Optimal temperatures for each stage of mushroom growth

Optimal temperature in mushrooms: why it is a key factor

Before delving into the specific temperatures for each stage, it is essential to understand the physiological and biochemical principles that inextricably link fungi to temperature. Fungi, as heterotrophic organisms, base their existence on a series of enzymatic reactions whose speed and efficiency are strictly modulated by the ambient temperature.

Each enzyme possesses an optimal operating temperature, and the combination of all these enzymes defines the thermal growth range for a given species. Outside this range, metabolic processes halt or become inefficient, leading to stunted development or death of the mycelium. Temperature also directly influences other critical physical parameters, such as relative humidity and the concentration of oxygen and carbon dioxide in the substrate and air, creating a system of interdependent variables that the grower or researcher must know how to manage in synergy.

The fundamentals of fungal physiology related to heat

The cell wall and plasma membrane of fungi are the first interfaces to interact with temperature variations. The membranes, composed of phospholipids and sterols, change their state of fluidity based on heat. At too low temperatures, the membrane becomes rigid and poorly permeable, hindering exchanges with the outside.

At too high temperatures, it becomes excessively fluid, losing its structural integrity and leading to the leakage of cellular content. Temperature, therefore, not only acts on the speed of reactions but also on the physical integrity of the organism. Understanding these mechanisms is essential to comprehend why, for example, a sudden thermal shock can be lethal even for an apparently vigorous mycelium.

Classification of fungi based on temperature: psychrophiles, mesophiles, and thermophiles

Not all fungi thrive under the same thermal conditions. Mycology classifies them based on their preferred temperature range. Psychrophilic fungi, like some species of the genus Clavaria that fruit in late autumn or early spring, have a growth optimum between 5°C and 15°C, with some strains able to metabolize even at temperatures close to zero.

Mesophilic fungi, which constitute the vast majority of cultivated and wild species (such as Agaricus bisporus or Pleurotus ostreatus), prefer temperatures between 20°C and 30°C. Finally, thermophilic fungi, like the famous Aspergillus fumigatus or some strains of Scytalidium thermophilum, can grow and reproduce at temperatures above 45°C, finding their optimum even above 50°C. This classification is not just a taxonomic curiosity but has immense practical implications for the choice of species to cultivate based on local climate or for managing parameters in a growth chamber.

Temperature in the spore germination phase

The journey of a fungus begins from a single spore, a microscopic entity endowed with enormous genetic potential but extremely vulnerable to environmental conditions. Germination is the process by which a spore, from a state of quiescence, "awakens" and begins to produce the germ tube that will give rise to the primary mycelium. This process is activated by a combination of factors, including the presence of nutrients, adequate humidity, and, not least, an optimal temperature. The temperature must not only be within a specific range to allow the activation of the spore's reserve enzymes but must also be sufficiently stable to not trigger mechanisms of secondary "dormancy".

Biochemical mechanisms of thermal activation of spores

Inside the spore, triglycerides and glycogen act as energy reserves. The increase in temperature to an optimal level activates enzymes such as lipases and amylases, which begin to break down these reserves into simple sugars and fatty acids, making them available for the synthesis of new membranes and cell walls. If the temperature is too low, these enzymes work at an infinitesimal speed, delaying or preventing germination. If it is too high, the enzymes themselves can denature, becoming permanently inactive. The thermal window for germination is often narrower than that for mature mycelial growth, making this phase particularly delicate.

Tables of germination temperatures for commercial and wild species

The following table reports the optimal and minimum/maximum temperature ranges for spore germination of some of the most common and studied mushroom species. The data are the result of a meta-analysis of numerous laboratory studies.

As can be seen from the table, species like Shiitake have a rather high germination temperature range, reflecting their adaptability, while the Morel (Morchella) prefers cooler temperatures, consistent with its spring habitat. It is interesting to note how the optimal germination temperature does not always coincide with the optimal temperature for mycelial growth or fruiting, a concept we will explore in detail in the following paragraphs.

Mycelial growth: temperatures for substrate colonization

Once the spore has germinated, the young mycelium begins its expansion into the substrate, a process known as colonization or the vegetative phase. This is the phase where the fungus invests most of its energy in exploring and degrading the substrate to absorb nutrients. An optimal temperature in this phase translates into a high colonization speed, which in turn reduces the risk of contamination by competing molds and bacteria, which often have faster generation times. Managing the temperature in this phase therefore means not only favoring your fungus but also disadvantaging potential antagonists.

Temperature optimization for rapid and healthy colonization

The temperature inside the substrate ("cake" temperature) is often 1-3°C higher than the ambient air temperature due to the metabolic activity of the mycelium itself. This is a critical factor to monitor, especially in intensive cultivations with large-volume substrates, where dangerous "hot spots" can be generated that exceed the maximum temperature tolerable by the fungus, causing mycelial death and the onset of rot. For most mesophilic fungi, the optimal substrate temperature for colonization is around 24°C to 27°C. At these temperatures, the activity of lignocellulolytic enzymes (such as laccases and peroxidases) is maximum, allowing efficient degradation of lignin and cellulose.

