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DoubleBlind: Image of oyster mushrooms against mold background. In this article, DoubleBlind explores contamination in the mushroom cultivation process, different types of mold, and how to avoid them.
DoubleBlind: Image of oyster mushrooms against mold background. In this article, DoubleBlind explores contamination in the mushroom cultivation process, different types of mold, and how to avoid them.

Mold on Mushrooms: What Every Shroom Grower Should Know about Contamination

Moldy mushrooms happen to the best of us—here's how to spot different kinds of mold that may infiltrate your grow.

Caine Barlow // November 16, 2020

Every mushroom grower at some point experiences the disappointment of moldy mushrooms. Even in professional environments, mushroom growers will occasionally have problems—for instance, when someone is careless, working when tired, or takes a shortcut and then two weeks later the spawn, or casing layer, is blue-green! The important thing to realize is: It happens! Try to identify the source of the contamination if you can, and try again.

All mushroom businesses have strategies to mitigate contamination, knowing that it is likely to occur and creating steps to minimize outbreaks when it does. That includes having a plan to contain and control contamination with log books, tracking which inoculant was used for which grow, and appropriately labeling syringes, jars, and containers.

Moldy mushrooms are not only annoying, and in larger scales costly, but they can also be a hazard to your health. Identify them early and dispose of them safely. This article will teach beginner growers how to know what to look for, and how to minimize the potential for moldy mushrooms. 

Cleanliness is the Key to Success

Growing fungi can be challenging, and it is easy to make simple mistakes. Unfortunately sometimes these mistakes can lead to contamination by a variety of molds. The “contaminants” in mushroom cultivation are unwanted fungi, bacteria, or insects. If you have followed all the steps in your Tek (methodology) correctly you have a super clean sterile environment meant for only the one fungus, be it Psilocybe cubensis or oyster mushrooms, making it a lovely place for other unwanted fungi or bacteria. When they start growing in the substrate (the food for the fungi) in which you are trying to grow your mushrooms, they compete for the same nutrients, and in some cases will also use the mushrooms and the mycelium as food.  

Contaminants can be introduced through a variety of sources, including, but not limited to, air currents, the inoculant (i.e. spore print/syringe), tools, the substrate, the growing environment (unclean jars or containers), pests such as flies, mites or insects, or the cultivator themselves. Proper preparation and being quick and efficient are crucial to success. Practicing a good lab technique, following a checklist, doing tasks stepwise, and cleaning between each step all go a long way towards minimizing contamination. Identify the potential sources of contaminants early if you can.  

For beginner cultivators, chances are you will be working in your kitchen or bathroom, so minimize moldy fruits or vegetables in your kitchen, and give surfaces a good wipe down with alcohol. Spores are everywhere, tiny particles carried on air currents and every time you move, open a container, or a housemate opens a door or window, there is going to be air movement and therefore spore movement. The use of a Still Air Box (SAB), or “glovebox,” will limit air movement. If you are using a grain based Tek, be sure to follow instructions—and if you’re uncertain, soak them for 24 to 48 hours minimum with a small squeeze of detergent to kill the endospores.

Use alcohol to clean everything (either isopropyl alcohol, ethanol, or methylated spirits, and dilute to 70 percent). Wipe everything down, keep your tools clean, and flame your tools between each step. Clean the area directly around where you intend to work, as well as the containers, and given that you are a vector for contamination, wear a mask and always use gloves. Food-safe gloves are the best (there are a variety available, and always get some that stretch well or fit well). There is nothing worse than spending three hours wearing gloves that are a size too small (been there, done that!).  

Keep in mind that contaminants may come from your spore print or syringe. The sterile technique practiced by the person who prepared the spore print/syringe will determine your success. This is especially true if the spores came from a wild specimen.

Create Favorable Growing Conditions

Regardless of your Tek (Spiderman Tek, PF Tek, or more advanced methods) or which stage you are at (colonization, bulk grow), keep an eye on development—not necessarily every day, but at regular intervals in order to watch the mycelium grow and learn how to identify changes. Keep a close eye on changes within the substrate: the appearance of slime, discoloration, or dusty textures within your PF Tek; the presence of odors; or those same dusty textures or discolorations in your bulk grow, which are all key indicators that something is not right.

