Identifying Contamination

Identifying comtamination and understanding how they occurred is an important skill for mushroom growers. Absolutely nobody who has tried to grow mushrooms can honestly say they have never experienced contamination – experiencing contamination, learning to identify it and learning how to prevent it are all part and parcel of the daily life of a mushroom grower. The good news is that growers can do a lot to reduce the risks of contamination, even if it will still happen from time to time. Fungi are highly sensitive to environmental impurities and the types of contamination, their causes, and potential remedies differ across substrates and grow setups.

“My grain spawn / grow bag / monotub has green mould on it, what caused it?”

Trichoderma contamination

This is typical of the sort of questions you will find asked on forums across the internet. User responses to this sort of question will vary from ‘blame anyone but yourself’ through to genuinely helpful suggestions, and can lead to confusion amongst newcomers to mushroom growing. Unfortunately, growers who look to blame others for their setbacks will generally take a long time to find real answers.

The reality is that contamination can occur in multiple ways, with multiple potential causes, but your first steps to a clean growth starts at home. Your starting point is to ensure that strict sterile working practices are followed, in every possible way, on all occasions. In addition to that, inoculate materials (liquid culture and especially spore syringes) need to be tested for potential contaminants prior to use.

Storage conditions, temperatures and air exchange need to be looked at carefully. For home growers, even things like pet, partner and child interactions need to be considered – for example, if your child sees your monotub, might he/she decide to have a look and feel inside and introduce airborne or contact contaminants? This is a very common hazard!

Issues with mushroom bags

Mushroom bags, like many things, can come from many sources. Most will be made of Polypropylene, but their quality can vary. Cheap bags, imported from China, are often said to be less than top quality. They are quite easily spotted – their filter patches are square on the outside but have a circular central hole shape. Other brands, such as Unicorn, have square filters overlaying square shaped holes.

Quality and bag thickness can become an issue because when bags undergo steam sterilization they can become a little brittle. As a consequence, lower quality bags can be susceptible to forming tiny holes, particularly on seams and folds. Users can inadvertantly ‘hole’ their own bags through energetic handling and shaking, so care is always advised. Similarly, dropping a filled bag can cause tears, splits or holes, particularly around joints.

Probably the easiest way of identifying contamination is looking for discolouration. Most forms of invasive moulds give themselves away with distinct and bright colours. Whether you’re looking at grain spawn, substrates, grow bags or monotubs, green, blue, grey, pinkish or black patches are indicators that they are hosting some form of contamination.

Identifying contamination can be tricky, especially for novice growers, who may confuse normal growth or environmental factors with contamination. What we will try to do here is explain how to visually identify common contaminants in each substrate, including notes on potential confusing factors.

Identifying Contamination in grain spawn

Identifying contamination in sterilised grain spawn you have either purchased or made yourself starts with inspecting your chosen grain prior to inoculation. If the grain looks clean and healthy, you’re off to a good start. The time contamination takes to become visible can vary according to type, but will be between 2-7 days, so if you want to be sure your grain is clean prior to inoculation, put it to one side for 7 days and then check it again. If there are no signs of contamination, you’re good to go.

Next, you need to check your chosen inoculate – whether that’s liquid culture or spore syringes. Identifying contamination in syringes is almost impossible unless it’s severe, so the ideal solution is to test a few drops in an agar petri dish (these can be purchased or made yourself). If it exists, most types of contamination should appear on the agar fairly quickly – generally within one week, but sometimes two. If your grain shows no signs of contamination and your LC is clean, you’re off to a good start.

It is important, when using liquid culture and spore syringes not to use too much, as excess liquid can trigger contamination in your spawn. As a general rule you’ll probably only need between 5ml and 10ml of liquid culture, or 2ml to 3ml of a spore syringe. Better to use less than too much – and you can always use more next time if it’s too slow for you.

Identifying Contaminants:

Bacterial Contaminants (e.g., “Wet Spot” or Sour Rot)

Identifying contamination- Bacillus

Description: The grain appears wet, slimy, or greasy, with an off-putting sour or foul odour. Affected grains may be discoloured (often brownish or darker than usual). This bacteria can exist in grains or substrate if they have not been properly prepared, but in the case of competent commercial suppliers this should not be an issue. The cause of sour rot can often be closer to home – maintaining high standards of hygiene throughout the mushroom cultivation process is essential in the fight against bacterial contamination. Inadequate sterile practices, such as not washing hands, using unsterilized tools, or working in a dirty environment can quickly introduce Bacillus and other contaminants. Wearing protective clothing including disposable gloves and face masks can help reduce the risk of passing contaiminants from your body.

Confusing Factors: Beginners might confuse condensation with bacterial contamination. Condensation itself is normal if moisture droplets are clear and grains look intact. Freshly sterilised grain can sometimes appear damp, usually caused by absorption of condensation, but such dampness will vanish fairly quickly. However, a greasy or slimy appearance allied to a sour smell confirms bacterial contamination.

Moulds (e.g., Trichoderma, Aspergillus, Penicillium)
Description: Moulds appear as fuzzy or powdery patches, typically green (most common with Trichoderma), black, or white. These patches can quickly spread and may look like thick cotton or coloured fuzz on the grain.
Confusing Factors: Some novice growers might confuse mushroom mycelium, which is typically bright white and has a thread-like or slightly fluffy appearance, with white mould. However, mycelium grows evenly and doesn’t have the colour variances that mould can develop as it matures.
Note: if this kind of mould appears, immediately isolate the grain and throw it away immediately. Mould spores can easily enter the surrounding air and contaminate surfaces, leading to it spreading very quickly.

