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Mushroom Production

Introduction

A large range of edible fungi are cultured. Agaricius bisporus is cultivated widely in western countries. In addition, Shii-take (Lentinus edodes), Straw mushroom (Flammulina velutipes), Oyster mushroom (Pleurotus ostreatus), and Chinese Black mushroom (Auricularia polytricha) are cultured in various Asian countries, and increasingly in western countries. The truffle industry is economically important in southern Europe and production is expanding in the southern hemisphere. Finally, edible mushrooms are picked from the wild, especially in Europe. LINK Many of the fungi are eaten fresh, but there is also a market for dried mushrooms and canned truffles.

Common or Button Mushroom

The identity of the fungi grown as commercial or button mushrooms in western countries is not altogether clear. The generally accepted name is Agaricus bisporus. However, breeding over the years appears to have resulted in the infusion of some genetic material from various sources.

The commercial mushroom is grown in a process that varies only slightly around the world. Essentially, straw is mixed with stable and poultry litter, mixed with appropriate minerals and then composted. LINK Spawn is added after the compost has cooled after the second phase. The compost is then laid out, usually in purpose built sheds in which the temperature is held at around 24 C. A layer of inert, alkaline material is added as a casing over the surface. Once mycelia reach the surface of the beds, the temperature of the shed is reduced to induce fruiting. Fruiting then proceeds over the next few weeks.

Commercial mushroom production is a highly intensive industry. Understandably, pests and diseases are a major problem. The principles of control do not differ from other intensive production systems.

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Shii-take and Log Culture

Shii-take is a highly prized mushroom in Japan. The range of variation in flavours and odours make this a valuable ingredient in the usually delicately flavoured Japanese dishes. The range comes from the genotype of the spawn, selection of logs used to grow the fungi, and conditions under which the logs are then incubated.  Shii-take is commonly grown on logs of deciduous trees. The logs are inoculated with spore suspension placed over the logs or by overgrowth from pre-inoculated fragments of timber. The base logs are not sterilised so the process has many potential hazards.

After extensive period of colonisation of the logs (laying), the induction of fruiting takes place when the logs are stood upright and the temperature drops. After a period of maturation, the temperature increases. Fruiting then takes place in spring, and continues for some years after.

Cropping takes place in the open, and traditional approaches continue to be used in Japan. Most of the crop is sold fresh, though some is dried, packaged and then sold throughout the year. The dried crop is most commonly exported from Japan. Artificial approaches to cropping have been developed. These use controlled temperatures, and selection of particular strains. In Australia, cropping more commonly uses these artificial environments and selected strains.

Auricularia and Pleurotis are also grown on timber logs. In Australia, Auricularia grows naturally on plum, fig and kurrajong trees, indicating a potentially huge potential host range for culture. Fruit bodies appear after periods of continuously wet and humid weather in autumn. Most Auricularia appears in shops in Australia in the dried form, often sourced out of China. The fungus grows on a range of trees and may be found after a period of warm and moist conditions.

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Culture on Organic Waste

Straw mushrooms are traditionally grown on straw from the rice paddy. The bundles of straw are drenched and then stood upright. The bundles are usually inoculated with spawn. Each bundle can produce fresh mushrooms for several weeks.

More recently, bundles of straw have been mixed with cotton or other organic waste, or cotton waste used exclusively. These alternatives often produce more mushrooms, probably due to the improved N:C ratio. LINK

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Ectomycorrhizal Fungi


Tuber melanosporum.

Fruiting bodies of a number of ectomycorrhizal fungi are consumed LINK The Black truffle (Tuber melanosporum) is perhaps the most highly prized of these fungi. Other truffle species of Tuber, Terfezia and Tirmania are also eaten. Field collections of morels (Morchella sp) and chantarelles (Cantharellus cibarius) are also highly prized. The latter fungi are found naturally in Australia, and sometimes appear in the shops in autumn.

Truffles have been harvested in European countries for many centuries. True Black truffles are found mainly in France, and other species of Tuber in France, Spain, Italy, many countries surrounding the Mediterranean sea, and China. The Black truffle is the most highly prized, because of its very strong flavour.

Culture of Black truffles is indirect, in that trees that support the fungi are managed with the aim of maximising the production of the fruit bodies. The soil of the truffiere tends to be alkaline, calcareous, and it must be well drained. Oak or Hazel trees are inoculated in the nursery. Saplings are planted out in the truffiere. The trees are then pruned to encourage lateral growth. After about seven (5 to 10) years, the fruit body appears in late winter to spring. The vegetation above the zone of fruiting tends to die off. Pigs or dogs have been used to detect ripe truffles, though more biochemical methods are being developed. The truffiere continues to produce abundantly for around 30 years.

In New Zealand and southern Australia, truffieres have been established. The inoculum and trees have been imported mostly from France. Initial harvests of truffles took place in New Zealand in about 1993, and in Australia in 1998. The truffles are being produced in the autumn of the northern hemisphere, and it is expected that large profits will ensue from the out-of season availability of this highly prized food.

Problems of production are legion. In Europe, many traditional truffieres are showing signs of reduced production. The reasons are unclear. In Australia, rotting truffles have been seen at the surface of compacted soils. The cause of compaction may be related to the level of intensity of production. Indeed, other ectomycorrhizal fungi are known to colonise the oak and hazel host trees in Australia. Further, the climatic conditions are not exactly the same in the southern and northern hemispheres. Thus we might expect selection of characteristics of fungi that benefit local conditions over time.

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Conclusion

A small but active market exists for edible fungi, in Australia for most fungi, and overseas for truffles. The systems to culture the fungi are well known and in most cases highly controlled. The product is nutritious and well regarded. That Australia has developed a culture where the product is purchased in shops is important. Unregulated picking from the field tends to reduce the crop, and among the Australian fungi are many that are toxic or hallucinogenic.

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References

Chang S-T & Miles PG (2004) Mushrooms: Cultivation, Nutritional Value, Medicinal Effect and Environmental Impact. 2nd ed. CRC Press.

Hall I., Brown G. & Byars J. (1994) The Black truffle. NZ Institute of Crop and Food Research, Wellington.

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