Yellow Mold: Slime Mold, Dry Rot or Aspergillus?

Search results file a garden curiosity, a decay fungus and a household mold under one colour. Confusing them costs money both ways: remediating a slime mold that would have dried up, or bleaching a sill plate being digested from inside.

What you are looking at: five yellows

The USDA Forest Products Laboratory Wood Handbook, chapter 14 says mold on wood shows as fuzzy or powdery growth in which green and yellowish hues are common, that fresh decay fungus growth is usually white or brown but may be strikingly yellow or orange, and that sapwood stains sometimes run to yellow. Colour cannot separate cosmetic from structural. Texture, substrate and behaviour can.

What you see	Texture and behaviour	Most likely
Bright yellow foam on mulch, appeared overnight	Wet and spongy; dries to a crust, then dark dust	Fuligo septica slime mold
Grey felt with lemon yellow and lilac patches on timber	Flat sheets; strands cross brick and plaster	Serpula lacrymans dry rot
Yellow or orange fan of threads on a joist	Fan or root-like strands pressed to the wood	Fresh decay fungus growth
Yellow-green to olive powder	Dry and dusty; colour comes from spores	Aspergillus or similar surface mold
Even yellow tint inside sapwood	No raised growth; will not brush off	Fungal or chemical stain

Also check for foam: BASF’s ENERTITE G data sheet says cured open-cell spray foam is yellow or off white (see mold in insulation).

The yellow growth matrix

One question sets the budget: is it eating the structure?

Yellow growth	Organism	Structural?	Action
Foam on mulch or soil	Fuligo septica slime mold	No	Scoop, bag, dry the bed
Foam on a deck board or post	Slime mold using wood as a perch	Not itself; the wet wood needs checking	Scoop, then probe and meter
Grey skin, lemon yellow patches, timber or brick	Serpula lacrymans dry rot	Yes, brown rot	Stop the water, open up, cut out decayed timber
Lemon yellow turning brown and grey	Serpula under stress or dying	Damage done; activity may be ending	Confirm the source is dry, assess what remains
Yellow or orange fan on a joist	Decay fungus	Yes, if wood is above fibre saturation	Probe, meter, fix leak, replace weak wood
Yellow-green powder on drywall or framing	Aspergillus or similar	No; the wet material may be	Clean or remove; fix moisture
Yellow tint inside sapwood	Stain	Only slightly, per the FPL	Leave or plane it

Only the two decay rows are a structural yes, which is why the probe and the meter below beat any photo comparison. For the neighbouring colours see green mold and orange mold.

Yellow slime mold on mulch and wood

Ohio State University Extension calls Fuligo septica one of the largest slime molds, conspicuous for its intense yellow, and most often found on bark mulch. It is not a true fungus: Ohio State classes slime molds as amoeba-like protists whose feeding stage oozes over surfaces eating bacteria, fungal spores and organic particles, then hardens into a spore crust that releases dark dust.

Ohio State says it is not toxic or dangerous to humans, animals or plants, and Iowa State University Extension says it typically dries into a white powdery mass within a few days. Washington State University Extension advises scooping it into a bag, because hosing it spreads the spores, and then drying the bed.

Yellow slime mold on wood

On a fence post or deck board, slime mold is using the wood as a surface, not food. The board has still been wet long enough to host it, so scrape it off and run the probe test below. Decking has its own page: mold on deck.

Serpula lacrymans: the yellow that eats buildings

The Historic Scotland Technical Advice Note 24, The Environmental Control of Dry Rot (2002), names Serpula lacrymans as by far the most important dry rot fungus in buildings and urges the Latin name because ‘dry rot’ is used loosely. It lists a rusty red fruit body, strands brittle when dry, and white or grey mycelium with pink or lemon tinges.

The yellow carries information. TAN 24 says the fluffy white growth seen in laboratories is soon tinged salmon pink, lilac or lemon yellow in buildings, that yellowing is a stress defence showing conditions are no longer optimal, and that colour moves from lemon yellow to brown and grey as stress rises. It warns colour alone is not a dependable field test.

Why masonry matters

The USDA Forest Products Laboratory Wood Handbook, chapter 14 explains that some brown rot fungi form water-conducting strands that carry water, usually from soil, into wood that would otherwise be dry, which is why it calls the name dry rot incorrect. TAN 24 adds that Serpula needs metal ions from masonry as well as wood, so it grows luxuriantly where timber meets brick, stone or plaster.

