Sarah Brown
Overlay is a common issue in mushroom cultivation, characterised by a dense, white, matted growth of mycelium that indicates faults in the growing process. While it is not a contaminant, it can hinder mushroom growth and requires timely intervention. The appearance of overlay may be due to prolonged colonisation of the casing layer, improper fruiting conditions, or dehydration. To fix overlay mushrooms, growers can adjust fruiting conditions by increasing fresh air exchange, maintaining relative humidity, and ensuring optimal air temperature. Scratching the overlay with a sterile tool can also help, but it is important to avoid touching the casing layer with bare hands to prevent contamination. While fixing overlay can be challenging, with proper care and adjustments, growers can improve the health of their mushroom crops and encourage fruiting.
| Characteristics | Values |
|---|---|
| Cause | Prolonged mycelial growth (colonization period) into the casing layer |
| Identification | Dense, white, matted growth of mycelia; cottony, cloudy, crust-like appearance |
| Fix | Adjust fruiting conditions; use fork-tek; use casing or pseudo-casing layer; increase fresh air exchange; maintain relative humidity at 90%; air temperature between 70-74°F (21-23°C) |
| Prevention | Initiate pinning at the proper time; avoid excessive CO2 levels |
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What You'll Learn
- Identify the problem: overlay and stroma have similar appearances but different causes
- Adjust the environment: increase fresh air exchange and maintain humidity and temperature
- Scratch the surface: use a sterile tool to gently scratch the overlay, allowing new colonisation
- Rehydrate: if the mushroom cake is dry, introduce moisture to the environment
- Avoid touching: human contact with the casing layer can cause contamination
Identify the problem: overlay and stroma have similar appearances but different causes
Overlay and stroma have similar appearances but different causes. Overlay is a dense mycelial growth that covers the casing surface and shows little or no inclination to form pinheads. It is caused by prolonged mycelial growth (colonization period) into the casing layer. It can also be caused by wrong fruiting conditions, such as excessive CO2 levels and insufficient fresh air exchange. It can turn into a dense, dry crust that blocks the mushroom cake and doesn't allow air and moisture to pass through.
Stroma, on the other hand, is caused by genetic changes. It is a dense, mat-like or cushion-like growth of mycelia that forms on the surface of composts or casings. This type of mycelium can absorb nutrients but cannot produce mushroom fruit bodies. While stroma is triggered by genetic changes, inappropriate fruiting conditions can also be a contributing factor.
Both overlay and stroma have similar visual symptoms, including a cottony, cloudy, mat-like or crust-like dense mycelium. However, it is important to distinguish between the two as it affects the course of action to resolve the issue.
To identify whether the problem is overlay or stroma, it is crucial to examine the growing conditions and the characteristics of the mycelium. By assessing the environmental factors, such as CO2 levels, air exchange, humidity, and temperature, you can gain insights into whether the issue is likely due to incorrect fruiting conditions (indicative of overlay) or genetic factors (suggestive of stroma). Additionally, observing the behaviour of the mycelium, such as its ability to form pinheads and produce fruit bodies, can provide further evidence to determine the specific problem.
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Adjust the environment: increase fresh air exchange and maintain humidity and temperature
Fresh Air Exchange (FAE) is a crucial component of mushroom cultivation. It refers to the process of replacing stale, CO2-heavy air with fresh, oxygen-rich air in your mushroom grow tent. Mushrooms, unlike plants, need a constant supply of fresh air to maintain their metabolic processes. Without a proper FAE system, your mushrooms could suffocate, leading to stunted growth or even death.
To increase fresh air exchange, you can use fans to ensure a continuous supply of fresh oxygen and maintain lower CO2 levels. The size and placement of fans are important considerations, as well as using a timer or controller to regulate their operation. You can also use carbon dioxide sensors to monitor the quantity of fresh air in your grow room.
Maintaining humidity is another critical factor in mushroom cultivation. The role of humidity is to create dew-point on the mushrooms, which is when moisture in the air condenses on the cap and then evaporates. Mushrooms require high humidity levels to start the formation of primordia, which eventually grow into mature mushrooms. Throughout development, mushrooms need humidity to ensure they grow to reach maturity.
To maintain the ideal humidity level, you can use a humidifier, misting spray bottles, pond misters, or simply opening windows and doors. It's important to monitor and adjust humidity levels to create an optimal environment for mushroom growth.
Temperature control is also essential for successful mushroom cultivation. Keeping the temperature between 18-24°C can help maintain optimal humidity levels. In general, the higher the temperature, the more water the air can hold. In temperate zones, mushrooms typically grow during the spring and autumn months, so replicating these conditions is ideal for year-round growth.
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Scratch the surface: use a sterile tool to gently scratch the overlay, allowing new colonisation
Overlay is a dense mycelial growth that covers the casing surface and shows little or no inclination to form pinheads. It is caused by prolonged mycelial growth (colonization period) into the casing layer. While it cannot be "cured", you can help by "scratching" it - using a tool to till the hardened casing and allow for new colonisation.
