fetid, fermentative odor
  • is colonized by bacteria
  • occupies the heartwood of some tree species on a normal basis:
    • conifers: firs and hemlocks primarily
    • hardwoods: elms, poplars, birches, oaks
  • also may form in response to wounding in sapwood

Is Wetwood a Disease?

Although causation was never demonstrated, wetwood was traditionally considered a tree disease caused by bacteria ​[3]​. This is primarily because bacteria are found within it.  It is sometimes called “bacterial wetwood”, as if to distinguish it from nonbacterial wetwood. All wetwood contains bacteria, so there is no distinction to be made.

The disease concept was reinforced by the fact that, in some cases, wetwood is associated with damage to the tree (slime flux; see The Bad below). 

Is it a disease? Let’s briefly review two common definitions of tree disease.

  • Any malfunctioning of host cells and tissues that results from continuous irritation by a pathogenic agent or environmental factor and leads to development of symptoms ​[1]​.
  • Any deviation in the normal functioning of a plant caused by some type of persistent agent ​[7]​.

Let’s first consider wetwood in the absence of slime flux (see below for supporting references).

Malfunctioning, Normal: In most species where it occurs, wetwood is the normal condition of the heartwood, and does no harm. In fact, when bacteria grow in the wetwood, it provides a benefit (decay inhibition). Therefore, we can’t say anything is malfunctioning or abnormal.

Symptoms: No external symptoms, and internal evidence is normal.

Causation: There has never been a demonstration via Koch’s postulates that bacteria cause wetwood. Quite the opposite. Two experiments showed that wetwood formed under conditions in which bacteria could not exist.

So, no, wetwood is not a disease. However, when it develops into slime flux (which generally occurs in only a limited number of species), it could certainly be argued that the bacteria are causing more damage than benefit, and slime flux can properly be called a disease.

The Good

Wetwood is generally beneficial. Here’s how it works:

  • Wetwood is often the normal condition of heartwood of mature trees in species in which it occurs ​[5, 9, 14, 15]​.
  • Wetwood also forms in response to wounding or infection ​[2, 5, 10, 15]​.
  • Wetwood can be formed under conditions that preclude bacterial growth (in other words, it is NOT caused by bacteria) ​[5, 16]​.
  • Wetwood appears to be wet in part because of accumulation of calcium and magnesium salts of low-molecular-weight organic acids, mainly acetic, propionic, and butyric acids ​[15]​. This lowers the osmotic potential.  A drier transition zone with living parenchyma separates sapwood from wetwood ​[5, 15]​ (see featured image above).  Water likely passes through the transition zone as a vapor to satisfy the osmotic potential in the wetwood. This was recently confirmed in Cryptomeria japonica ​[12]​.  Those results suggest that, over time, water potential of wetwood may not remain more negative than in sapwood, but it is particularly so during winter, when sapwood completes its transition to heartwood.

Wetwood protects the tree from decay fungi:

  • Wetwood is colonized by facultatively and obligately anaerobic bacteria ​[3, 9, 18, 19]​ that consume almost all available oxygen and bring the oxygen content far too low for fungal growth ​[8, 13, 17]​.
  • The bacteria also produce volatile, low-molecular-weight organic acids: acetic, propionic and butyric acids ​[11, 15, 17]​.  These acids are responsible for the odor and kiln corrosion.
  • The combination of anaerobic conditions and inhibitory organic acids largely prevent fungal growth and wood decay in intact wetwood ​[4, 13, 17]​.

Thus, wetwood is formed by the tree itself during heartwood formation and as a response to wounding. It is a favorable environment for the growth of bacteria that create conditions inimical to the growth of root- and butt-rot fungi.

Rather than a disease, wetwood appears to be generally a mutualistic symbiosis.  Trees create conditions favorable for bacterial growth, while bacteria create conditions that defend the tree from decay fungi.  However, wetwood does occasionally develop into slime flux, which can be damaging to trees and thus a disease.

Is it a coincidence that wetwood often occurs in tree species that have heartwood with no effective decay-inhibitory extractives, like Abies (true firs), Tsuga (hemlocks), Betula (birches) and Populus (poplar) spp.? It may well be that this is an alternative strategy used by some tree species to inhibit decay of heartwood and injured wood.

The Bad

Wetwood in Populus angustifolia (narrowleaf cottonwood).

Wetwood has a bad reputation in the logging and wood products industries. Let’s face it: it stinks, is loaded with bacteria, and pressure may build up, squirting the foul liquid on a hapless logger (leading to the appellation piss-fir in some parts).  

It is also associated with a variety of problems during wood products production ​[14]​:

  • Wetwood is more difficult to dry than normal wood and requires more energy.
  • Wood dries unevenly and may warp and twist.
  • During kiln drying, acid vapors may cause kiln corrosion.
  • It is associated with ring shake and honeycomb, two lumber defects.  Ring shake in elm leads to the term “onion elm” in the lumber trade.

Another bad outcome of wetwood, which occurs in a small subset of species, is a condition known as slime flux ​[3, 6, 10]​. This occurs when the gasses produced anaerobically by the bacteria cause pressure to build up, expelling the liquid, which may damage living tissues on its way out. In landscape trees, the liquid may ooze from pruning wounds, cracks, etc., become colonized by a dog’s breakfast of microbes, become slimy, and may kill sapwood and bark that it contacts.  This happens in a minority of affected species.

And the Ugly

There’s nothing ugly about it, but I had to complete the movie title!

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