Pleurotus spp Oyster Mushrooms

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Unlike most of the cultivated mushrooms, which represent one species, a group of species of the genus Pleurotus are commercially cultivated and referred commonly as oyster mushrooms. Pleurotus ostreatus (Jack. ex.Fr.) Kummer, is best known species among oyster mushrooms and the specific epithet "oyster" obviously refers to its Oyster-shell like appearance of the fruitbodies. Pleurotus spp. are most versatile of all the mushrooms, representing about fifteen species capable of growing over a wide range of temperature (5°C to 30°C) and on almost all the lignocellulosic wastes; P. sajor-caju, P. florida, P. ostreatus, and P. flabellatus are most popular commercial species. It is a primary rot fungi and can degrade moistened substrates directly and does not require precomposted substrates like secondary rot fungus, e.g.,

A. bisporus. Ease with which oyster mushrooms can be grown has manifested itself in the production statistics where the production of oyster mushroom registered 442% increase during the period 1980-1991 (Chang 1993).

Production of the oyster mushroom also involves the main steps of mushroom growing described earlier: selection of the substrate and its pretreatment, spawn preparation, spawning, incubation for spawn-run, and providing conditions for fruiting of mushrooms, i.e. cropping. Oyster mushrooms can be grown on a large variety of lignocellulosic wastes depending upon the availability and cost (Poppe 2000) but the cereal straws (wheat and paddy) are the most common substrates in many countries (Figure 3); cottonseed hulls are also popular in the United States. Substrate pretreatment is mainly aimed at moistening and so-called pasteurization/ sterilization to give advantage to the mushroom mycelium and avoid contamination with moulds specially Trichoderma spp. Most commonly used pretreatments are hot water dip, pasteurization with steam, chemical pasteurization and steam sterilization (Jandaik 1997). Zadrazil and Dube (1992) have described a method for special substrate preparation for oyster mushroom. Pretreated substrate is mixed with grain spawn at 2 per cent by wet weight of the substrate and then filled in suitable containers, most commonly polybags. Bottle cultivation of the oyster mushroom is done in Japan. In some countries delayed-release nutrients, mostly formaldehyde-treated or polymer-coated soybean meal, is added in the substrate to increase the yield but the method involves risk of rise in bed temperature and contamination with moulds. Growth parameters for cultivation of Pleurotus differ from species to species, especially temperature requirement and general parameters and have been described by Jandaik (1997). There are wide variations among the growers about the method and style of opening of the containers for fruiting: complete removal of polycover, only top open like Japanese method of bottle cultivation, and slashes or holes in polycover. Average commercial yields obtained are 1 ton fresh weight of mushrooms per ton of dry weight of the substrate. One peculiar and serious problem with the cultivation of the oyster mushroom is spore allergy, which sometimes develops among workers for which facemasks are generally used during the operations.

6.2 Lentinula edodes (Shiitake)

Shiitake is the second most important commercial mushroom; it contributed 25.4% of total mushroom production in 1997 (Table 1). Of late, production of this mushroom has become very popular due to not only its unique taste and flavor but also its unique medicinal properties, such as antitumor, hyocholesterolemic, and antiviral properties; Lentinan, a polysaccharide, is now an established immunomodulator (Mizuno 1995); Though Japan is the leading producer of shiitake its cultivation first started in China near 1100 AD, and the technology was perhaps passed on to Japanese growers by the Chinese.

Cultivation technology of the L. edodes has been described in detail by various authors (Harris 1986; Royse 2001). Traditionally shiitake has been grown on natural logs of various species of trees but currently oak (Quercus) logs are most popular. The type, size, and quality of logs used have

Figure 3 Pleurotus florida on wheat straw blocks.

been described by Royse (2001). Generally, logs of 7-15 cm dia are cut into 1 m lengths. Holes are then drilled; one row of holes is drilled for each 2.5 cm of log diameter and are evenly spaced length-wise every 15 cm along the row. Holes are plugged with wood piece spawn or sawdust spawn, and then finally sealed with hot wax; plug spawn is however preferred for varied reasons. Spawn run may take 6-18 months which depends upon many factors. Logs, after the spawn-run, are transferred to a growing yard, which should be cooler and humid than the spawn-run area. One interesting treatment given to induce fruiting in logs is "shocking treatment" where logs are banged with a hammer or dropped on end (Chang and Miles 1989). Production is very good during the spring and fall. Some growers, however, use green houses for winter production when the prices are considerably higher. In the green house cultivation technology, logs are generally soaked in water and vibrated mechanically prior to keeping in the houses. After taking the first flush, logs are reincubated for about 3 months and the process is repeated up to five times. Yields obtained from log system may be as high as 33%; best production occurs in second and third years. Shiitake production drops and is no longer possible after the bark is lost.

To make the shiitake cultivation more environment-friendly, synthetic log production system was developed where sawdust is the main ingredient; however, straw and corncobs are also used. Basal ingredients are supplemented with some starchy substance like cereal brans, maize, and some chemicals like calcium carbonate, calcium sulfate

(gypsum), and sugar. Ingredients are mixed in a mixer and moistened to a level of 60%. Moistened substrate is generally filled in polypropylene bags (2.5 Kg). Little holes are either made in the polybags or special breather patch made of micro-porous plastic is preprovided. The bags are then sterilized for 2h at 121°C in large autoclaves and, after cooling, seeded with spawn. The bags after heat sealing are shaken to evenly distribute the spawn; sawdust spawn or cereal grain spawn is used in this system. Spawn-run at 21°C with 4h of light per day takes 18-23 days for optimum growth. Colonized blocks are taken out by slicing and peeling off the polycover and kept for 4 weeks in the environment conducive for browning of the exterior surface i.e., at temperature of about 19°C and 2000-3000 ppm CO2 and are watered once daily; and humidification may also be resorted to. As the browning process nears completion, pinheads start to form about 1 -2 mm beneath the surface. Primordia development is stimulated by soaking the blocks in cool water (12°C) for 3 -4 h; soaking is required for second and third flushes also. Mushrooms are ready for harvesting approximately after 7-11 days of soaking (Figure 4). Shiitake are harvested by gentle twisting by hand and stem cut with sharp knife like that for button mushroom. After harvesting, blocks are soaked again for 12 h, which may be 18 h in third soaking; flush breaks in shiitake are 16-20 days long. The total production cycle on synthetic logs is just 3 -4 months and biological efficiency achieved is also very high (75-125%) as against cycle of 5-6 years and B.E. of 33% in natural log cultivation (Royse 2001).

Mushroom Histology
Figure 4 Lentinula edodes on synthetic (sawdust) block (experimental crop).

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