Emma Wilson
Mushrooms are susceptible to a wide range of contaminants, including bacteria, fungi, insects, and other pests. Contamination can occur at any stage of the cultivation process and can be challenging to manage once it takes hold. The most common contaminants are moulds, which can spread quickly and are often difficult to eliminate. Bacterial contamination is also prevalent and typically identified by a slimy appearance and an odour of fermentation or decay. Other contaminants include insects such as mushroom flies and nematodes, which can transmit diseases and directly inhibit the growth of mushrooms. To prevent contamination, a sterile environment and processes are crucial. This includes proper disinfection and sterilisation of equipment and substrates, as well as personal hygiene practices to reduce the presence of microorganisms.
| Characteristics | Values |
|---|---|
| Cause | Fungi, bacteria, yeasts, mould spores, animal pests, poor hygiene, flies, poor sterilization, incorrect sterilization, poor sterile workflow, working with weak mycelial cultures, unsterile equipment, contaminated water, contaminated air, human vectors |
| Appearance | Green, Blue, Grey, Black, Yellow, Orange, White patches or discolourations, slimy patches, crusty or gel-like substance on grain surface, white foam, hair-like structures with a tiny head, soft and squishy mycelium block |
| Odor | Sour, alcohol, fermentation, decay |
| Type of Fungi | Trichoderma, Aspergillus, Penicillium, Verticillium, Rhizopus, Fusarium, Bacillus, Cobweb, Mycogone, Overlay, Nematodes, Sciaridae, Lycoriella, Megaselia |
| Prevention | Sterile environment, sterile process, sterile equipment, sterile lab work, clean body, clean air, lower humidity, provide air circulation, hydrogen peroxide spray, salt, flame-sterilized spoon, soaking grains before sterilization, laminar flow hood, still air box, sanitization |
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What You'll Learn
Common contaminants and how to spot them
The most common contaminants in mushroom cultivation are bacteria, mould, and animal pests. Fungi, yeasts, and other organisms can also be contaminants. The most common type of bacterial contamination is known as "wet spot" or "sour rot", which is characterised by a dull grey slime with a sour-smelling odour. Bacterial contamination can also cause yellowing of the substrate.
Mould is another common contaminant. Green mould (fungus) is the most common infection, turning the mushroom bread green and producing a white foam on top. Trichoderma, a group of green mould fungi, is present in all soils and is one of the most common forms of contamination seen in mushroom cultivation. It produces white mycelium that rapidly covers substrates before producing emerald-green spore-bearing structures. Cobweb mould is not a single species but a closely related group of mould species that cause soft rot in mushrooms. Aspergillus and Penicillium are other types of mould that can contaminate mushroom crops.
Fungi can also be contaminants, with distinct borders between two mycelia where they meet, known as sectoring. Fusarium, which has the same white colour and growth speed as mushroom mycelium, turns bright pink, purple, orange, and yellow after a few weeks.
Animal pests such as mushroom flies and nematodes can also cause contamination. Mushroom flies are small insects that are usually dark in colour, while their larvae are lighter and feed on mycelium. Nematodes attack the mycelium, turning it dark and causing the fungi to malform or inhibiting their growth.
To spot contaminants, look out for discolouration such as yellow or brown spots or streaks, slimy patches, and unusual odours. A magnifying glass can help to identify hair-like structures of fungi known as sporophores.
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How to prevent mushroom contamination
Mushroom contamination can occur due to various factors, including air, water, substrate, equipment, and human handling. Here are some detailed steps to prevent mushroom contamination:
Sterilization and Cleanliness:
Maintaining a sterile environment is crucial for preventing mushroom contamination. Before starting, ensure your hands are clean and free from any potential contaminants. Wash your hands thoroughly with soap and water, or use rubbing alcohol to sanitize them. Similarly, any tools or equipment that will come into contact with the mushrooms or their growing environment should be disinfected. This includes knives, scissors, and any other cutting or handling implements. Sterilize these tools with hot soapy water or rubbing alcohol.
Additionally, the substrate, or growing medium, should be properly prepared and sterilized. High pressure and temperature for an extended period can help eliminate dormant spores and competing organisms. This can be achieved using a stovetop pressure canner capable of reaching 15 PSI.
Air and Ventilation:
The air in the cultivation area can be a source of contamination, as mold spores and other invisible contaminants may be present. To mitigate this, ensure the area is well-ventilated. You can also use a laminar flow hood or a still air box (SAB) to provide a clean stream of air for your work, effectively reducing airborne contaminants.
Water:
Water can introduce bacteria, fungi, and other harmful microorganisms into the growing environment. Always use sterilized water that is free from contaminants. Reverse osmosis systems and UV light sterilization can help ensure the purity of the water used for mushroom cultivation.
Storage and Timing:
Storing your mushroom kit for too long can affect the vitality of the mycelium and increase the chances of contamination. It is recommended to start growing as soon as you receive your kit. Additionally, be mindful of the temperature and humidity levels. Mold thrives in warm and humid conditions, typically between 25-30 degrees Celsius. If you live in an area with high temperatures and humidity, consider growing mushrooms during cooler months or in a well-ventilated space.
Personal Protective Equipment (PPE):
Human handling is a significant source of contamination. When handling mushroom cultures, wear gloves, masks, and hairnets to minimize the transfer of contaminants.
By following these steps and adhering to strict hygiene and sterilization practices, you can significantly reduce the risk of mushroom contamination and increase the chances of successful cultivation.
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What to do if your mushrooms are contaminated
If your mushrooms are contaminated, there are a few things you can do to address the issue and prevent further contamination. Firstly, it is important to identify the type of contamination you are dealing with. Common contaminants include bacteria, mould, and fungi.
