What Is the Real Difference Between Mold and Mildew?: Answered by a Scientist

If you've been suspect of mold in your home, you're probably familiar with the word mildew. The odds are you've likely spent hours on end googling “mold vs. mildew” only to still be left with questions. So, what's the actual difference? Is there a difference?

Although different scientifically, the terms mold and mildew are used day-to-day interchangeably, as if they mean the same thing. The word mildew is tossed around by realtors, inspectors, property managers—often to make a problem seem less severe (Let’s face it: the word mildew just doesn't get the same bad rap as mold does)—or just by everyday people who have been misinformed. While many articles claim their difference is based on color, or texture, or appearance, etc., this is wrong; the real distinction is much more scientific.

To start, let's discuss the similarities between the two to understand how they may be mistaken for each other and how they've come to be referred to commonly as the same thing. Molds and mildews both share characteristics of fungi, the same kingdom that the mushrooms we eat belong to. Fungi are a huge mixing pot of a bunch of different organisms that feed by absorbing nutrients from organic matter. The two also reproduce similarly by making spores, which are tiny microscopic little units that create new individuals, making the growth and spread of molds and mildews possible. Think like how plants use seeds to reproduce. They totally stole this idea from fungi.

The two also share similar structures and can look identical. For example they both produce hyphae, which are long hair-like threads used by the fungi to feed and grow. They also can be a rainbow of different colors and different textures as the diversity of the two groups is so large. This makes distinguishing between the two almost impossible to the naked eye and requires a microscope.

With so many similarities you might be skeptical of a distinct difference still, however thanks to how complex living things are, there's still room for many distinct differences. To start, most mildews are actually not classified as true fungi, while molds are. They are part of a fungi-like group of microorganisms called Oomycetes (Downy Mildews), which was ironically actually once considered a part of the fungi group by scientists. There are some mildews however that are considered true fungi, such as Erysiphales (Powdery Mildews) Confusing, I know.

A maybe more useful explanation is that the difference between them is that mildews are often obligate parasites, while most molds are saprophytic. In understandable terms, mildews require a live host to feed on, while molds prefer feeding on dead things. This means that mildews require a live host like a plant on which it can feed and grow, while molds can grow on basically any organic material; for example, wood, building materials, or fabrics, in which it uses oxygen to break down and take nutrients from. Because mildews often require a live host, many of the most common types of mildews are plant diseases. Think of the great potato famine. Yep! That was a type of mildew. Because of these specific needs of mildews, the growth of an actual mildew in the home is not possible- unless of course you have a garden in your living room.

While it can be easy to feel defeated and confused when it comes to this topic, just remember you are not alone, especially considering the majority of articles online are misleading and not scientifically informed. However, with a newly found scientific understanding of the difference between the two, you can be confident that whatever moldy looking substance is growing in your home, it is not mildew.

https://ucmp.berkeley.edu/chromista/oomycota.html#:~:text=The%20Oomycota%20were%20once%20classified,of%20cellulosic%20compounds%20and%20glycan.

https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/erysiphales

https://portal.ct.gov/CAES/Fact-Sheets/Plant-Pathology/Powdery-Mildew#:~:text=These%20fungi%20are%20all%20obligate,others%20are%20fairly%20host%2Dspecific.