Oyster mushroom diseases are issues that affect the shape and appearance of the caps and stems, giving them a less marketable look. These defects almost always result from microclimate imbalances, which may be caused by:
- Miscalculated or poorly installed ventilation, humidification, or heating/cooling systems.
- Sudden changes (fluctuations) in temperature and humidity.
Climate fluctuations in the chamber can sometimes be triggered by user settings. For example, using a timer to control humidification is not ideal in a growing chamber.
The best approach to humidification is to have part of the nozzles (or an ultrasonic humidifier) running continuously to maintain a consistent humidity level of around 82-84%. Another set of nozzles can then be set to activate based on a humidity sensor, turning on when levels drop to 84% and switching off once optimal humidity is reached.
If climate controllers are not in place, you may not even realize that temperature and humidity fluctuations are happening in your chamber.
Mushroom responses to microclimate shifts become visible after a few days. From the appearance of the mushrooms, you can often identify the specific climate factor causing the defect.
Table of Contents
Bacterial lesions
- Moldy oyster mushrooms
- White fuzzy mold on oyster mushrooms
Curved and wavy oyster mushroom caps with pimples on the edges
Many mushroom growers refer to any infection on dead mushrooms as "bacteriosis." However, this isn’t accurate.
True bacteriosis in oyster mushrooms is caused by Pseudomonas bacteria, specifically known as Brown Blotch Disease (BBD). You can read more details on bacteriosis in a separate article.
Now, let’s examine the photo.
Why is there mold on these mushrooms?
Initially, there was a lack of humidity in the air, which caused the mushroom in the center to dry out, leaving its cap a lifeless brown color. The two mushrooms seen higher up and to the left in the photo began to develop later, once humidity levels increased. However, the humidity was either raised too high or too rapidly.
In response, the mushrooms turned their caps upward, exposing the hymenophore (gills) to help them release moisture.
But these sharp changes in humidity stressed and ultimately killed the mushrooms.
As a result, condensation began to form on the dead mushrooms, creating the perfect environment for mold to grow.
White fuzz on oyster mushrooms may look like mold, but it often isn’t.
If oyster mushrooms were harvested when the caps were fully mature, they tend to store less well than younger, firmer mushrooms. White spots, therefore, often appear on these mushrooms after they’ve been in the fridge for a few days.
If you bought oyster mushrooms from a store and see a thin layer of white fuzz in the center of the cap, which easily wipes off with a finger, it’s likely just aerial mycelium starting to grow.
These mushrooms are safe to eat.
However, if the white fuzz covers most of the cap and cannot be easily removed, it’s a sign that the mushrooms have likely spoiled.
In this case, it’s best not to eat them.
What could be causing this?
This could refer to mushrooms stored in the refrigerator that have developed a fuzzy, white coating on the stems similar to the mycelium seen on their caps.
I received a few photos where the white fuzz on the stem is clearly visible; it resembles oyster mushroom mycelium. The sender noted that this fuzz developed on stems of mushrooms stored in a bag in the refrigerator.
Sometimes, mold can start growing on the stems. But in the photos I received, the white, delicate fuzz is clearly oyster mushroom mycelium, not mold. This type of aerial mycelium usually appears on mushrooms when there’s high humidity and carbon dioxide in the fridge. However, if the coating is thicker or has an unpleasant odor, it’s more likely to be mold.
When airflow is too strong, mushroom caps can dry out, crack, and turn yellow even with optimal humidity.
The edges may turn brown and feel dry or warm to the touch.
Cracking may occur along the cap edges, causing them to tear. Sometimes this happens after harvesting but often occurs directly on the mushroom bags.
Caps can also crack under normal airflow if the humidity periodically dips, like when water is used on the floor.
Maintaining stable humidity in the chamber is key—ideally, it should be consistently between 83% and 90%, depending on the chamber’s temperature and the specific strain.
If the airflow is slightly above optimal, cap edges may start to curl down to shield the hymenophore (spore-bearing surface, gills) from drying. This effect is especially noticeable if the humidity is also low.
Some oyster mushroom strains naturally have brown caps, particularly many Pulmonaris strains. Read more here.
You’ll find the brown color of a healthy mushroom cap is distinctly different from one that has been stressed or weathered.
The edges of the mushroom caps become wavy and/or form small growths, as shown in the photo.
If the oyster mushroom cap bends downward, this clearly indicates a slight lack of humidity (80-83%) in the growing room.
