Red Mold: Fungus, Serratia Film or Iron Stain? How to Tell

Most pages ranking for this search treat red mold as one organism with one cleanup. Four unrelated things look red on a household surface, and two are not fungi at all. Identity first saves scrubbing a water stain for a month, or writing a remediation scope for a film a bathroom cleaner removes.

What you are looking at: four things that read as red

Start with where the red sits and how it feels, not its exact shade. Each cause has its own home turf and texture.

What it is	Where it shows up	Texture and colour	Quick tell	Source
Serratia marcescens (a bacterium)	Toilet bowls, sinks, tiles, shower stalls, tubs, pet dishes	Pink or red slimy film, often a line at the water level	Wipes off; feeds on soap or fatty residue	NDSU Extension
Iron staining from water	Fixtures, toilet tanks, anywhere water dries	Rusty, reddish or orange stain, no fuzz	Follows the water path; bleach does not lighten it	EPA secondary drinking water standards
Iron bacteria	Well pipes, pumps, plumbing fixtures	Sticky rusty, yellow, brown or grey slime; oily sheen	Usually private wells; a lab water test confirms	Minnesota Department of Health
Wood extractive bleed	Cedar, redwood, walnut, mahogany	Flat yellow to reddish-brown stain	Appears after rain, condensation or a leak	Wood Handbook, Chapter 16
Red-pigmented fungi	Damp wood sapwood, other organic material, food	Fuzzy, powdery or (yeasts) glossy colonies	Raised growth, often mixed with other colours	USDA Forest Products Laboratory Wood Handbook, Chapter 14; Mycology Online's Fusarium description

Bright orange powder on fire-charred wood is covered on our orange mold page.

The five-minute sort: which red is this?

Work through these in order.

1. Is it at a water line on a fixture? A pink-red band in a toilet, tub, sink or shower is the textbook Serratia location, per NDSU Extension. Why this differs: generic advice sends you hunting for a leak; here the food is soap residue, so cleaning and drying habits are the fix.
2. Does it smear? Slime that wipes off is a biofilm, a flat stain is chemistry, a raised fuzzy spot is more likely fungal. Why this differs: texture sorts causes faster than shade.
3. Put one drop of household bleach on it. Per the Wood Handbook, Chapter 16, a drop or two of 5% sodium hypochlorite usually bleaches mildew in a few seconds, and discoloration that does not bleach is probably dirt, extractives bleed or iron stain. Why this differs: it is a test on a coin-sized spot, not a treatment; killing is covered on does bleach kill mold.
4. Does the red follow the water? Iron above 0.3 mg/L gives reddish or orange staining under the EPA secondary drinking water standards. Why this differs: no cleaner fixes a water supply; a lab water test does.
5. Is it on a porous building material? Red growth on drywall paper, ceiling tile or carpet is a mold question again, and EPA's Mold Cleanup in Your Home says absorbent materials may have to be thrown away. Why this differs: now the material decides the scope and colour stops mattering.

Bleach drop result	Most likely	Next step	Source
Lightens within seconds	Mildew or other surface fungus	Clean, then find the moisture that fed it	Wood Handbook, Chapter 16
No change, flat and rusty	Iron stain or iron in water	Water test; oxalic acid lifts iron stain on wood	Wood Handbook, Chapter 16; EPA secondary drinking water standards
No change, reddish-brown under paint on cedar	Water-soluble extractives	Stain-blocking primer, then topcoat	Wood Handbook, Chapter 16
Slime lifts and the surface comes clean	Bacterial film such as Serratia	Chlorine-bleach cleanser; keep the surface dry	NDSU Extension

Red mold vs pink mold: usually the same film

NDSU Extension calls the organism a pink or red slimy substance, so both searches usually point at Serratia marcescens, a bacterium that grows on phosphorus or fatty residue such as soap. Our pink mold page covers it in full.

Question	Serratia film (pink or red)	Red-pigmented fungus
Kingdom	Bacterium	Fungus (mold or yeast)
Typical spot	Water lines, grout, drains, pet dishes	Damp organic material such as wood or food
Look and feel	Wet, slimy, smears	Fuzzy, powdery or glossy colony, raised
Published fix	Chlorine-bleach cleanser; dry after use	Moisture control and removal of the growth

For a toilet tank, NDSU Extension says to add a quarter cup of bleach and let it sit 15 to 20 minutes, and not longer, because bleach damages rubber valves and seals. On other surfaces it suggests 10 to 20 minutes of contact.

