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Where you find it tells you more than its color does

What Is Orange Mold? What That Orange Growth Usually Is

Orange growth can be mold, slime mold, iron bacteria, or plant rust. How to tell what you're seeing, why color doesn't show danger, and what to do.

8 min read

"Orange mold" isn't a specific type of mold. Orange or yellow-orange growth around a home can be several different things, and where you find it is the best clue. On damp walls, wood, or fabric, it may be mold; extension services list orange among the colors mold can take. Orange slime in a toilet tank in a home on well water is often iron bacteria, which Penn State Extension calls more of a nuisance than a health hazard. Bright yellow blobs on mulch are usually slime mold, which isn't a fungus and doesn't harm people, pets, or plants. Orange spots on junipers and apple leaves are often rust fungi. Whatever it is, color doesn't tell you how risky it is. NIOSH says a mold's color doesn't necessarily show whether it is more or less dangerous.

The earlier version of this article named Trichoderma and Fusarium as toxic orange molds and linked orange mold to neurological symptoms. Neither claim holds up. The University of Adelaide's mycology reference describes Trichoderma colonies as white turning green, and Fusarium as ranging from whitish to yellow, pink, red, or purple. Health reviews haven't found adequate evidence linking indoor mold to neurological symptoms.

Orange Growth on Walls, Wood, or Fabric Indoors

Indoor mold comes in many colors. NDSU Extension says mold may change surfaces to "white, green, brown, black or orange," and University of Minnesota Extension lists orange among the colors of indoor mold.

If you see orange spots or fuzz on drywall, wood trim, window frames, ceilings, or stored items, treat it like any other mold. You don't need to know the species. The CDC says it isn't necessary to determine what type of mold you have, and that all molds should be treated the same with respect to potential health risks and removal.

  1. Find and fix the moisture. Look for leaks, condensation, and poor ventilation. The EPA warns that cleaned-up mold will most likely return if the water problem isn't fixed.
  2. Size it up. The EPA says most people can handle an area smaller than about 10 square feet themselves.
  3. Protect yourself. The CDC recommends at least an N95 respirator, gloves, and goggles. People with asthma, allergies, lung disease, or weakened immune systems shouldn't do the cleanup.
  4. Clean hard surfaces with detergent and water and dry them completely. If you use bleach, use no more than 1 cup per gallon of water, and never mix it with ammonia or other cleaners.
  5. Remove what can't be cleaned. The EPA says porous materials such as ceiling tiles and carpet may have to be thrown away if they become moldy. Don't paint or caulk over mold.

For larger areas, hidden mold, or growth that keeps returning, see who to call for a mold inspection. To keep it from coming back, see what humidity level helps prevent mold.

Orange Slime in a Toilet Tank, Sink, or Water

Iron bacteria. In homes with private wells, Penn State Extension explains that iron bacteria use dissolved iron or manganese in groundwater to grow, "producing a bacterial slime along the way." Signs include "orange or opaque slimy strands inside toilet tanks or orange-colored water," along with reddish, yellow, brown, or gray deposits. Penn State says they are "typically considered more of a nuisance than a health hazard." The Minnesota Department of Health adds that while iron bacteria don't directly cause illness, they can create conditions where disease-causing organisms may grow.

What to do:

  • Confirm it. The Minnesota Department of Health says you can confirm iron bacteria by having your water tested at a laboratory.
  • Treat the well. Penn State says shock chlorination of the well is frequently the first method people try, but warns that doing it often can damage private water systems. A water treatment professional can help with persistent problems.

Iron staining without slime. Dissolved iron alone can leave stains. The Minnesota Department of Health says iron causes "yellow, red, or brown stains on dishes, laundry, and plumbing fixtures" and "does not usually present a health risk." Options include water softeners and iron filters.

Pink-orange film around drains and toilet bowls is often a different bacterium, Serratia marcescens. See pink mold in the bathroom for how to clean it.

Yellow or Orange Blobs on Mulch or the Lawn

If you find a bright, spongy blob on landscape mulch after wet weather, it's likely a slime mold. University of Missouri Extension describes the common "dog vomit slime mold," Fuligo septica, as looking like "bright yellow scrambled eggs" when fresh and turning brown as it dries.

Despite the name, it isn't a mold in the usual sense. Missouri Extension says Fuligo septica, like all slime molds, "is unrelated to fungi," and "does not harm plants, people or animals." You can leave it alone. University of Arkansas Extension says it will go away by itself. If you'd rather remove it, NC State Extension suggests tossing it under shrubs or onto a compost pile with a garden fork. Replacing the mulch may not help, since it's likely to return.

Orange Spots on Trees and Plants

Bright orange spots or growths on plants are often rust diseases. University of Minnesota Extension describes cedar-apple rust, which forms "orange, gummy, tentacle-like projections" on junipers and red cedars, and bright orange-red spots on apple and crabapple leaves. It says infections usually don't significantly affect the health of the tree. These plant diseases aren't the same thing as mold growing indoors.

Orange Mold on Food

Throw out most moldy food, whatever the color. A USDA fact sheet, *Molds on Food: Are They Dangerous?*, explains that some molds produce mycotoxins, poisonous substances that can make people sick, and that heavy growth sends root threads deep into food. Its guidance:

  • Discard soft foods such as bread and baked goods, soft fruits and vegetables, soft cheeses, yogurt, jams and jellies, and cooked leftovers.
  • Trim hard cheese and firm fruits and vegetables by cutting off at least 1 inch around and below the mold spot.
  • Don't sniff moldy food, which can cause respiratory trouble.

Is Orange Mold Toxic?

Color can't answer that. NIOSH says certain molds can produce toxins, but "the molds themselves are not toxic or poisonous," and color doesn't necessarily show how dangerous a mold is.

What's well established is that mold growing indoors can affect some people. The CDC says mold can cause a stuffy nose, sore throat, coughing or wheezing, burning eyes, or skin rash. People with asthma or mold allergies may have severe reactions, and people with weakened immune systems or chronic lung disease may get lung infections.

Claims about neurological harm aren't supported. The Institute of Medicine's review, summarized in the Surgeon General's 2009 call to action, found the evidence insufficient to determine whether damp indoor environments or mold are linked to neuropsychiatric symptoms. For more on what the evidence does show, see what kind of mold is dangerous in a house.

Frequently Asked Questions

Is orange mold dangerous?

It depends on where it is and who's exposed, not the color. Mold growing indoors should be removed and its moisture source fixed. Slime mold on mulch and iron bacteria in well water aren't considered health hazards.

Why is there orange slime in my toilet tank?

If you have a private well, iron bacteria are a common cause. They feed on iron or manganese in groundwater and leave orange, slimy strands. Have your water tested to confirm, and ask a water treatment professional about shock chlorination or filtration.

Is the orange or yellow slime mold in my mulch harmful to my dog?

University of Missouri Extension says Fuligo septica doesn't harm plants, people, or animals. Still, it's reasonable to discourage pets from eating anything growing in the yard, and to call your veterinarian if your pet seems unwell.

Should I test orange mold to find out what it is?

Usually not. The CDC doesn't recommend mold testing, and the EPA says sampling is usually unnecessary when mold is visible. Knowing the species doesn't change the cleanup.

Can I cut orange mold off bread or fruit?

Not off bread or soft fruit. The USDA advises discarding them because mold can spread below the surface. You can trim at least 1 inch around mold on hard cheese and firm produce.

Sources

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