White mold vs efflorescence vs mildew: how to tell them apart

What is white mold?

“White mold” is not a single species. It is a description of any mold that looks white, cream or pale gray, often fuzzy, cottony or web-like. EPA’s mold guide makes the point with a photo of mold on firewood: “Molds come in many colors; both white and black molds are shown here.” CDC also says mold “can look like spots of many different colors.”

Indoors, white mold tends to show up on organic materials that stay damp: wood framing and attic sheathing, the paper face of drywall, stored cardboard, fabric, leather and furniture in humid basements or closed-up rooms. EPA’s guide includes photos of mold growing on a suitcase stored in a humid basement and on ceiling tile.

White mold is not safer or more dangerous than other colors. CDC’s guidance is that you do not need to know the type of mold; if you see or smell it, remove it and fix the moisture. The health effects EPA describes, allergy symptoms, irritation and asthma attacks in sensitive people, apply regardless of color. For background on the mold that gets the most attention, see our black mold guide, and for all mold topics, our mold hub.

The tricky part with white growth is that two common look-alikes are not mold at all: efflorescence on masonry and, on plants outdoors, powdery mildew.

What is efflorescence?

Efflorescence is a deposit of mineral salts left behind when water moves through concrete, block or brick and evaporates at the surface. North Dakota State University (NDSU) Extension explains that when a basement is not drained or sealed well enough, moisture seeps slowly through walls and floors. As the water evaporates, salt crystals grow until you can see them.

NDSU says these deposits can take many forms depending on which minerals are present, including sodium, calcium, magnesium, carbonate, chloride and sulfate. Soluble salts such as sodium and magnesium sulfates often look white and fluffy, and can appear “moldy” in cracks in floors or walls. Calcium and magnesium carbonates form light-colored, powdery deposits that build up over long periods.

NDSU Extension’s agricultural engineer put the problem simply: as people search their homes for mold, “what they find may not necessarily be mold.” Efflorescence shows up most on concrete and masonry, especially in basements, crawl spaces and on foundation walls, because those materials let water pass through them. If you see a white deposit on wood or drywall, efflorescence is much less likely. Our basement mold guide covers moisture problems in below-grade spaces.

How do you tell white mold from efflorescence?

NDSU Extension gives a simple test you can do with a dropper or spoon:

1. Put a few drops of water on the deposit. Soluble salts dissolve easily in water.
2. If it does not dissolve, try a few drops of vinegar. Carbonate deposits dissolve in a dilute acid such as vinegar but not in water alone.
3. If it dissolves in neither water nor vinegar, it may be mold.

Wear gloves, and if there is a chance it is mold, an N95 respirator, as CDC recommends for mold cleanup. The table below combines NDSU’s test with what EPA and CDC say about mold.

Is white mold the same as mildew?

Not exactly, though the words overlap. EPA says “mildew” refers to certain kinds of mold or fungus, and that the term is often used generically for mold growth, usually with a flat growth habit. EPA notes mildew often lives on shower walls, windowsills and other places with high moisture.

In everyday use, “mildew” tends to mean thin, flat growth on a surface, matching EPA’s description, while “mold” covers the broader group. That is a rule of thumb, not a scientific line. Both are fungi, both need moisture, and EPA’s cleanup advice is the same: fix the water source, scrub hard surfaces with detergent and water, and dry them completely.

The difference that matters is the material underneath. Mildew on a nonporous surface like tile, glass or a painted sill usually wipes off. Mold that has grown into porous materials like drywall, ceiling tile or carpet may be impossible to remove completely, according to EPA, and the material may need to be replaced. Our full guide to mold vs mildew covers the comparison in detail.

What about white powdery mildew on plants?

If the white powder is on leaves outdoors or on houseplants, it is almost certainly a plant disease, not household mold. University of Minnesota Extension describes powdery mildew as a common fungal disease that causes white to gray powdery spots, blotches or felt-like mats on leaves, stems and buds. Infected plants may look as if they were sprinkled with baby powder.

University of Minnesota Extension says powdery mildew thrives in humid conditions, is worse on shaded plants with little air movement, and spreads by wind-borne spores. It notes that on trees and shrubs, powdery mildew does not cause significant damage to the plant’s health and does not require management, though it can spoil the look of ornamentals such as roses.

For your house, the takeaway is that powdery mildew on the garden does not mean you have a mold problem indoors. Powdery mildew is a disease of living plants, a separate issue from mold growing on damp building materials. If you have white growth inside the house, use the tests and steps on this page instead.

What does white growth tell you about moisture?

Whichever it turns out to be, white growth is a moisture signal.

Efflorescence means water is moving through masonry. NDSU Extension says the usual causes in basements are high water tables or saturated soil pushing moisture through floors and walls that are not drained or sealed well enough. NDSU adds that groundwater with high sulfate, more than 150 parts per million, can corrode concrete over time by breaking down the cement and forming expanding crystals in its pores. NDSU’s recommended fix is a proper drainage system around the foundation and floor that moves water away quickly.

White mold means a material has stayed damp long enough to support growth. EPA says that if wet materials are dried within 24 to 48 hours after a leak or spill, in most cases mold will not grow. Common sources include roof leaks, plumbing leaks, condensation on cold surfaces, and high humidity in attics, crawl spaces and closed-up rooms.

EPA’s humidity target is below 60 percent, ideally 30 to 50 percent. CDC recommends no higher than 50 percent. An inexpensive humidity meter tells you where you stand. If water is coming in from outside, our guide to water damage categories and classes explains how pros assess it.

How do you remove white mold or efflorescence?

Efflorescence: because it is salt, you do not need a mold cleaner or bleach. NDSU Extension says soluble salts dissolve easily in water, and carbonate deposits can be removed with a dilute acid such as vinegar. Brush or wipe it off and let the surface dry. Expect it to return until the water problem is fixed.

White mold: follow EPA and CDC mold guidance. EPA says areas under about 10 square feet can usually be handled yourself. Fix the moisture source first. Wear at least an N95 respirator, gloves and goggles (CDC). Scrub hard surfaces with detergent and water, then dry completely. EPA does not recommend bleach as a routine practice; if you use it, CDC says no more than 1 cup per gallon of water and never mix it with ammonia. Throw away moldy porous items such as carpet or ceiling tiles that cannot be cleaned. EPA also warns against painting or caulking over mold. Our step-by-step guide on how to get rid of mold covers the details.

For areas larger than about 10 square feet, hire a professional. In Florida, Florida Statutes section 468.8411 defines mold assessment and remediation as work on growth larger than 10 square feet, the threshold where state licensing applies. Our mold licensing by state guide explains how to check credentials.