Mycelium is the vegetative part of a fungus, appearing as a network of white, thread-like structures. While healthy mycelium typically presents as white, a yellowing hue can sometimes develop. This color change can be a normal part of the fungal life cycle, or it might signal an underlying issue that requires attention. This article explains the various causes of mycelium yellowing and suggests steps to take.
The Nature of Mycelium and Its Metabolites
Healthy mycelium appears as a vibrant white network, often with a cottony or rope-like texture. This network represents the active growth of the fungus, breaking down materials to absorb nutrients. As mycelium grows and processes its environment, it produces metabolic byproducts. These byproducts, often referred to as “myc piss” or exudates, are a natural part of the fungal metabolic process.
They consist of enzymes, acids, and other organic compounds that aid in substrate decomposition and nutrient absorption. The liquid can appear yellow to amber, and sometimes even orange or brown, depending on the fungal species and substrate. This yellowing indicates an active and healthy mycelial network.
Contamination and Other Causes of Yellowing
While some yellowing is normal, other instances can indicate problematic conditions such as contamination or environmental stress. Bacterial growth is a common culprit, often manifesting as slimy, yellowish or brownish patches on the substrate. These areas may also produce an unpleasant, sour, or even rotten odor. Bacillus species are frequently associated with “wet spot” or “sour rot,” causing a slimy, wet appearance, particularly in grain substrates.
Environmental stressors can also cause mycelium to yellow or produce more exudates. An overly dry substrate can stress the mycelium, leading to increased metabolite production as it struggles to extract moisture. Conversely, excessive moisture can create anaerobic conditions that promote bacterial growth, which then causes the mycelium to yellow in response. Nutrient imbalances in the substrate might also stress the fungal network, potentially leading to a yellowish discoloration.
How to Identify the Cause
Distinguishing between harmless metabolic yellowing and problematic discoloration requires careful observation of several cues.
Visual Cues
Benign yellowing from metabolites typically appears as transparent, yellowish to amber liquid droplets, without any associated slime or cloudiness. This liquid might collect diffusely or in small pools. In contrast, bacterial contamination often presents as cloudy, slimy patches, sometimes with distinct, aggressive borders, and the mycelium itself may appear dull or discolored.
Olfactory Cues
Healthy mycelium has an earthy, mushroomy, or sometimes slightly sweet scent. A foul, sour, cheesy, or ammonia-like odor strongly suggests bacterial contamination.
Timing and Context
Metabolites are common during colonization or when the mycelium is fighting off minor issues. Rapid, erratic growth patterns or cloudiness in liquid cultures can point towards mold or bacterial contamination.
Remedial Actions and Prevention
For benign yellowing from metabolites, no specific action is usually needed, as it signals a healthy, active network. Small adjustments, such as increasing fresh air exchange if pooling is significant, might be considered.
Remedial Actions for Contamination
If the yellowing is indicative of contamination, isolating or disposing of the affected substrate or cultures is important to prevent spread. Contaminated containers should not be opened indoors to avoid releasing spores or bacteria into the environment.
Prevention
Prevention is the most effective approach to minimize yellowing caused by contamination or stress. Maintaining rigorous sterile technique during inoculation and handling is paramount. This includes using disinfected tools and surfaces, practicing good personal hygiene like washing hands and wearing gloves, and ensuring the grow environment is clean. Providing optimal environmental conditions, such as appropriate temperature (25-30°C or 77-86°F), humidity, and adequate fresh air exchange, helps mycelium thrive and resist contaminants. Using properly prepared and sterilized substrates also reduces the risk of unwanted microbial growth.