How Does Wood Decay Spread in Outdoor Wood?

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How does wood decay spread in outdoor wood? Decay is caused by fungi that feed on the wood, and their spread depends first on moisture. Outdoor wood gets wet and dries again over and over, which gives fungi repeated chances to establish themselves. This explainer answers the question directly, explains the moisture mechanism and the main decay types, and lists what the sources do not settle. It draws on a 2020 review of wood-water relationships and fungal decay, and a 2023 study of white and brown rot fungi, both hosted on the NIH PubMed Central site.

Quick answer

• Moisture is the starting condition: “A key prerequisite for fungal degradation of wood is the presence of moisture.”
• The amount and form of water matter, because “Content, state and distribution of moisture in wood are keys for fungal activity.”
• Decay falls into groups described as “brown-rot, white-rot and soft-rot decay.”
• White rot and brown rot break down different wood components, which the 2023 study examines in detail.

Why moisture is the starting point

Fungi need water, and the 2020 review puts that point at the center of its argument. It states that “A key prerequisite for fungal degradation of wood is the presence of moisture.” It also notes that “Among the latter is its susceptibility to fungal decay,” which refers to the way wood’s water relationships determine how vulnerable the wood is. Moisture is therefore the condition the sources treat as the trigger.

The review stresses that the form of the water matters as well as the amount. It says that “Quantification and localization of capillary and cell wall water – especially in the over-hygroscopic range – is considered crucial for determining minimum moisture thresholds ( MMThr ) of wood-decay fungi.” In other words, the place where water sits inside the wood decides whether a given fungus can grow there at all.

Why outdoor wood is a special case

Outdoor exposure changes how moisture behaves. The review explains that “In contrast, wood in outdoor applications undergoes frequent changes of wetting and drying, and sorption of water in the vapour phase can be easily overruled by liquid water uptake due to precipitation or condensation.” Rain and condensation are the two sources of liquid water the review names.

The review also notes that the research record on thresholds is mixed. It says that “various minimum moisture thresholds ( MMThr ) were determined for different combinations of wood and fungal species.” The review notes that earlier set-ups “differed in external moisture supply and the way of infecting the wood specimens.”

The three decay groups

Wood-decay fungi have been sorted into groups. The 2020 review says that “Wood-decaying fungi have traditionally been assigned to three major groups referring to the macroscopic and microscopic degradation pattern they form in wood,” and it names them as “brown-rot, white-rot and soft-rot decay.” The grouping describes what the decay looks like, and it also gives a rough guide to the chemistry involved.

The 2020 review adds a caution about the labels. It states that “the terms brown-, white- and soft-rot are still widely used because they are providing information about general decay mechanisms.” The labels are useful, but the review treats them as broad descriptions rather than exact boxes.

White rot and selective breakdown

The 2023 study describes white rot in detail. It says that “Members of white rot fungi are able to decompose all structural components in lignocellulosic materials due to them having a complete extracellular enzyme system.” That enzyme system includes “a wide variety of oxidative enzymes, such as Li-peroxidase, Mn-peroxidase and laccase.” The 2023 study says that this system “endows them with the ability to efficiently convert lignin into H 2 O 2 and CO 2 .”

White rot does not always break down everything at the same rate. The study explains that “selective rot initially degrades hemicellulose and lignin, while cellulose is relatively retained.” It also describes a second pattern, in which “simultaneous rot degrades cellulose, hemicellulose and lignin in a rather uniform depletion.” So the decay pattern depends on the fungus and the wood.

The experiment in the 2023 study tested this on wood. It reports that “The results showed that the white rot fungus Trametes versicolor had a selective biodegradation in softwood, which preferentially convert wood hemicellulose and lignin, but cellulose was retained selectively.” The same fungus showed a different pattern in hardwood, where it “achieved simultaneous conversion of cellulose, hemicellulose and lignin.”