If the humidity drops even lower, the oyster mushrooms become light and dry. The hymenophore (gills) thins out and may turn white or yellowish.
The fungus ages quickly to release spores faster. In this case, the mushroom consists only of skin and gills; it does not contain any mushroom flesh, which is why it is light.
As a result, the caps curl up, exposing the hymenophore. See the image below.
First, purchase an anemometer to measure the airflow rate by attaching the device directly to the outlet of the nozzle cup in the air duct. You can see what I mean by "nozzle cup" in this photo.
If you have a two-zone growing system, the airflow speed should be 26.4 to 32.8 feet per second (8 to 10 m/s).
Second, check the humidity in the chamber with a different device, not the one currently in your chamber, or re-calibrate your existing device.
This can be done either in a metrology laboratory or by using a highly sensitive, but somewhat expensive aspiration psychrometer.
The clusters you see in the photo on the right better indicate the lack of moisture than any device.
Oyster mushrooms turning yellow and drying out is a sign of insufficient moisture.
If at the same time your psychrometer shows 89%, this clearly indicates the need for verification.
There should be several devices measuring humidity in the chamber, placed directly in the rows and not far from the growing clusters.
Third, check the direction of the airflow from your cups. If they are directed to the sides or have fallen out of the duct, correct these issues.
Also, inspect the recirculation filter—if it is dirty, the humidity in the chamber may drop.
The appearance of mushrooms varies depending on how much the carbon dioxide level is exceeded and at what stage the primordia were affected.
In this case, oyster mushrooms may twist, elongate into a tube, or press against the plastic on the block. To describe the appearance of such fruiting bodies, various comparisons are used:
All these deformities arise from excessive CO2, and the degree of distortion depends on its concentration and other related factors. When mushrooms exhibit these conditions, it is essential to check the proper installation and operation of the ventilation system.
Typically, several factors contribute to this situation: along with increased CO2 levels, a low rate of moisture evaporation in the mushrooms may also be observed.
When airflow is insufficient, it cannot effectively remove carbon dioxide and the moisture cloud accumulated from the mushrooms respiration.
As a result, symptoms may vary, and clusters of different ages may appear differently.
Oyster mushrooms with thick stems and small caps usually indicate that a high concentration of carbon dioxide was present during the early stages of primordia development.
All defects characterized by thin or thick stems and small caps are discussed in the article “The Effect of Carbon Dioxide”
A common cause of abnormal shapes and yellowing in mushrooms is an waterlogged substrate. For instance, twisted oyster mushrooms may have their entire fruiting body turn inside out (bend) and become wavy, with the cap turning yellow.
The hymenophore (gills) may grow abnormally, remaining white or also turning yellow.
If mushrooms bend solely due to increased carbon dioxide, they will have normal flesh.
However, if there is excess water in the substrate, the oyster mushroom will feel non-elastic and watery to the touch.
If you squeeze it, water may flow out, and the flesh may break apart between your fingers. Often, the mushroom emits an unpleasant odor. Such oyster mushrooms cannot be eaten or pickled.
For more information about the problem of oversaturated substrates, click here.
White growths or a white mesh on the caps of oyster mushrooms, resembling semolina or netting, form under low temperatures combined with high levels of CO2 in the growing chamber.
This white substance is known as secondary mycelium, where hyphae develop on the cap in response to adverse environmental conditions. Small primordia may even emerge on the mushroom cap from these hyphae.
These mushrooms are safe to eat, provided you can assure buyers that they are perfectly fine.
A white powdery residue on mushrooms can also be from spores. It has a different appearance, resembling dust, and can settle not only on the mushroom cap but also on surrounding surfaces.
For more information on the risks associated with fungal spores, click here.
White fuzz can also appear on small mushrooms, resembling tiny scales that can be easily wiped away with a finger.
Growths or warts along the edge of the cap that look like pimples occur with a significant increase in humidity (by 3-5%) alongside an increase in CO2 concentration.
More details about such mushrooms in a separate article.
In extreme cold (below 10°F / -12°C), outdoor air holds very little moisture. If mostly cold outside air is used with a low rate of recirculation, a paradox occurs: humidifying nozzles operate at maximum, yet humidity remains low.
This happens because the airflow doesn’t have enough time to absorb moisture, as the flow speed is high and air interaction with the nozzles is brief.
To address this, many growers hang additional nozzles directly in the grow chamber, near the ceiling. When these nozzles operate periodically—controlled by a timer or humidity sensor—it can result in a wavy edge on the mushroom caps or even make the entire cap wavy.