Scratching is a risky procedure as it opens up another route for contamination. If you do decide to scratch, make sure your tools are sterile (boiled or sterilised with alcohol) and that you are clean. Do not touch the exposed substrate with your hands. You should only scratch the casing layer, not the grain.
To scratch, stick the fork lightly into the myc layer and use a twisting motion to gently break up the myc and casing layer. Do this all over, then mist lightly and keep up with fanning. You only want to break up the overlay, so scratch just the surface. You can then tamp it up with a dry towel.
After scratching, you may need to rehydrate the mushroom cake. You can then make a casing or pseudo-casing layer.
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Rehydrate: if the mushroom cake is dry, introduce moisture to the environment
If your mushroom cake is dry, it's time to introduce some moisture to the environment. The process of rehydration is simple and can be done in a few different ways. The main purpose of rehydration is to compensate for moisture loss. Mushrooms absorb water from the cake, and a lot of moisture can evaporate from the top layer, so it is important to ensure the cake doesn't dry out.
Firstly, check your cakes from all sides. If there is a dry crust of overlay on the cake, use a sterile fork to gently scrape it off. This will help to provide access to air and water for the mushrooms to develop further. Remember, the cake won't absorb water through the overlay. Next, rinse each cake thoroughly under running water. Tap water or clean drinking water is suitable for this. You can also place the cake in a container of cold water to rehydrate. It is recommended to fully submerge the cakes in water, as this provides better results. If your cake is floating, you can weigh it down. Leave the cakes to soak for 3 to 4 hours, or even up to 24 hours for a thorough rehydration. During this time, you can prepare the plastic container the cakes will go back into. It is important to use a clean container to minimise the risk of contamination.
After the cakes have soaked, you can place them back into the container. If you wish to focus the growth on larger mushrooms, place the grow block back into the plastic bag. Alternatively, you can leave the bag off and allow mushrooms to grow from all over the block. You can also mist the surface of the cake until it is well soaked, triggering a new flush of fruiting. This method is less risky than full immersion but may not have the same benefits.
Rehydration stimulates a new flush of fruiting and helps to harvest more mushrooms. It also washes out waste products and promotes beneficial enzymes. It is a simple process but can greatly improve the health and yield of your mushroom cakes.
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Avoid touching: human contact with the casing layer can cause contamination
When cultivating mushrooms, it is imperative to avoid touching the casing layer with your hands. The human body is one of the greatest sources of contamination in mushroom cultivation. Human hands and clothes are rife with bacteria and spores that can ruin an entire crop. Before working with mushrooms, it is advisable to take a shower, use hand sanitiser, and put on clean clothes, such as a lab coat or scrubs. It is also recommended to wear gloves, masks, hair nets, and clean shoes to reduce the risk of contamination.
To prevent touching the casing layer, use a sterile fork (treated with alcohol) or another tool to scratch the surface. Sterilise all equipment, tools, and surfaces before and after use to prevent contamination. This includes treating forks or other tools with alcohol and boiling water. Practising good hygiene and maintaining a clean workspace are crucial to reducing the risk of contamination.
Fungi reproduce through spores, which are microscopic cells released from mushrooms. These spores can remain viable for long periods, and even indefinitely in some cases. The best way to reduce undesired spores in your workspace is to use a laminar flow hood, which includes a filter pad, fan, and High-Efficiency Particulate Air (HEPA) filter. Additionally, proper ventilation is essential in mushroom cultivation. Without adequate ventilation, the growing environment becomes humid and stagnant, creating an ideal breeding ground for contaminants.
Contamination can be challenging to spot, especially in the early stages. Some signs of contamination to look out for include foul odour, discoloration (yellow, green, or black spots), mould growth (white or green fuzzy patches), slimy texture, and stunted or abnormal growth. If you notice any of these signs, take immediate action to prevent further contamination. This may include ditching contaminated tubs or jars, disinfecting the growing area, and adjusting growing conditions.
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Frequently asked questions
Overlay is a condition that occurs when there is an overly colonized casing layer. It is caused by prolonged mycelial growth and indicates faults in the growing process.
Overlay is often bright white and dense, with a cottony, cloudy, or crust-like appearance. It can also be identified by an excessive CO2 level and a lack of pinhead formation.
You can fix overlay by adjusting fruiting conditions:
- Increase fresh air exchange by gently introducing fresh air every 4-6 hours.
- Maintain relative humidity at around 90%.
- Ensure the walls of the fruiting chamber are covered with small water drops.
- Keep the air temperature in the fruiting chamber between 70-74°F (21-23°C).
- Use an aquarium air pump for constant FAE in the grow box.
While it is not advisable to touch the casing layer, you can scratch off the overlay with a sterile tool. Make sure your tools are sterilized with alcohol or boiling water, and do not touch the exposed substrate with your hands. However, this method will set you back about a week as the mycelium has to recover and re-colonize the casing layer.
To prevent overlay, initiate pinning at the proper time and avoid conditions that favour overlay formation, such as excessive CO2 levels and improper humidity or temperature.