Red mold in shower, toilet and sink

Two causes account for most bathroom red. Pink-red slime is the Serratia case. A rust-coloured stain running from a faucet or ringing a toilet tank is usually iron in the water. The EPA secondary drinking water standards sets federally unenforced levels for these nuisance effects, three of them by stain colour:

Contaminant	Secondary level (mg/L)	Noticeable effect above that level, as EPA prints it
Copper	1	metallic taste; blue-green staining
Iron	0.3	rusty color; sediment; metallic taste; reddish or orange staining
Manganese	0.05	black to brown color; black staining; bitter metallic taste

On a private well, the Minnesota Department of Health adds iron bacteria: yellow, orange, red or brown stains, an oil-like sheen and sticky rusty slime on pipes, pumps and fixtures, confirmed by a laboratory water test. That is water-treatment work, not mold work. Caulk and grout growth in general sit on our mold in shower page, and one rule carries over: EPA's Mold Cleanup in Your Home says do not paint or caulk over moldy surfaces.

Red mold on wood: stain, extractive or decay

Wood can turn red with no fungus involved. Per the Wood Handbook, Chapter 16, coloured extractives sit in the heartwood of western redcedar, redwood, walnut and mahogany. When a leak, rain or condensation wets the wood, water carries them to the surface and leaves a yellow to reddish-brown stain; the handbook names bathrooms among rooms where it bleeds through paint.

Fungal red is possible too: the USDA Forest Products Laboratory Wood Handbook, Chapter 14 says sapwood stains are mostly bluish to brown, but yellow, orange, purple and red are sometimes encountered. Sanding and depth are covered on mold on wood.

Red on wood	How to tell	Fix the handbook gives	Why this differs from generic advice
Extractive bleed (cedar, redwood)	Flat, appears after wetting, does not bleach	Clean, stain-blocking primer, cure, topcoat	More coats of the same paint will not stop it
Red or orange fungal stain in sapwood	Colour inside the wood; brushing does not remove it	Dry the wood; a pigmented finish can hide it	Stain fungi affect strength only slightly; moisture is the defect
Surface mold with red tones	Fuzzy, brushes off softwood, lightens with bleach	Remove growth; dry to 20% or less per the handbook	On oak the stain may go too deep to sand out
Iron stain from fasteners	Blue-black on cedar or oak; brown rust halo on pine	Oxalic acid, then rinse	It returns unless the iron source goes

Moisture is the number that matters. Per the USDA Forest Products Laboratory Wood Handbook, Chapter 14, fully air-dried wood usually does not exceed 20% moisture content, a reasonable margin against fungal damage, and serious decay needs wood above the fiber saturation point, an average of 30%. Per the same handbook, iron stain needs wood above about 16% to move. A pin meter tells you more than the colour does.

The genuine red fungi, and why the genus rarely matters

Mycology Online, run by the University of Adelaide, describes colony colour on laboratory media; a lab names a genus from structures under a microscope, not from shade.

Organism	Colony colour as described	Type	Source
Fusarium	Whitish to yellow, pink, red or purple; one complex deep pink to red with a carmine red reverse	Mold	Mycology Online's Fusarium description
Rhodotorula	Coral red to salmon or slightly orange, glossy	Yeast	Mycology Online's Rhodotorula description
Aureobasidium	Cream or pink, later becoming brown or black	Mold (a so-called black yeast)	Aureobasidium description

For the cleanup, the name changes nothing. CDC says you do not need to know the type of mold, and the EPA Mold Course, Chapter 3 says sampling is unnecessary in most cases when mold is visible. The generic method and EPA’s 10 square foot line live on DIY mold remediation.

Red on bread and food

A red patch on bread can be Serratia rather than mold, as the photograph shows. For the kitchen decision it does not matter: the USDA FSIS ‘Molds on Food’ chart says to discard bread and baked goods because porous foods can be contaminated below the surface, and notes that moldy foods may carry invisible bacteria alongside the mold.

Confirming it, and what the lab charges

When the sort leaves doubt, the EPA Mold Course, Chapter 3 describes the cheap tool: clear tape pressed on the spot and examined under a microscope, which can also separate mold from soot or dirt. An iron-related bacteria test on water answers the other red question.