Bacteria are relatively easy to spot, being slimy in appearance. Most mycelium is white, including the mycelium of the contaminants, so you have to wait for spores to develop before you can identify any contaminants. Different contaminants are more likely to appear at different stages. The contaminants you get in your jars are going to be slightly different from what grows on the casing layer of your bulk grow due to the different growing conditions.

Contamination is often the result of unfavourable growing conditions, maybe too much moisture or too much carbon dioxide (CO2). Unfortunately, these are best solved through experience and following instructions, so don’t use shortcuts and don’t rush what you are doing (i.e. let the substrate drain properly, squeeze out enough water, etc.). Keep your jars at room temperature while they incubate—too cold and your mycelium will grow slowly, but too hot the heat will favor mold growth.

If you spot contamination in your jars, the contents of the jar cannot be salvaged as spores and unwanted mycelium are likely to have spread through the remaining substrate. If you find contaminants on your bulk grow, people often suggest you can cut away the contaminated parts. If you catch it early, this is possible, but keep in mind that in a lot of cases if you can identify something as a contaminant then chances are spores have already been spread, and the whole grow may be contaminated. If found while the mushrooms are growing, using water containing 1 percent of a 3 percent hydrogen peroxide solution to spray over the contaminated area may help reduce the problem somewhat by killing the spores—it will not kill the mycelium, but should allow the mushrooms to mature.

When you want to discard the entire contents of the jar, tray etc., wear a mask, since the mold could be harmful to you if you get a good lungful. Your outdoor compost is a great place to dispose of contaminated substrates, but regardless of where, make sure you do it outside—don’t spread spores in your house! Cleaning post contamination is best done using alcohol, or at a minimum using vinegar while wearing a mask and gloves. Do not use bleach as fungi can live on bleached substrates.

Some Discoloration is Normal

Depending on what you are growing, some discoloration is normal. Some mycelium, as with Shiitake or Ganoderma, takes on a brown color as it matures; Psilocybe mycelium on occasion is known to bruise blue. Depending on what you are growing, check the notes on growth behavior to see if the discoloration is normal.

Many fungi with maturity begin to exude colored liquids. This is perfectly normal and is a regular part of their metabolism. Essentially this liquid is mushroom wee. This “wee” often means the mycelium is ready to fruit. Overproduction of this liquid can indicate the presence of contaminants, the mycelium creating either antibiotics or other compounds poisonous to competing fungi or bacteria.

Heavy deposits of spores on some mushrooms should also not be confused with contamination; it is a common occurrence both in nature and in confined growing environments.

Common Contaminants

As a beginner, the following list of molds are the easiest to identify, and the most likely to encounter. Most are very common and will happily exist in and around your house—so chances are, you probably know them well from having found them growing in your fruit bowl, on some vegetables, in your fridge (especially yoghurts), on cheese, or on bread. Some others may come from airborne spores originating in soil.  

  • Blue-green mold – Penicillium sp
  • Black-yellow-green mold – Aspergillus sp.
  • Green mold – Trichoderma sp.
  • Cobweb mold, Downy mold – Hypomyces rosellus
  • Brown mold – Botrytis sp.
  • Lipstick mold – Sporendonema purpurescens
  • Pink Mold – Trichothecium sp.
  • Red bread mold, or Pink mold – Neurospora sp.
  • Black Bread mold, or Black Pin mold – Rhizopus sp.
  • Dry Bubble – Verticillium sp.
  • Wet Spot / Sour Rot – Bacillus sp.
  • Bacterial Blotch – Pseudomonas sp.

The Three Main Molds: Blue-Green, Dark Green, Black

Blue-Green Mold: Penicillium sp.

Penicillium sp. is the genus that includes species often found growing on your cheese or bread. Some varieties of Penicillium are also used to create the coloured veins through your blue cheese. The genus is also famous as the source of Penicillin.

Like many molds Penicillium has a white mycelium, which makes it difficult to distinguish from the mushroom mycelium you are trying to cultivate. Penicillium starts as small circular colonies, but fortunately, it sporulates fairly quickly, appearing as a granular or powdery bluish-green mold, often with a broad whitish rim of new growth. When it sporulates in jars, it can spread rapidly and can take over the entire substrate. It has a musty dirt smell.

Penicillium can grow on a variety of substrates, including rice flour, grains, coco coir, bulk substrates, and wood. It grows on carbohydrates and is not a parasitic mould. It can also be found in a variety of teks (including PF Tek and variations of it).