This effect is especially noticeable if the air in the ducts isn’t humidified after passing through the heat exchanger. As outside air is warmed up, it becomes much drier. This dry, warm air then flows into the grow chamber, reducing the humidity even further.
In such conditions, it’s common to see scales on the mushroom cap, as shown in the photo—these are dead, exfoliated skin cells. These dandruff-like scales are especially noticeable on the left side of the cluster.
1. Adjust Ventilation: Aim to use the minimum amount of outside air to help maintain humidity. Monitor cap shape to ensure it remains acceptable for sale.
2. Manage Temperature Gradually: If it’s difficult to heat the grow chamber during extreme cold, lower the temperature gradually by 1–2°C (1.8–3.6°F) every 4–5 hours.
3. Adjust Humidity Accordingly: Reduce humidity according to the specific requirements of your mushroom strain.
When temperature, moisture, and CO₂ levels are balanced, mushrooms grow slowly but develop a beautiful, marketable shape.
Coral-like formations on oyster mushrooms are often due to insufficient light and poor conditions during the early primordia stage in the incubator.
In the first two photos, the tiny pins have developed without air access.
The later photos (3-4) show how these shapes changed once the plastic was opened and a small draft introduced.
Many growers note that these formations resemble a hedgehog or, as some describe, a head of cauliflower (see photos below).
It’s worth noting that there is indeed a mushroom that naturally forms similar shapes—Hericium erinaceus, also known as "lion's mane" or "monkey head," sold by labs.
However, if you planted oyster mushroom grain spawn and ended up with coral-like or spiky white growths, this likely results from an imbalanced microclimate.
High CO₂ levels, lack of ventilation, and absence of light at the primordia stage can all impact oyster mushroom shape. Even if such clusters are later moved to a growing chamber, they won’t develop properly.
To fix this, remove these abnormal formations. If the microclimate is corrected, typical mushrooms should appear within 3-4 days.
If you notice that your typically gray oyster mushrooms have developed a bright blue hue, it often results from a sudden drop in temperature in the grow room. This temperature drop needs to be sharp to trigger the blue coloration.
The younger the mushrooms are when the temperature decreases, the greater the likelihood of them changing color.
What happens to the primordia in this situation?
There are two possible outcomes:
1. If the primordia turn blue, they may have a chance to survive and will grow into a gray-blue color. It's crucial to gradually raise the temperature in the growing chamber after the drop; a sudden increase can cause the mushrooms to drown in excess moisture.
2. If the primordia turn brown or yellow, it indicates that they are dead.
Typically, blueing results in mushrooms that have an unattractive appearance. You may observe clusters where many caps do not open, and some mushrooms appear dry and shriveled.
If the growing chamber is heated by a stove or wood-fired boiler, oyster mushrooms can be poisoned by flue gases. The mushrooms may take on an unusual light brown color that is not characteristic of the strain.
Continued exposure to smoke can lead to:
Flue gases can enter the chamber in two ways:
Mushrooms affected in this way should not be eaten. The discoloration of the cap is likely caused by the partial dissolution of sulfur oxides in the humid air of the growing chamber. This condensate settles on the caps and contains trace amounts of acids formed from sulfur oxides, which disrupt the development of the mushroom gills, leading to the death of the fungus.
Black spots on oyster mushrooms can be in the form of streak.
Dark, radially spreading stripes can appear on the caps of oyster mushrooms due to fluctuations in moisture levels in the growing room.
These stripes form as a result of ruptures in cell layers that occur during periods of lowest humidity.
On some mushrooms, the stripes turn black, which can deter buyers who may view them as a defect.
In the photo, you can see a cluster that developed under conditions where humidity fluctuated between 80 and 86% (relative humidity). The disk humidifier was only activated twice a day, with additional moisture provided by wetting the floors.
Also note the downward-curved, wavy edges of the oyster mushroom caps.
If you encounter this issue, it’s essential to adjust the humidification system to prevent humidity fluctuations.
Additionally, temperature fluctuations may also contribute to these dark streaks, as rapid shifts can disrupt the normal development of cell layers in the mushroom cap, further increasing the chances of pigmentation changes.
In the article, I’ve provided a brief overview of the most common types of oyster mushroom deformities.
If you encounter any issues with the appearance of your oyster mushrooms, feel free to reach out to me for a consultation!
You can find the consultation fees here.
