Mycelium is the vegetative part of a fungus, forming a network of branching, thread-like structures called hyphae, often found underground or within a substrate like soil or wood. Temperature profoundly influences the existence and proliferation of these fungal networks. Understanding its impact on mycelial growth and survival is central to controlling fungal populations.
Mycelium’s Temperature Sensitivity
Each mycelial species has an optimal temperature range for healthy growth. For many fungi, this ideal temperature for active growth is typically 25 to 30°C (77 to 86°F). When temperatures deviate from this range, the mycelium experiences stress, which can inhibit its normal activity. Growth can significantly slow down above 30°C and may cease for many species around 35°C (95°F). Unstable temperatures can also lead to reduced yields in cultivation or increase the risk of contamination.
Killing Mycelium with Heat
Elevated temperatures kill mycelium by causing proteins to denature and cellular structures to break down. For complete sterilization, temperatures typically range from 121°C to 132°C (250°F to 270°F). This is often accomplished using an autoclave, which applies heat under pressure to create a sterile environment for mycelial growth.
Pasteurization involves heating the substrate to more moderate temperatures, generally 60°C to 71°C (140°F to 160°F), for one to two hours. This method aims to reduce undesirable microbes while preserving some beneficial microorganisms. Common fungal contaminants like mold can be killed at temperatures above 60°C (140°F), though maintaining these temperatures for two to four hours is often necessary for effective eradication. The duration of heat exposure is as important as the temperature itself, as prolonged exposure to lower heat can achieve effects similar to shorter periods at higher temperatures.
Mycelium and Cold Temperatures
Freezing temperatures (below 0°C / 32°F) generally slow down mycelial growth and can induce a state of dormancy. While ice crystals can physically damage fungal cells, many species have evolved mechanisms to survive freezing. Freezing temperatures often do not outright kill mycelium, especially if exposure is not prolonged or if cryoprotectants are present.
Some ectomycorrhizal fungi can survive temperatures as low as -48°C, and mycelium has shown recovery after being frozen in liquid nitrogen. Freezing a substrate before inoculation is generally safe. However, freezing a substrate already colonized by mycelium is not advised, as this can damage or destroy the living fungal tissue.
Factors Influencing Mycelial Survival
Mycelial survival at extreme temperatures is influenced by several factors. The specific fungal species plays a role, as some naturally tolerate heat or cold better. For example, fungi from warmer environments may exhibit higher heat resistance.
Moisture content within the substrate also impacts survival; dry mycelium may show increased resilience to high temperatures, while water affects freezing tolerance due to ice crystal formation. The duration of exposure to an extreme temperature is also a determinant. Prolonged exposure, even to non-lethal temperatures, can eventually lead to cell death. The type of material supporting mycelial growth can also influence its resilience to temperature treatments.
Applying Temperature Knowledge
Understanding the temperatures that kill mycelium is important in various applications, particularly in mushroom cultivation. Sterilizing or pasteurizing growth substrates effectively eliminates or significantly reduces competing microorganisms, allowing the desired mushroom mycelium to colonize the substrate without hindrance. In the food industry, heat treatments help prevent fungal spoilage, preserving food quality and extending shelf life.
Temperature knowledge also helps manage unwanted fungal growth, such as mold in homes. Temperatures exceeding 60°C (140°F), sustained for several hours, can effectively kill mold spores. However, while heat can eliminate living mold, it does not remove dead spores or resolve underlying moisture issues that are often the primary cause of mold growth.
Controlling temperature is also important in preventing diseases and pests that can harm cultivated fungi. These diverse applications show how precise temperature management is key to effectively controlling fungal populations.