Different Colors Of Mold

What do the different colors of mold mean?

I took this from an article by Michael Taylor that appeared in the TheConversation on July 26, 2023. It’s a graphical break-down of different colors of fungal growths – from blues in  fruit bowls to blacks staining  showers.

• Scientists believe that humans breathe in up to 10 billion mold spores every day.
• Some fungi can be toxic to humans while other types won’t bother us at all.
• The following shows some of the colors of molds and what they mean.

It’s something that all of us dread seeing in our home – mold.

These fungal growths thrive in damp conditions and spring up in numerous places, whether it be on a wall, a carboard box or even in dust.

While sometimes you can’t see them at first, spores can crop up in a variety of different colors including black, yellow and sometimes purple.

But what do all of these mean?

You ingest and inhale thousands of tiny life forms on a daily basis.

The air and surfaces around you are home to multitudes of bacteria, fungi, viruses, mites, algae and protozoa. Your skin isn’t much better, with a complex ecosystem of organisms called commensals which aren’t necessarily good or bad, but will shift in their composition depending on where you live, the products you use and the pets you have.

Most of these creatures are generally undetectable due to their microscopic size and low concentrations. But when they find a niche they can exploit, you might notice them by their smell, or the appearance of unwanted staining and color changes. A lot of this fungal growth is what we call mold.

Were you ever disappointed when you lifted an orange out of the fruit bowl to discover the bottom half is covered in a velvety blue-green growth?

Do you wonder what the myriad different colors of mold that appear on your stuff tells you about the world you try not to think about?

Black

Toxic black mold can develop in the home due to a flood or chronic damp conditions.

Often black staining is quite a disturbing occurrence. The concept of toxic black mold is one many people have become aware of due to flood impacts. A quick online search will likely terrify you, but not all black discoloration is due to the same organisms, and almost none of it will outright cause you harm.

You have seen Stachybotrys, known as toxic black mold, turn up on building materials that were wet a long time.

When the grout in your shower turns black though, that’s a different fungus called Aureobasidium. It’s slimy, sticky and somewhere between a filamentous mold, which grows threadlike roots through whatever it’s eating, and a yeast, which prefer a free-floating, single-celled style of life. Bleaching will often kill Aureobasidium, but the dark pigmentation will likely hang around – harmlessly, but stubbornly.

The mold colonizing the grout in your shower is unlikely to be toxic. In fact, you can kill it with bleach, but the harmless pigment may linger behind.

Blue

Mold growing in your fruit bowl is related to the one that gave us penicillin. The dusty appearance are spores waiting to be disturbed and spread all over your other fruit.

That blue orange I mentioned before, you can thank Penicillium for that. The organism that gives us blue cheese and the antibiotic penicillin is also responsible for producing a dense growth of mold that almost looks like smoke when disturbed, spreading millions of spores onto the rest of your fruit bowl.

Penicillium is a big group with hundreds of species, ranging from recognized pathogens to species yet to be named. However, the ones that turn up in our homes are generally the same “weed” species that simply cause food spoilage or grow in soil.

Yellow and Orange

Yellow molds can leave a stain behind even once the spores are gone.

We often think of fungi as organisms that thrive in the dark, but that’s not always true. In fact, some need exposure to light – and ultraviolet (UV) light in particular – to complete their life cycle.

Many plant pathogens use UV light exposure as a trigger to produce their spores, and then protect their DNA by hiding it behind melanin-containing shells.

Stemphylium and Epicoccum turn up in our homes from time to time, often hitching a ride on natural fibers such as jute, hemp and hessian. They produce a spectrum of staining that can often turn damp items yellow, brown or orange.

Green

Trichoderma is present in all soils, and will grow fast if the conditions are right. 

We’re all fairly familiar with the green spots that turn up on moldy bread, cake and other food items. Often, we try to convince ourselves that if we just cut off the bad bit, we can still salvage lunch. Sadly, that’s not the case, as the roots of the fungi – collectively called mycelium – spread through the food, digesting and collecting sufficient nutrients to pop out a series of tiny fruiting bodies which produce the colored spores you see.

The green tuft is often from a group of fungi called Aspergillus. Under the microscope they look rather like the puffy top of a dandelion gone to seed.

Like Penicillium, Aspergillus is another big fungal group with lots of species that turn up virtually in every environment. Some are heat tolerant, some love acid and some will happily produce spores that stay airborne for days to months at a time.

In the green gang is also a fungus called Trichoderma, which is Latin for “hairy skin”. Trichoderma produces masses of forest-green, spherical spores which tend to grow on wet cardboard or dirty carpet.

Pink, Purple and Red

Red oncom, a traditional staple food in West Java, Indonesia, is made with Neurospora. 

There are plenty of different colors of mold in this category. There is also a common bacterium that makes the list.

Neurospora, also known as the red bread mold, is one of the most studied fungi in scientific literature. It’s another common, non-hazardous one that has been used as a model organism to observe fungal genetics, evolution and growth.

Fusarium is less common indoors, being an important crop pathogen, but will sometimes turn spoiled rice purple. It also occasionally turns up on wet cement sheet, causing splotchy violet patches. Fusarium makes large, sticky, moon-shaped spores that have evolved to spread by rain splashes and hang onto plants. However, it is fairly bad at getting airborne and so doesn’t tend to spread very far from where it’s growing.

Finally in this category, that pink scum that turns up around bathroom taps or in the shower? It’s actually a bacterium called Serratia. It will happily chew up the soap scum residue left over in bathrooms, and has been shown to survive in liquid soaps and handwash.

Some of the pink stuff in your bathroom isn’t even mold – it’s bacteria. 

White

An example of Isaria farinosa growing out of its host.

When fungi were first being classified and were eventually given their own phylogenetic kingdom, there were lots of wonderful and not strictly categorical ways we tried to split them up. One of these was hyaline and non-hyaline, essentially referring to transparent and colored, respectively.

One of the interesting non-pigmented molds you may well catch sight of is a thing called Isaria farinosa (“farinosa” being Latin for “floury”). This fungus is a parasite of some moths and cicadas and is visible as brilliant white, tree-shaped growths on their unfortunate hosts.

So when you notice the world around you changing color, you can marvel with your newfound knowledge at the microscopic wonders that live complex lives alongside yours. Then maybe clean it up, and give the fruit bowl a wash.

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