Shiitake mushrooms growing on an oak log, the most common wood used as a substrate in production of this species.
Image: Justin O'Dea, Washington State University
Background
Shiitake is one of the most-cultivated species of edible mushrooms worldwide, and in a given year may constitute the greatest volume of all mushrooms produced globally. China produces the vast majority of shiitake worldwide, while production in the US is relatively low compared to Asian countries. Shiitake cultivation was referenced in China as early as the 12th century, but larger-scale production did not begin until the mid-1940s in Japan, beginning the country’s association with shiitake culture.
Biology & Ecology
Shiitake is native to east Asia and naturally occurs as a saprotrophic, white rot fungus of the wood of broadleaf trees. The species has a natural affinity for the wood of chinquapin (Castanopsis) species and other genera of the beech (Fagaceae) family from which the common name is derived. The name “shiitake” (pronounced shee-tah-keh) comes from Japanese word consisting of chinquapin or shii (椎) and take (茸) which denotes it as a mushroom of the wood of that species.
Mycelial growth of the shiitake fungus occurs between 5-32°C but this varies by strain; optimal growth generally falls between 22-27°C. Temperatures above 35°C, can be damaging or lethal; likewise, shiitake growth has been observed to cease when moisture content of its substrate falls below 24% (fresh weight basis). Optimal substrate moisture content for shiitake growth generally falls between 40-48%, after which growth rates begin to be attenuated in accordance with moisture levels restricting gas exchange otherwise needed for metabolic processes. Shiitake requires low levels of light to fruit, although mushrooms will grow without light once fruiting begins. Shiitake’s natural fruiting pattern is autumn and spring-oriented. Fruiting is primarily induced by shifts in temperature and wetting-drying cycles; this is one of the key biological characteristics that has allowed shiitake to become a widely cultivated species. Fruiting may occur anywhere between 5-25°C depending on strain, but generally occurs within a range of 10°C for a given strain. Strains developed for fruiting in warmer temperatures tend to be more aggressive decomposers and dependent on pronounced shifts in temperature and wetting-drying cycles to induce fruiting. More recently studies have substantiated observations that shiitake fruiting can also be positively affected by agitating the colonized substrate with physical force or electricity.
Shiitake mushrooms are versatile in thier use. They have a long shelf life as a fresh product but can also be easily dried for long term storage and to strengthen thier flavor and aroma.
Characteristics
Shiitake is an adaptable species that has been developed to yield well and produce reliably. Strains have been developed to produce in different types of production systems with defined production metrics that help make mushroom yield and quality control more predictable. Beyond shiitake’s cultivation appeal it can also be dried easily for long-term storage, maintain considerable shelf life as a fresh product, and ability to satisfy a wide range of culinary, nutritional, and medicinal markets. In culinary applications, it's use is versatile as well with a distinct but agreeable flavor, aroma, and texture. It is also highly nutritious, and not known to incite many adverse reactions (allergies etc.) when eaten, with the exception of a small percentage of the population that can develop shiitake flagellate dermatitis in reaction to the polysaccharide lentinan if shiitakes are eaten raw or undercooked.
Cultivation
Traditional larger-scale production of shiitake has been on small-diameter logs which are inoculated with shiitake spawn on hardwood dowels or a sawdust that is inserted into holes drilled into the sapwood ~3-8 weeks after felling during the dormant season, depending on species and timing of cutting. Logs are primarily sourced from Quercus, Castanopsis, and Carpinus spp. that are ~8-20 cm diameter and cut to ~120 cm lengths. Shiitake harvests from inoculated logs typically begins 1-2 years later after the spawn run of a given strain of shiitake has completed within the log; mushrooms then generally begin producing 2-3 harvests per year for 3-5 years, and are grown under forest canopy.
Shiitake is still produced in traditional outdoor systems, although indoor production on hardwood sawdust-based substrates of various forms has become the dominant method for producing shiitake as controlled-environment mushroom growing facilities and systems have become more common worldwide. Indoor production has effectively broadened the geographic range of where shiitake can be produced, provided there is ample, suitable sources of substrate nearby. As long as growers have sufficient means and/or access to indoor growing facilities, shiitakes can be produced more rapidly and in greater quantities than from traditional systems, and add additional opportunities for mushroom quality control through the controlled environment and custom substrate formulations.
Timber requirements for shiitake logs in traditional production are relatively low compared to timber harvests for wood products; they are often consonant with forest management plans where the logs are cut from forest thinnings, or are 2) sourced on-site by farms with forestland otherwise unused for producing other forest products. Production space varies by farm size but utilizes ~20-25 m-2 of space under forest canopy per 100 logs. Substrates for indoor shiitake production often utilize byproducts of timber and agricultural industries. These can sometimes be abundant and resource-use efficient as long as these industries are stable enough to minimize fluctuations in price and supply. Grain is sometimes used as a component of shiitake substrate, but as a non-byproduct, grain is vulnerable to becoming unaffordable if price premiums rise.