Black Mold: Aspergillus sp.

Black Mold: Aspergillus sp.

Aspergillus sp. often have the appearance of a green or black mold, but some species can be yellow, brown, or blue. The mycelium of Aspergillus can be light grey in color with a similar appearance to mushroom mycelium. Sometimes colonies of Aspergillus may form a ring with a dense mycelium at the edge. They can have a musty, oily smell.

Keep in mind that some Aspergillus sp. are poisonous. There are a variety of species all having different colours which can make identification difficult: Aspergillus niger is black; Aspergillus flavus is yellow; Aspergillus clavatus is blue-green; Aspergillus fumigatus is greyish-green, and Aspergillus veriscolor exhibits a variety of colours (yellow to pink to green).

Aspergillus prefer a neutral to basic pH. Some species are thermophilic, hence why it is essential to follow the recommended times required to sterilize grains, or bulk grain/wood-chip mixes.

Green Mold: Trichoderma sp.

Green Mold: Trichoderma sp.

Trichoderma sp. are a very aggressive group of fungi. Trichoderma are used in agricultural mulches as they help create a protective environment for plants. They love warm, humid environments with little air movement, and given the opportunity, will rapidly outrun other mycelium. Trichoderma have an aerial, cottony mycelium whereas Penicillium has a flatter mycelium with a grainy appearance.

Trichoderma mycelium is often light grey, growing in circular colonies, with rapid growth soon producing forest-green or olive-green spores. Sometimes the mycelium can be yellow or green, often with a distinct ring surrounding the colony. Sporulation may happen late, making it difficult to spot the contamination or the degree of contamination.

Trichoderma will parasitize fungi, both mycelium and mushrooms. Parasitised mushrooms will have dry brownish blotches or sunken lesions on the cap or stem. The mushrooms will become covered by fine, downy mildew that may eventually become greenish from spore production.

It can be found in a variety of teks (PF Tek and variations of), on grains, and on bulk substrates.

Other Common Molds

Cobweb Mold, Downy Mildew: Hypomyces rosellus (formerly Dactylium dendroides)

Cobweb Mold, Downy Mildew: Hypomyces rosellus

Cobweb Mold, Hypomyces rosellus, is a fast-growing fungus that is cobweb-like in appearance. It often begins as small scattered patches that spread rapidly over the surface of the substrate. It is initially grey and can develop a whitish appearance, with aerial mycelium. Cobweb mold is pathogenic on many fungi, and often found on wild mushrooms, therefore easily spread to spore prints and spore syringes when using foraged specimens. It is particularly problematic when attempting to do tissue cultures of wild mushrooms onto agar.

Cobweb mold favors high humidity. It rapidly overruns any tek (PF Tek and variations of), within a day or two, and if introduced to bulk grows will overrun the casing layer enveloping all the mushrooms present. The mushrooms will become covered with a fluffy mass of delicate mycelium, which will result in a soft rot.

Brown mold: Botrytis sp.

Botrytis cinerea is well known as the mold that grows on grapes and fruit—encouraged on some wine grapes as it imparts a particular flavour. Botrytis mycelium is white at first, then turns grey. It is fast-growing, aerial, then turns a dull golden brown to cinnamon brown as the spores mature. 

It can spread on casing layers, preferring a mixture high in woody tissue, and thrives in high humidity with a moderate temperature. 

Lipstick mold: Sporendonema purpurescens (formerly Geotrichum candidium)

Lipstick mold: Sporendonema purpurescens

Lipstick mold is white at first, sometimes with a “frost-like” appearance, then forming white balls of mycelium. It tends to colonize casing layers, but can colonize other substrates. As it develops the mycelium becomes pink, then red as the spores mature. As it ages, the color fades to a dull orange. It grows slowly, and is an uncommon problem. 

Pink Mold: Trichothecium roseum

Trichothecium roseum causes pink rot of apples and is a parasite of fleshy fungi. The mycelium is initially white, with its colonies growing quickly, becoming pink or peach-coloured as spores are produced. Trichothecium resembles Penicillium in its flat granular powdery appearance.

Red Bread Mold, or Pink Mold: Neurospora sp.