On the outbreak you see	TAN 24 Table 3 reading
Fluffy, like cotton wool	Certainly alive
Flat and grey, like thin sheets	Probably not actively growing
Dark yellow or brown	Dying or dead
Smells of mushrooms	At least part may be alive
No damp and no stagnant air nearby	Almost certainly not active

For a scope, the key line is the British Wood Preserving and Damp-Proofing Association position TAN 24 quotes: finding and fixing the dampness, and drying it out, is the fundamental part of a successful treatment. TAN 24 also records that some firms cut back sound timber up to 1 metre beyond decay. The general mold, stain and decay tests for timber are on mold on wood.

Yellow-green Aspergillus and other surface molds

The Crop Protection Network and Purdue Extension bulletin BP-83-W describe Aspergillus flavus as an olive-green, powdery mold whose spores disperse like dust. The Wisconsin State Laboratory of Hygiene fungal glossary says it prefers water activity of 0.75 to 0.8 and has been recovered indoors from carpet and building materials: a mold of damp materials, not proof the wood beneath is decaying. Colour does not change the cleanup method, which lives on DIY mold remediation; naming the species rarely changes scope (see mold testing).

Yellow mold in a crawl space

The USDA Forest Products Laboratory Wood Handbook, chapter 14 says condensation can seriously decay crawl space framing, that its primary source is soil moisture, that a polyethylene barrier on the soil isolates it, and that joists need at least 18 inches of clearance for inspection. Three yellow patterns follow: foam on bare soil is slime mold; yellow-green powder across many joists is surface mold from humid air; a yellow fan or felt on one sill plate or rim joist bearing on damp masonry is the one to probe now. Method: crawl space mold remediation.

Surface mold or decay: the probe and the meter

The Federal Highway Administration probing guidance describes probing with an awl, pocketknife or pick hammer: decayed wood is soft, resists less, and a lifted sliver breaks abruptly where sound wood splinters. A screwdriver gives the same signal. The limits: it finds near-surface decay, struggles on soft species like western red cedar, and water-softened wood can mimic decay.

Then meter it. A firm face proves little: according to the FPL Wood Handbook, lab toughness losses ran from 6% to more than 50% once decay had removed just 1% of the wood’s weight, a stage visible only under a microscope.

Wood moisture content	Meaning	Per the source
At or below 20%	Safe margin; existing decay will not progress	FPL Wood Handbook
Above 20%	Timbers at risk from dry rot	TAN 24 Table 2
Above about 30%	Serious decay possible	FPL Wood Handbook
Above 60%	Likely safe from dry rot, not other organisms	TAN 24 Table 2
Humid air only	Not wet enough for decay; permits some mold	FPL Wood Handbook

Inside, the FPL Wood Handbook says brown rot leaves wood browner and cracked across the grain into cubes, while white rot bleaches it and feels spongy. TAN 24 says all dry rot fungi cause brown rot.

The decision path, in six questions

Answer in order; cleaning first destroys the evidence.

1. What is it on? Mulch, soil or lawn: slime mold. Scoop, dry, done, unless it is on wood.
2. Foamy or powdery? Overnight foam is slime mold; dry powder that wipes off is surface mold; felt, skin or strands go to step 3.
3. Does it cross masonry? Strands from timber onto brick or plaster are the Serpula pattern. Stop cleaning; get a structural assessment.
4. Probe the wood. Splinter means sound. Abrupt break or soft point means decay.
5. Meter it. Per the FPL, at or below 20% decay will not progress; above about 30% it can be serious. Find the water before pricing timber.
6. Size it. EPA 402-K-01-001 treats surface mold under 10 square feet as a small job with minimum protective equipment and no containment. Decay in a bearing member is a structural repair at any size.

Removal, case by case

The reason column is the point: generic spray and scrub advice fails on two of these cases. Kill versus remove is covered on how to kill black mold; the chemistry does not change with colour.