Yeasts
Description: Yeast contamination appears as a milky, slimy coating on the grains, often with a sweet or sour smell. This slime has a sticky, translucent quality, unlike the fuzziness of mould. Yeasts are types of fungi, but are more like bacteria in this context. They are killed off by heat during even basic sterilisation even quicker than endospores. Yeasts are normally introduced through bad inoculate (liquid culture or spores) or inadequate sterile procedures.
Confusing Factors: Wet grains may initially look similar, but true yeast contamination has a distinct odour and sticky appearance. Condensation doesn’t produce this sticky, slimy look.

Identifying Contamination in substrates

Identifying contamination in substrates before you use them is also esseential. The same safety check recommended for grain applies also to your substrate – leave it untouched for a week to check for pre-existing contamination. If anything later occurs, at least you can remove the substrate from your list of possible suspects.

Identifying Contaminants:

Cobweb mould in monotub

Trichoderma (Green mould)
Description: Trichoderma starts as white mycelium, like mushroom mycelium, but quickly turns green as it matures. It has a fine, powdery or fuzzy texture and spreads in circular patches or clusters.
Confusing Factors: Early-stage Trichoderma is white and could be mistaken for mushroom mycelium. However, it turns green within a day or two, unlike mushroom mycelium, which remains white. The speed of colour change and green spore production are key identifiers.
Note: if this kind of mould appears, immediately isolate the substrate and, ideally, throw it away straight away. Mould spores can easily enter the surrounding air and contaminate surfaces, leading to it spreading very quickly.

Cobweb mould (Dactylium)

Description: Cobweb mould is thin, wispy, and greyish, growing much faster than mushroom mycelium. It looks like fine, spreading webs or clouds on the surface of the substrate.
Confusing Factors: Cobweb mould is often mistaken for mushroom mycelium at first. However, cobweb mould grows significantly faster and has a grey hue, unlike the dense, white growth of healthy mycelium.

Bacterial Blotches
Description: Appears as darkened, moist, or slimy patches, usually accompanied by an unpleasant sour or rotting smell. The substrate in affected areas may become mushy.
Confusing Factors: Condensation droplets may be misinterpreted as bacterial growth. Unlike bacterial blotches, condensation doesn’t produce any smell or sliminess. Bacterial contamination also creates a more localized discoloration and odour.

Identifying Contamination in All-in-One Grow Bags

The same safety checks recommended for identifying contamination in grain and substrate also apply to all in one bags which contain those two components together – leave them untouched for a week to check for pre-existing contamination. If anything later occurs, at least you can remove the substrate from your list of possible suspects.

Black bread mould

Common Contaminants:

Trichoderma (Green mould) – as above

Black mould (e.g., Aspergillus spp.)
Description: Black mould appears as dark or black spots or patches. It is powdery or sooty and may be localized or spread over the substrate. It looks quite similar to cobweb mould (see monotub notes below) but with little black pinheads on the strands.
Confusing Factors: Black mould is usually distinct and hard to mistake for mycelium. However, dark patches could also be condensation shadows, so examine closely to ensure the darkness is caused by actual mould.

Wet Rot/Bacterial Slime
Description: Wet rot appears as slimy or wet patches with a sour smell, often brown or discoloured. These areas may feel sticky or mushy if touched.
Confusing Factors: Moisture from condensation can sometimes pool slightly in bags but should not be accompanied by smell or discoloration. Wet rot always has a foul odour and localized sliminess.

Identifying Contamination in Monotubs

Identifying Contamination:

Trichoderma (Green mould) – as above
Description: Appears first as white, fluffy patches that soon turn bright green. It spreads quickly, forming round or irregular green spots or patches. When rehydrating monotubs, trichoderma is quite a common occurrence, generally caused by excess water remaining in the tub.
Confusing Factors: Green mould in monotubs can resemble algae growth, but algae usually don’t grow indoors. The key indicators for Trichoderma are rapid spread, bright green colour, and powdery texture.

Cobweb mould
Description: Cobweb mould looks like fine, grey, wispy threads, often covering large areas within days. It’s thin and looks much like spider webs, spreading faster than typical mushroom mycelium.
Confusing Factors: Cobweb mould can look like early mushroom mycelium, but it’s more diffuse and lacks the density of healthy mycelium. It can also be easily tested by applying hydrogen peroxide—cobweb mould dissolves on contact, while mycelium does not.

Tips for newcomers identifying contamination and confusing factors:

Healthy uncontaminated spawn

Condensation: Clear condensation on jars or monotub walls is normal and differs from contamination, which often has colour (green, black, or grey) or odour.
Mycelium Metabolites: Yellowish fluid appearing on the substrate or jar can resemble bacterial contamination but are often harmless and odourless.
Healthy Mycelium: Mushroom mycelium is dense, white, and often grows in a web-like or thread-like pattern. Any unexpected colours (especially green, black, or grey) generally indicate contamination.
Odour: Contaminants, especially bacterial and mould contaminants, emit strong odours. Mushroom mycelium has a mild earthy smell; anything sour, rotten, or chemical is a red flag for contamination.

Following these visual and sensory indicators can help mushroom growers in identifying contamination, minimizing spread and preserving healthy growth. Contamination is something that anyone considering mushroom cultivation needs to guard against. It can never be repeated often enough that protection starts at home – with maximum attention given to sterile working conditions and techniques.