Interaction between substrate temperature and substrate composition

The ideal temperature for colonization can vary slightly based on the nature of the substrate. Denser substrates with a high C/N ratio (carbon/nitrogen), such as wheat straw, tend to overheat more easily due to the greater microbial and fungal activity required for their degradation. In contrast, softer, more aerated substrates, like hardwood sawdust, dissipate heat more easily. An experienced grower adjusts the ambient temperature based on the type of substrate used: for a dense substrate, it might be necessary to maintain an ambient temperature 1-2°C lower than the theoretical optimum to compensate for the internally generated heat.

Tables of colonization temperatures for common substrates

The difference between the substrate temperature and the ambient temperature is evident, especially for fast-growing species like Pleurotus. For Shiitake, colonization is a very long process, and maintaining a constant temperature throughout the period is crucial to prevent premature and incomplete maturation of the block.

Incubation and maturation: the role of thermal gradients

The term "incubation" in mushroom cultivation often refers to the colonization period, but technically it also includes a subsequent phase of "maturation" or "consolidation" of the colonized block. In this phase, the mycelium, although having visually colonized the entire substrate, is not yet ready to fruit. It must complete its physiological maturation, accumulating reserves and triggering the genetic pathways that will lead to fruiting. Temperature plays a crucial role in this stage as well, often different from that of colonization.

Thermal differences between colonization and maturation

For many species, a slight lowering of the temperature is the environmental signal indicating the transition from the vegetative to the reproductive phase. This simulates, in nature, the arrival of a cooler season after summer. For example, for Agaricus bisporus, after colonizing the compost, the temperature is lowered from about 24°C to 16-18°C and a casing layer is applied. This thermal shock, along with other factors like increased CO2 and humidity, is fundamental for inducing primordia formation. Maintaining the temperature too high in this phase can lead to a "vegetative" mycelium that continues to grow in the casing without forming mushrooms, a phenomenon known as "overgrowth".

Fruiting: the thermal spectrum for sporocarp formation

Fruiting is the most spectacular moment in the mushroom's cycle, but also the most complex from an environmental management perspective. Temperature in this phase influences not only the speed of development but also the morphology, size, color, and even the nutritional composition of the sporocarp. Careful temperature management during fruiting is what separates a high-quality harvest from a poor one.

Primordia initiation temperature (pinning)

The formation of primordia, the small "pins" that will become mushrooms, is the most critical phase of the entire fruiting process. Each species has a very precise thermal window for initiation. For Pleurotus ostreatus, for example, initiation requires a thermal shock, with temperatures dropping sharply by 5-10°C, ideally towards 10-15°C. For Shiitake, initiation is often triggered by day/night temperature swings (thermal amplitude) of about 5-7°C, mimicking autumn conditions. For Champignon, as mentioned, a constant lowering to 16-18°C is sufficient.

Sporocarp development and maturation temperature

Once the primordia have formed, the temperature is generally raised slightly to favor stem elongation and cap opening. However, this development temperature is often lower than the colonization temperature. A temperature too high in this phase causes overly rapid development, with mushrooms of soft consistency, long thin stems, and small deformed caps. Conversely, a temperature too low excessively slows growth, increasing the risk of primordia abortion and parasitic attacks.

Tables of fruiting temperatures for main cultivated species

The table shows how thermal strategies for fruiting are very diverse. Shiitake benefits from a strong thermal amplitude, while Champignon and Hericium prefer a constant temperature. These differences are the result of millennia of adaptation to their natural habitats of origin.

Temperatures for harvesting and post-harvest storage

Temperature management does not end with the harvest of the mushroom. Mushrooms are extremely perishable products, with very high metabolic and respiratory activity even after detachment from the mycelium. The storage temperature directly influences the speed at which mushrooms lose weight (dehydration), brown (enzymatic activity), and develop degrading microorganisms.

The "cold chain" for the quality of the fresh product

Immediately after harvest, mushrooms should be brought as quickly as possible to their optimal storage temperature, which for most species is between 1°C and 4°C. At this temperature, respiratory activity is drastically reduced, as is the activity of polyphenol oxidase enzymes responsible for browning. Every hour of delay in cooling results in a significant loss of shelf-life and commercial quality. For particular species like Pleurotus, which are slightly more sensitive to cold, the temperature can be maintained at 4-6°C to avoid cold damage.

Temperature: always keep it under control

In conclusion, it is essential to emphasize that temperature never acts in isolation. Its effect is in continuous interaction with relative humidity, CO2 concentration, light intensity and quality, and air flow. An increase in temperature, with absolute humidity constant, decreases relative humidity, increasing the risk of dehydration for primordia.

Similarly, high temperature accelerates mycelial respiration, increasing CO2 production, which if not adequately removed can inhibit fruiting. The successful grower is therefore one who does not limit themselves to controlling individual parameters, but who understands and manages the ecosystem in its complexity, using temperature as one of the main tools to guide the fungus through its desired life cycle.