Brown rot and carbohydrate loss

Brown rot works differently. The 2020 review describes its typical pattern: “Brown-rot fungi prefer conifers and degrade hemicellulose and cellulose while leaving a modified (brown) lignin-rich residue behind.” The residue is the one the review describes as left behind after the carbohydrates are degraded.

The mechanism is still being studied. The 2020 review states that “The details about brown-rot mechanisms are still under discussion, but it is generally agreed that brown-rot fungi use a two-step oxidative-enzymatic mechanism.” The 2023 study reports a related result, noting that “Both brown rot fungi species preferentially converted carbohydrates” in the wood samples it treated.

Non-enzymatic breakdown

Some breakdown does not rely on enzymes alone. The 2023 study notes that “fungi are also capable of converting cellulose and lignin non-enzymatically by Fenton chemistry and make wood fibers brittle.” The study lists this as a separate route alongside enzymatic breakdown, and it does not test outdoor boards.

For related reading, see why untreated wood turns gray outdoors and how white rot fungi break down wood lignin.

How we chose these sources

The two sources cover different levels of detail. The 2020 review is a broad survey of wood-water relationships and fungal decay, and it includes a discussion of outdoor wetting cycles. The 2023 study is an experimental paper on white and brown rot fungi acting on softwood and hardwood. Both pages returned HTTP 200 on direct requests, and both are hosted on the NIH PubMed Central site. The 2023 study is about bioenergy pretreatment as well as decay, so its findings are used here only for the decay mechanisms.

What we could not verify

The 2020 review states that “The limitations of the various methods and experimental set-ups to investigate wood-water relationships and their role for fungal decay are manifold.” The thresholds that define when decay can start are therefore still uncertain, and the review calls for combining methods “to provide new insights and eventually a breakthrough.”

The 2023 study is a laboratory experiment with biomass pretreatment, and it does not test a garden board or deck. Its results describe the chemical selectivity of each fungus, not the time it takes for a particular outdoor board to fail.

What the sources do not say

The sources do not give one moisture percentage that stops decay in every outdoor board. The 2020 review describes thresholds as varying by “wood and fungal species,” and it describes the definition of fibre saturation as “somewhat diffuse.” The sources also do not give a spread rate in meters per year for outdoor wood, and they do not recommend a treatment for any particular structure.

Frequently asked questions

Does wood decay need liquid water?

Not always, according to the 2020 review. It says that “In contrast to the above-cited studies, no decay was observed at MC below fibre saturation,” and it notes that some decay has been reported “at RH below 100%” in older work. The review treats the question as still under study.

Why does outdoor wood decay more than indoor wood?

Outdoor wood experiences repeated wetting. The 2020 review says that “wood in outdoor applications undergoes frequent changes of wetting and drying,” and that liquid water uptake can overrule vapor-phase sorption.

What is the difference between white rot and brown rot?

White rot attacks lignin along with the sugars, while brown rot removes most of the carbohydrates. The 2023 study says that “selective rot initially degrades hemicellulose and lignin, while cellulose is relatively retained” for one white rot pattern, and the 2020 review says brown rot leaves “a modified (brown) lignin-rich residue behind.”

Does the wood species change how decay spreads?

Yes, according to the 2023 study. It says that “The biodegradation path and mechanism of wood varies depending on diverse fungi and tree species.”

Is there one moisture threshold for all fungi?

No source supports that. The 2020 review says that “various minimum moisture thresholds ( MMThr ) were determined for different combinations of wood and fungal species.”

Sources

• Wood-water relationships and their role for wood susceptibility to fungal decay. Applied Microbiology and Biotechnology, 2020. https://pmc.ncbi.nlm.nih.gov/articles/PMC8326242/ (HTTP 200, checked 2026-10-28)
• Fungal Selectivity and Biodegradation Effects by White and Brown Rot Fungi for Wood Biomass Pretreatment. Polymers, 2023. https://pmc.ncbi.nlm.nih.gov/articles/PMC10144154/ (HTTP 200, checked 2026-10-28)