When the conditions are right, mycelium takes control of the substrate, synthesizing its elements into new materials with natural tensile strength and other useful qualities. But before that happens, spores need to grow.
Upon first glance, a mushroom appears as an intricate web of root-like filaments. But mycelium is actually the primary “plant” portion of the organism, and a fruiting body represents less than 5% of its life cycle.
Substrate
In the wild, mushroom mycelium forms an underground network that doesn’t just feed the mushrooms growing on its surface. It also helps reduce competition from invasive species and supports the host plant’s immune system with unique compounds. The mycelium explores its surroundings and secretes digestive enzymes that break down dead organic material (or sometimes living organisms) into smaller parts that can be absorbed as nutrients for the entire mycelia community.
The fungi’s metabolic processes require oxygen and carbon dioxide, and mycelia transport these gases through their substrate. As a result, the structure of substrate is a critical factor for mushroom cultivation—it must be relatively dry to encourage hyphal growth and hydration but still have a compact and aerated structure that facilitates gas transport.
Depending on the species, different nutrients are required for optimal mycelium development and fruiting. For instance, straw mushrooms (V. volvacea) grow best in nutrient-enriched substrates that have high carbon to nitrogen ratios (C/N), while button mushrooms (A. bisporus) need less-enriched substrates with lower C/N ratios (around 20). Enriching the substrate with various minerals and supplements like gypsum, limestone, Epsom salt, powdered oyster shells, and manure is a common practice for improving the mycelium’s ability to absorb important elements.
Another essential feature of an adequate substrate is the granules’ size and shape. These properties influence transfer phenomena at the solid-solid, liquid-solid, and gaseous-solid interfaces and are of particular importance for enhancing mycelium’s hydration and colonization capabilities.
When the granules are too large, they interfere with hydration and prevent aeration. This is a problem that can easily occur with the use of sawdust, which is often used as an inoculation substrate in home mushroom cultivators’ experiments. When using larger sawdust granules, the time until colonization can increase significantly and contamination rates are higher. At Fungi Ally, we have found that using a mixture of sawdust and compost for our inoculation trials results in the most productive colonies. We find that inoculating 20 five-pound bags of shiitake spawn per 200 pounds of bulk substrate allows for colonization to occur within 8 weeks and does not increase contamination rates.
Hyphae
Fungi are a kingdom of organisms that produce threadlike filaments called hyphae. In a fungal mycelium, these filaments are interconnected in a dense network known as the thallus. These hyphae provide food and moisture to the fungus. They also serve to anchor the fungus in the soil, providing a solid base for its growth.
Mycelium has the ability to navigate around impenetrable objects such as a tree root, and it can also fuse together following complex morphogenetic programs to generate structures like perithecia, lichenous thalli or fruiting bodies (basidiocarps).
Unlike a plant root, hyphae do not increase in diameter, instead they branch repeatedly and form an intricate radially expanding network called the mycelium. This is what makes mycelium so useful to agriculture, as it is able to grow in all directions without needing to break the surface of the substrate, making it possible to grow in almost any condition.
Each individual hypha is interrupted at regular intervals by cross-walls called septa. These septa are formed when the protoplasm in a hypha is drawn backward from its growing tip and divided into new sections or cells. The process of splitting apart a hypha into multiple cells, and then returning those cells to the growing tip to divide again, is called coenocytic growth. Fungi that are coenocytic have a higher rate of nuclear division than fungi with single septate cells.
When a hyphae is in the process of splitting into cells, the walls of that cell are slightly permeable. This allows water to flow through the wall and across the cytoplasm, allowing the hypha to absorb nutrients from its surroundings.
To do this, a hypha secretes enzymes into its food source. These enzymes break down biological polymers into monomers that the fungus can absorb through facilitated diffusion and active transport. The hyphae then close the ecological energy cycle by absorbing these molecules into its mycelium, where they are converted to protein and other essential building blocks that the fungus uses to create new growth.