Test	What it answers	EMSL code	Published price	Turnaround
Direct exam of tape lift, swab or bulk	Is this fungal, and roughly what genus	M041	$68 to $328	2 weeks down to 3 hours
Iron related bacteria (IRB)	Is the slime iron bacteria	M121	$105	2 weeks
Culturable fungi, genus ID from bulk or swab	Which genus grows from it	M007	$113	2 weeks

Per that list, speed is most of the price. Whether to sample at all is covered on mold testing.

Signs it is bigger than the spot

Sign	What it suggests	Source
Red slime or stains on several fixtures, on a private well	Iron bacteria in the supply, not a cleaning problem	Minnesota Department of Health
Red or orange stain on every fixture the water touches	Iron above the secondary level; test the water	EPA secondary drinking water standards
Red growth on drywall paper, ceiling tile or carpet	Porous material that may need removal	EPA's Mold Cleanup in Your Home
Musty smell with no visible source	Hidden growth worth investigating	EPA Mold Course, Chapter 3
Red wood above about 30% moisture content, or soft when probed	Conditions for decay, not just stain	USDA Forest Products Laboratory Wood Handbook, Chapter 14
Patch larger than about 10 square feet	Beyond EPA’s homeowner guidance	EPA's Brief Guide to Mold

The EPA Mold Course, Chapter 3 lists the back of drywall and the top of ceiling tiles among places mold grows unseen.

For contractors

On a red mold call, identity comes first: two of the four causes are not remediation scope. ANSI/IICRC S520 (fourth edition, 2024) is the procedural standard for mold remediation, and a 2024 Cal State LA mold assessment quotes its conditions: Condition 1 is normal fungal ecology, Condition 2 is settled spores or fragments from a Condition 3 area, and Condition 3 is actual growth, active or dormant, visible or hidden. A Serratia film or an iron stain is none of those.

Item	What to record	Why an adjuster or owner asks
Identity	Location, texture, bleach drop result, photos	Shows whether the red was fungal before scope was written
Water source	Well or municipal; staining on other fixtures	Iron points to treatment, not remediation
Moisture readings	Pin meter on wood, mapped by location	Separates stain from active moisture and decay risk
Material class	Porous, semi-porous or non-porous per item	Drives remove-or-clean decisions under EPA guidance
Lab results, if taken	Tape lift or bulk with chain of custody	Answers ‘was it mold’ if the cause is disputed

Write a bathroom Serratia clean as a cleaning line, not remediation; on wet porous drywall, record material, area, readings and cause, and drop the colour.

Questions people actually ask

Is red mold dangerous?

Colour does not tell you. CDC says mold can look like spots of many colours and that you do not need to know the type, so red growth gets the same treatment as any other: fix the moisture, then clean or remove by material and area. Much indoor red is Serratia or iron staining, not mold. See the scope note below.

What causes red mold in a shower?

Usually Serratia marcescens, a bacterium that NDSU Extension says forms pink or red slime where soap or fatty residue sits on a damp surface. Iron in the water gives a flat rusty stain instead. A chlorine-bleach cleanser and drying surfaces after use handle the first; a water test and treatment handle the second.

Is red mold the same as pink mold?

Usually, yes. NDSU Extension describes Serratia marcescens as a pink or red slimy substance, so both searches tend to mean the same bacterial film. Genuine red fungi exist, such as some Fusarium species and the yeast Rhodotorula, but they form fuzzy, powdery or glossy colonies on damp organic material, not a smear at a water line.

What does red mold on wood mean?

On cedar or redwood it is often water-soluble extractives, which the Forest Products Laboratory says leave a yellow to reddish-brown stain after the wood gets wet and do not bleach out. Red fungal stains in sapwood also occur. Measure moisture: per the same handbook, serious decay needs an average of 30% or more.

How do I get rid of red mold?

Match the fix to the cause. Serratia film: a chlorine-bleach cleanser and dry surfaces, per NDSU. Iron staining: test and treat the water. Extractive bleed on painted wood: a stain-blocking primer, per the Wood Handbook. Red fungal growth: remove it, dry the material, and replace porous items EPA says may need to go.

Should I test red mold?

Usually not for visible growth: EPA’s mold course says sampling is unnecessary in most cases when mold can be seen. A tape lift helps when you cannot tell mold from a stain or soot, and an iron-related bacteria test answers the water question. Per the EMSL 2026 list, a tape lift exam costs $68 to $328.

More in our mold hub.