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Traditional production of shiitake on logs can either be outdoors, or in shaded hoophouses. The species has a native affinity for the wood of trees in the beech family, but it can be produced on the wood of a wide range of different broadleaf tree species; the image shows shiitakes growing on logs from gray birch, red alder, sweet cherry, and bigleaf maple. Log-grown shiitake production systems are relatively accessible requiring relatively low capital and equipment needs at startup, but can be labor intensive and limited in production capacity and mushroom quality control compared to indoor production systems. Image: Justin O'Dea, Washington State University |
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Growing shiitake in indoor controlled-environments on sawdust-based substrates has become the dominant method of production worldwide. Indoor production has increased the versatility of where and how shiitake can be produced, along with additional quality controls, mechanization, and increased output volume, but also requires a considerable financial investment in the production facility and increased need for strict sanitation measures in production. Image: Justin O'Dea, Washington State University |
Market & Economics
A postcard from Morotsuka, Miyazaki Prefecture, Japan promoting the locally-produced shiitake, it's history in the region, and and it's place within Japanese culture.
In many Asian countries, there is a reliably high, consistent demand for shiitake due to it's use in many asian cuisines, and historic presence within certain cultures. For instance, in Japan, dried shiitakes are also commonly given as gifts. Currently there is much research emphasis on further elucidating nutraceutical benefits of shiitake mushrooms, which may further bolster market demand. Nonetheless, mass production of shiitake can inadvertently cause market prices to be vulnerable to the effects of overproduction, and render shiitake production economically unsustainable for growers, depending on their production circumstances.
Traditional shiitake production on logs has largely been declining due to market flooding from indoor produced shiitake in regions where substrate and labor costs are relatively low. This production method is generally difficult to scale-up because certain production aspects are not easily mechanized, but it nonetheless often an accessible production option for beginning and smaller scale growers. Mushroom quality from outdoor shiitake production systems on logs is being studied to help ascertain if there are certain favorable characteristics that would substantiate market differentiation for forest-grown shiitake. These producers may additionally need to differentiate in other ways to remain economically sustainable, either through value-adding with innovative marketing, processing, and/or focusing more on more direct markets. This may help keep traditional shiitake production systems viable for 1) small-scale growers lacking indoor production facilities and/or 2) for farms that are otherwise diversified enough to buffer against fluctuations in shiitake profitability, but the sustainability of these traditional production systems within commodity markets is likely to remain tenuous in the long-term.
Indoor shiitake production systems are likely to continue to increase due to the increased yield potential and versatility that these systems provide in how and where shiitake can be produced. Indoor shiitake systems have been developed to accommodate other substrates which can augment primary hardwood substrates, along with nutrient supplements. Indoor shiitake production nonetheless is dependent on growers having the means to invest in an indoor production facility, equipment, and infrastructure, which often requires considerable up-front capital and/or leveraging other held assets to support the investment. Indoor production also depends on an adequate and economical supply of shiitake substrates and other resources needed to maintain production levels capable of producing a positive return on investment.
Issues
Some of the most important production issues related to shiitake are contamination of substrates with other fungi or bacteria. Traditional shiitake producers mainly focus on retaining the log’s bark integrity, inoculation timing, and the production site and layout to mitigate effects of competing fungi, pests, and pathogens. The most economically important and widespread pathogen of cultivated shiitake in any production system are Trichoderma spp., commonly referred to as green mold. Other pest species reported to affect shiitake in Japanese outdoor production are Trametes species, Corticum species, Diatrype species, Hypoxylon species, Bulgaria inquinans, Graphostroma platystomum, Schizophyllum commune, Pycnoporus coccineus, Porodisculus pendulus, Hypocrea peltata, Porostereum crassum, and Merulius tremellosus. Various bacteria, particularly Peudomonas spp., may also become pathogenic in indoor shiitake production. Because indoor shiitake production is a highly concentrated production system, these systems can be vulnerable to rapid spread of pathogens or competing fungi that take advantage of the host abundance and/or optimized production environment. Indoor facilities therefore mitigate this risk through sterilization of substrates, use sterile environments during inoculation, and use strict sanitation measures throughout the production process.
Other production related issues noted for in Japan on logs is production season extension, particularly during summer when high temperatures commonly preclude fruiting. In some areas of Japan also oak log substrates may be limited. Shiitake strain access can also be an issue for producers. Shiitake strain development is constrained by a lengthy timeline and investment, developed lines are often kept proprietary by industry and/or governmental policy. This aspect can be challenging for growers, and make the strain development timeline asynchronous with the response timeline desired by growers to meet their current needs. Generally there is an abundance of research exploring indoor shiitake production on different locally-available substrates so that the species can be produced in a wide variety of locations worldwide.