The mycelium is also important for nutrient recycling. It helps to decompose organic matter in the environment, transforming it into organic nutrients that can be absorbed by other organisms in the soil food web. The mycelium also helps to keep a healthy balance of carbon and nitrogen in the soil. Too little carbon will cause a fungal mycelium to stop growing, while too much nitrogen can lead to mycelial deterioration and delays the formation of mushroom caps.
Compost
In nature, fungi exist in a complex network of mycelia. These webs of fine, thread-like filaments extend outward from the mushroom fruiting bodies to pick up water and nutrients from their surroundings. Fungi can be found growing in soil, plant matter and wood. They also form webs in compost piles to help break down the cellulose and lignin rich material of old leaves, branches and straw.
The best material for mushroom cultivation is a mix of wood shavings and straw, sawdust, manure or animal waste. These organic materials provide the essential carbon to the mycelia and contain a rich supply of nutrients. The ideal mixture is slightly acidic to inhibit fungal decomposition and has a high moisture content, which encourages mycelial growth.
While it is important to have the right mix of ingredients for the mycelium to grow, it is equally critical to have the correct environmental conditions. The temperature must be constant and the humidity high. The pH should be between 5.5 and 6.8, and the nutrient content must be balanced. The mycelium needs enough nitrogen to make proteins that are essential to the development of structures and molecular tools, yet not so much that it is inhibitingly toxic. In addition, mycelial cells require adequate oxygen for respiration. Without oxygen, mycelia halt and contaminant organisms that are better adapted to anaerobic conditions take over, deplete the mycelium of energy and secrete substances that change the environment to the point the mycelium can no longer survive.
The compost used for cultivation must be uniform in density and texture. If too coarse or sandy, the mycelia cannot develop and the mushroom yield will be poor. The compost must be aerated, with forced air passing through a system of tunnels, bunkers or aerated floors. In a commercial operation, this is typically accomplished by using an in-floor aeration system with a plenum or series of nozzles. In a home-scale operation, a simple system of holes drilled into the compost floor or in the lid provides sufficient aeration and prevents contamination by contaminant organisms that can easily settle in a wet compost.
Mushrooms
As long as the environment is right, a mushroom can continue to grow and produce its spores. These spores are spread or dropped from the cap of the mushroom into a substrate, usually compost. The mushroom grows a small structure, called a primordium, on the substrate and then enlarges it into an egg-shaped form with a stem and gills that grow from beneath its cap. This stage is known as the button phase. The button is surrounded by a veil of mycelium that is sometimes referred to as the universal veil. The mushroom may remain in this state for hundreds of years, producing a ring of fruiting bodies as it expands underground.
Mushrooms are also able to communicate with each other via their mycelium networks. They can transfer carbon-based materials to trees and other plants in their vicinity, helping them to grow. They can even share nutrients with each other, helping them to survive in the face of drought and a lack of sunlight. In this way, researchers have likened the mycelial networks to a kind of underground internet and even a more-than-human consciousness.
A grower cultivates mushrooms by first sterilizing the substrate he is using to grow them. This can be done by heat (autoclave), pressure cooker, or chemical methods. After the substrate has been sterilized, he adds mushroom spawn to it — with a great example being with one example being found on the Gro Magik website. This is usually purchased from a company that produces it in a sterile lab to ensure it contains no outside contaminants. The spawn is then incubated. During the incubation process, the mycelium grows to cover all of the substrate. This is known as leap off, and it usually occurs about four days after inoculation.
Once the mycelium has completely covered all of the substrate, it can begin to fruit. This can be triggered by environmental conditions such as specific temperatures, carbon dioxide levels, or humidity. In addition to triggering fruiting, the mycelium can be pushed into a production cycle by placing the colonized substrate bags in a room that has high humidity and a drop in temperature.
The spores can then develop into mushrooms that grow from the rhizomorphs on the surface of the button. Once the mushrooms have formed, they can be eaten, and their spores can be used to start another crop of mushrooms.