Laying the logs
After inoculation, it is necessary to encourage the growth of the fungus through the log while discouraging weed fungi. Logs are laid side by side, propped up at a slant in a well-drained, shaded area with single logs placed crosswise between rows (Fig. 4). One may also want to cover the logs with a porous material such as burlap or straw mats to protect from excessive heating due to direct exposure to sunlight and to favour moisture retention while still allowing adequate ventilation and wetting during rainfall.
If excessive dehydration occurs, e.g., under 30 percent moisture content (dry weight basis), the logs should be watered. Growers usually learn how to determine if a log has enough moisture simply by hefting it. When logs are watered, they should be thoroughly soaked and then allowed to dry out for a few weeks between waterings. Continuous wet conditions favour surface contamination by weed fungi. If conditions are excessively hot and moist, the cover over the logs should be removed to promote surface drying. To encourage uniform water distribution, which promotes uniform growth, the logs should be turned (reverse the ends) every 2 to 4 months.
Optimum conditions in the laying yard are temperatures between 15° and 28°C (59° and 82°F) and a relative humidity of 80 to 85 percent. In practice, most failures in shiitake cultivation in Japan have been traced to incorrect conditions in the laying yard that favour competition from weed fungi.
Raising
Shiitake is capable of fruiting only after the fungus has completely colonized the log (1 to 2 yr.). At this time, a fuzzy white fungal growth can be seen at the cut ends of the log in the sapwood area (whitecolored wood near the log surface (Fig. 3)), especially just under the hark.
From this time on, conditions should be altered to favor fruiting. To fruit, the fungus requires abundant moisture, sufficient air movement, and shaded exposure to light. Fruiting is favored by cool temperatures, near 8° to 22°C (46° to 72OF). Cool nights followed by warm days and a constantly high relative humidity of at least 85 to 90 percent are optimal.
To provide these conditions and facilitate harvesting mushrooms, the logs should be uncovered and stacked in rows along boards in an upright position on well-drained, shaded ground (Fig. 5). Each log is separated by the width of another log placed on the opposite side of the board. This configuration creates rows from which mushrooms can be picked from either side. Fruiting occurs primarily in the wet, cool seasons — spring and autumn. Once shiitake begins to fruit on a log, it generally continues to do so during spring and autumn for an additional 3 to 7 years.
If a summer has been particularly dry, the logs may be too dry to support fall fruiting. For fruiting, log moisture content should be over 40 percent, the higher the better. To increase water content, overhead sprinklers can be used; or to conserve water, the logs can be soaked in a stream or tub of water for 1 to 3 days. In commercial production, dehydration followed by soaking in cool water (13° to 20°C, 55° to 70OF) is often used to stimulate fruiting. Logs that have become dehydrated usually produce bumper crops within a week of being soaked. Soaking also tends to eliminate certain insect pests. Logs in the raising yard should be turned, end for end, every 2 to 4 months to ensure even moisture distribution.
Surface contamination of logs in the raising yard may occur. Especially the older logs may become contaminated with a blue or green surface mould. Surface moulds are particularly damaging to mushroom cultivation because mushroom growth is prevented from starting at the log surface. To prevent the spread of surface moulds and other competitive fungi, any log found either badly contaminated (more than 10% of the log surface contaminated) or producing other mushroom species should be discarded immediately.
Logs that have lost their bark should also be discarded. The disposal site should be in a location separate from the cultivation site. Burying, or preferably burning, the contaminated logs is a simple and effective method to prevent the spread of competitive micro-organisms. A relatively dry log surface will help discourage growth and spread of surface moulds. Therefore, if logs are watered artificially, they should be watered thoroughly for a relatively short period, e.g., 1 to 3 days, followed by longer drier periods; e.g., 3 to 4 weeks. Light, frequent waterings should be avoided.
In Japan, fungicides or insecticides are occasionally used to kill surface contamination or insect pests. However, no fungicide or insecticide has yet been registered with the U.S. Environmental Protection Agency for this purpose. Indoor (e.g., greenhouse) cultivation of shiitake can be used to produce mushrooms in seasons other than spring and fall or to intensify mushroom production.
Generally, logs at the raising stage are placed indoors at 10° to 20OC (55° to 70°F). Prior to the time fruiting is desired, they are usually kept drier than normal. Fruiting is then stimulated by water soaking and maintaining a constantly high relative humidity as described earlier. This procedure can be repeated as often as every 2 to 3 months.
Shiitake requires light to fruit. However, the light requirement is relatively low. If a greenhouse is used, the glass/plastic should be shaded. If shiitake is grown in an otherwise dark chamber, lighting to provide approximately 30 foot-candles of light must be used to ensure optimal fruiting. Increasing the light intensity over this level probably will not give any further improvement. A light/dark cycle (for instance, 9 hr. of light per day) may be preferable to continuous lighting.
Artificial light may be from fluorescent bulbs (including plant-growth bulbs) or tungsten filament bulbs (e.g., approximately two 40-watt fluorescent bulbs or two 100- watt tungsten filament bulbs at a distance of 2 to 3 meters (6 to 10 ft.). When the cultivation method is optimal, mushroom yields are high. One hundred pounds of logs will yield as much as 9 to 35 pounds of fresh mushrooms over a 4- to 6- year production period. Because the fresh mushrooms usually contain 90 percent moisture and bed logs are approximately 50 percent moisture, optimal yields on a dry weight basis can be 2.5 to 10.5 percent.
Author: Gary F. Leatham. The author is a Research Microbiologist, USDA Forest Serv., Forest Prod. Lab:, P.O. Box 5130, Madison, WI 53705. The Laboratory is maintained in cooperation with the. Univ. of Wisconsin, Madison.