Red Bread Mold is fast-growing, with creeping aerial mycelium that becomes bright pink as the spores mature. Contaminated cultures or bulk grows should be destroyed immediately, with care not to disturb the spores in your growing area: It can grow through the polyester pillow stuffing or micropore take.

Black Bread Mold, or Black Pin Mold: Rhizopus sp.

Black Bread Mold, Rhizopus sp., via Wikimedia Commons

Chances are you have seen Black bread mold, a fast-growing fungus with a dense mycelium, white at first, producing aerial hyphae. The mycelium becomes grey and then takes on a black appearance overall as spores are produced. It appears like a “forest of black-headed pins.”  

Dry Bubble, or Brown Spot: Verticillium sp.

Dry Bubble, or Brown Spot: Verticillium sp.

Dry Bubble begins as a white mycelium present on the casing layer, which in time turns a grey yellow color. It is characterised by deformed mushroom pins:  These turn a grey/brown color and become leathery in texture. If the mushrooms are infected at a later stage, they’ll result in crooked mushrooms with tilted caps, bulging stems especially at the base with peeling flesh.

Common Bacterial Contaminants

Wet spot, or Sour Rot: Bacillus sp.

Wet spot, or Sour Rot: Bacillus sp

“Wet spot” or “Sour Rot” is caused by Bacillus bacteria. It is a mucus-like slime, which is dull grey to brown. It is characterized by a slimy “wet” appearance and has a strong odor that can be described as smelling like rotting apples or dirty socks. Bacillus breeds very quickly and is called “Wet Spot,” as it makes uncolonized grain appear excessively wet. 

Bacterial Blotch: Pseudomonas sp.

Bacterial Blotch: Pseudomonas sp.

Bacterial blotch appears on the mushroom caps as yellow spots or lesions, circular or irregular in shape. They are superficial; reproducing rapidly in overly humid environments where the mushrooms may be wet. The caps tend to become chocolate brown and slimy with age. This bacterium has a mucus-like slime that is dull grey to brown. It also has an unpleasant odor.

It is advised to remove the infected fruit bodies and minimize humidity (less than 92 percent).

A Note about Flies

During the bulk grow is when your growing project is very susceptible to spores or bacteria in your misting water, a lack of or too much airflow, as well as insects. Fungus gnats, vinegar flies, and other winged insects will be attracted to the smell of the mycelium, the mushrooms, the substrate, and the exudates (secretion from the mushrooms). Fungus gnats are especially attracted to the mycelium, while vinegar flies are attracted to the exudates. They will introduce spores and bacteria to your bulk grow.

Fungus gnats are insidious; they are quick breeding and crafty. In addition to laying their eggs in the casing layer, with their larvae hatching and feeding on the mycelium, they serve as vectors for contamination. People lose entire crops to the blighters. Word on the street is that they can squeeze through polyester pillow stuffing! If you are using a shotgun chamber, instead of the pillow stuffing to fill the holes, double up the micropore tape over the holes to allow airflow and to stop insects from entering the growing chamber.

More Advanced Methods

In more formal environments, the growing process involves starting with Petri dishes of agar, sections from which can be transferred to liquid culture. The liquid culture is used to inoculate the spawn, and finally to bulk substrate. Agar allows you to isolate the mycelium without contaminants, but also to select mycelium with preferred growth characteristics.  Used in conjunction with liquid culture, the combination of both techniques will enable you to easily see any contaminants before inoculating spawn.

Working with agar also allows you to quickly become familiar with a variety of contaminants. But if you are working directly from spore syringes, how well your mushroom grow precedes is dependent on the cleanliness of the mushrooms used to make the spore print, how clean the process used to make the spore print, and then the making of the spore solution in the syringe.  

Keep in mind the success of your project is dependent on how clean you can keep each step of the process, and how quick and efficient you can be during each step. Every grower experiences contamination, so when it happens, backtrack and identify where it happened, and then try again. Just be sure not to lose confidence.

Caine Barlow is a Mycologist and Fungi Educator based in Melbourne, Australia.  He gives regular talks on mycology, fungi conservation and teaches gourmet mushroom cultivation.  He is member of the Australian organisations Entheogenesis AustralisThe Australian Psychedelic Society, and MYCOmmunity Applied Mycology. He is a “Trusted Identifier” on The Shroomery and a moderator on many Australian facebook fungi groups. 

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