If you are dealing with bacterial contamination, look out for slimy appearances on the mycelium or grains. This slime may appear as yellow or brown rings around the grains or as a crust or gel-like substance on the grain surface. Bacterial contamination can often be accompanied by an odour of fermentation or decay. If the contamination is limited to a small area, you can try to separate the contaminated grains from the healthy ones using an alcohol-sanitized or flame-sterilized spoon.
If you are dealing with mould contamination, the most common type is green mould, specifically Trichoderma. This mould produces white mycelium that rapidly covers the substrate before producing green spore-bearing structures. If you catch it early, you can try to remove the contaminated area and treat it with a hydrogen peroxide spray to kill the mould spores.
If you are dealing with fungal contamination, common types include Aspergillus, Penicillium, and Rhizopus. Aspergillus can appear as black, gray, blue, yellow, or green patches. Penicillium produces a green or blue mould. Rhizopus develops hair-like sporophores with a tiny head and has a sour odour. To address fungal contamination, you will need to remove the affected area and improve the sterilization of your equipment and environment.
In general, preventing contamination requires creating a sterile environment and following sterile processes. This includes proper sterilization of substrates, equipment, and your own hands and body. Additionally, maintaining good air circulation and humidity levels can help prevent mould growth.
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The impact of contamination on mushroom growth
Contamination can have a devastating impact on mushroom growth, and it is the leading cause of crop failure. It can come from the air, water, soil, or even the grower themselves. The most common contaminants are bacteria, mould, and animal pests. Fungi spores are everywhere, and it only takes one to contaminate an entire mushroom culture.
Bacterial contamination is also common and often identified by a slimy appearance on the mycelium or grains. It can cause the substrate to become sludgy and thick, and it is often accompanied by an odour of fermentation or decay. One of the most common forms of bacterial contamination is "wet spot" or "sour rot", characterised by a dull grey slime with a sour-smelling odour.
In addition to mould and bacteria, animal pests such as mushroom flies and nematodes can cause significant problems during mushroom cultivation. They transmit diseases, feed on the mycelium, and directly inhibit the growth of mushrooms.
To limit the impact of contamination on mushroom growth, early detection is crucial. Signs of contamination include discolouration, such as yellow or brown spots, and a slimy appearance. Odour can also be a helpful indicator, as contaminated cultures may smell like fermentation or decay. Once contamination is spotted, growers must act quickly to separate or remove the affected areas and prevent further spread.
To prevent contamination, it is essential to maintain a sterile environment and follow proper sterile techniques. This includes practising good hygiene, using sterilised equipment, and ensuring substrates are properly prepared. While perfect sterility is nearly impossible, taking extra precautions can significantly reduce the risk of contamination and improve the chances of successful mushroom growth.
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Sterilization techniques and their effectiveness
Sterilization is critical in mushroom cultivation to protect the mushrooms and the economic viability of the process. Mushrooms are highly susceptible to contamination due to their unique growth process. They absorb nutrients from their surroundings, making them vulnerable to absorbing harmful substances as well. Various sterilization techniques are available to mitigate this risk, including pasteurization, autoclaving, and chemical sterilization. Each method has its advantages and challenges, and the choice depends on factors such as the scale of the operation, available resources, and the specific mushroom species.
Pasteurization
Pasteurization is a process of heating the substrate (the growth medium for mushrooms) to temperatures between 65°C and 85°C for 1.5 to 2 hours. This technique reduces the population of competing bacteria, giving the mushroom species an advantage. It is often used for substrates like wood chips, straw, or coffee grounds. However, pasteurization does not eliminate all impurities, and some mushrooms require sterile conditions for growth.
Autoclaving
Autoclaving involves using a pressure cooker or an Instant Pot to sterilize containers and agar, killing any contaminants. This method is crucial before inoculating the containers with mushroom spores or mycelium. It is important to handle the containers with care and maintain a clean environment to minimize the risk of contamination.
Chemical Sterilization
Chemical sterilization methods include using isopropyl alcohol, hydrated lime (calcium hydroxide), or dish soap to eliminate contaminants. Isopropyl alcohol is effective for cleaning surfaces and gloves to prevent the introduction of contaminants. Hydrated lime is mixed with water and added to the substrate to raise the pH, creating an environment unsuitable for contaminants. Dish soap can be used to reduce contaminants on straw by mixing it with hot water and maintaining a temperature of 160-180°F for 1-2 hours.
Other Techniques
Other techniques to prevent contamination include the use of laminar flow hoods, which create a clean air environment by filtering out bacteria and fungal spores. Additionally, maintaining a clean and controlled environment is essential, as contaminants thrive in unclean conditions. Proper handling of equipment, assuming objects are contaminated if unsure, and practicing good aseptic techniques are crucial aspects of successful mushroom cultivation.
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Frequently asked questions
Discoloration is one of the most obvious signs of contamination. If you spot green, blue, grey, or black patches on your mushrooms, they are likely contaminated. However, blue stains may indicate bruising, and yellow patches may be a sign of the mycelium getting old. Other signs include foul odour, slimy texture, abnormal growth, and white or green fuzzy growth, which could be mould.
Mushroom contamination occurs when unwanted microorganisms invade the growing environment and compete with the mushrooms for resources. Poor hygiene, incorrect sterilisation, poor ventilation, proximity to a contaminated area, and outdoor contamination are all common causes of contamination.
If you spot any signs of contamination, it is best to discard the contaminated mushrooms and separate them from the non-contaminated ones. You should then sanitise the affected area and inspect your workspace for the source of the contamination.