Case	What to do	Why this differs from generic advice
Slime mold on mulch	Scoop, bag, rake the bed dry	Hosing spreads spores, per WSU; it is not a fungus
Slime mold on wood	Scrape off, probe and meter	The board’s moisture is the real finding
Surface mold, small area	Fix moisture; scrub hard surfaces with detergent and water; dry	EPA says moldy porous materials may have to go
Surface mold on wood that probes sound	HEPA vacuum once dry, clean, hold below 20%	FPL: softwood surface growth often brushes off
Decay fan, wood soft	Stop leak, cut to sound wood, sister or replace	FPL: visible decay means greatly reduced strength
Serpula lacrymans	Remove water, ventilate, replace decayed timber, isolate new timber from damp masonry	TAN 24: ventilation quickly inactivates it once timber is off damp masonry

Yellow mold on food

Food follows the USDA Food Safety and Inspection Service, by food type rather than colour: cut at least 1 inch around and below the spot on hard cheese and firm vegetables; discard soft cheese, bread, jams, nuts and peanut butter. Fridges are covered on mold in refrigerator.

When it is bigger than the visible spot

• Strands or sheets crossing brick, block, stone or plaster.
• A probe that sinks, or a sliver that snaps short, in a sill, rim joist, joist end or post base.
• Framing above about 30% moisture, or still above 20% after the source is fixed, per the FPL.
• A rusty red fruit body, a TAN 24 Serpula feature.
• A mushroom smell in a closed space (TAN 24 Table 3).
• Springy floors, cubical cracking, or regrowth after cleaning.

For contractors: scope, documentation, policy wording

Slime mold is a short service ticket; surface mold follows the EPA area guidance; decay is carpentry plus moisture correction, and documentation decides whether that invoice survives.

Record	Capture	Why the adjuster asks
Identification	Substrate, strands, fruit body, photos	Justifies decay repair versus cleaning
Moisture map	Pin readings with a dry reference, RH, temperature	Shows the FPL lines and drying completion
Probe log	Member, tool, result	Separates cosmetic from strength loss
Water source	Leak, condensation, ground or masonry wicking	Policy wording turns on cause
Cut back extent	Measured to sound wood, photographed	Defends replaced linear feet

The ISO HO 00 03 sample form hosted by the Insurance Information Institute lists ‘smog, rust or other corrosion, or dry rot’ among Coverage A exclusions and separately excludes ‘mold, fungus or wet rot’, except where hidden within walls, ceilings or floors and caused by an accidental discharge of water from plumbing, heating, cooling or an appliance. That is sample ISO wording; the customer’s own policy, their state insurance department and a licensed professional decide what a claim pays.

Questions people actually ask

Is yellow mold dangerous?

It depends which yellow. Ohio State University Extension says yellow slime mold on mulch is not toxic or dangerous to people, animals or plants. Serpula lacrymans dry rot is dangerous to the building because it destroys timber. Yellow-green Aspergillus is a household mold handled by normal EPA cleanup rules.

What is the yellow foam growing in my mulch?

Almost certainly Fuligo septica, the dog vomit or scrambled egg slime mold. It feeds on bacteria in the mulch, not on plants or the house. Iowa State Extension says it usually dries to a white powdery mass within days. Scoop it into a bag; Washington State Extension says hosing spreads the spores.

What does yellow mold on wood mean?

It is surface mold, slime mold resting on the board, or a decay fungus; the Forest Products Laboratory notes fresh decay growth can be strikingly yellow. Probe it: a splinter means sound wood, an abrupt break means decay. Above about 30% moisture, according to the FPL, serious decay is possible.

Is yellow mold the same as dry rot?

Sometimes. Serpula lacrymans shows lemon yellow tinges when stressed, according to Historic Scotland’s Technical Advice Note 24, alongside grey skin, lilac patches and strands crossing masonry. Yellow growth without strands, on wood that probes sound, is more likely surface mold.

Why is there yellow mold in my crawl space?

The Forest Products Laboratory says soil moisture is the primary source of crawl space condensation. Foam on soil is slime mold, yellow-green powder on many joists is surface mold, and a yellow fan on one sill or rim joist near masonry should be probed for decay first.

How do I get rid of yellow mold for good?

Remove the water. Dry the bed for slime mold, keep indoor humidity below the EPA’s 60 percent ceiling for surface mold, and for decay fix the leak, cut back to sound wood and keep framing at or below the 20% moisture content the Forest Products Laboratory treats as a safe margin.

More in our mold hub.