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Shiitake Mushroom Production:Obtaining Spawn, Obtaining and Preparing Logs, and I

by Stephen M. Bratkovich    

In nature, the shiitake fungus propagates and spreads from spores produced by the mushroom. However, for cultivation, spore germination is too unreliable. Instead, logs are inoculated with actively growing fungus. The fungus is first adapted to wood by growing it directly on small pieces of wood. Active fungal cultures intended as inoculum for mushroom cultivation are called spawn. Because the quality of the crop can be no better than the spawn, growers must use viable shiitake spawn of a good variety in pure culture, free of weed fungi and bacteria.

Spawn Strain Characteristics

Different cultivars or strains of shiitake may perform differently under different conditions. Initially, growers should try more than one strain to ensure success. Also, growers can extend the growing season by using strains that fruit under different environmental conditions. For example, a grower in southern Ohio could use a cold-weather strain for spring and fall production, and a heat-tolerant strain for summer production.

The following strain characteristics need to be considered when ordering spawn from commercial suppliers:

• Preference for type of wood
• Resistance to weed fungi
• Speed of colonization (time of first fruiting)
• Ease of fruiting
• Season of fruiting
• Ability to stimulate (force) fruiting
• Required temperature for fruiting
• Size, shape, color and flavor of mushrooms
• Mushroom storage characteristics

The individual grower will need to experiment with different strains and decide which work best in a particular situation.

Sources of Spawn

There are numerous spawn suppliers throughout the United States and Canada. In addition to experimenting with different strains, growers are encouraged to purchase spawn from more than one spawn supplier. Many spawn suppliers also sell equipment and supplies related to mushroom growing; product catalogs are often available from them upon request. Fact Sheet F-39 Shiitake Mushroom Production: Introduction and Sources of Information and Supplies, includes an up-to-date list of spawn suppliers. It is available from your county office of Ohio State University Extension.

Forms of Spawn

Shiitake spawn can be purchased on two mediums: sawdust and wood dowels. It is usually supplied in either sealed plastic or glass containers. The spawn should be moist, white (sometimes with a brown crust) and appear rather fuzzy. Good quality spawn smells mushroom-like, not mildewy or mold-like. Weed fungi and bacteria are controlled by not damaging or opening the spawn container until use of the entire contents.

Spawn Storage

Spawn must be kept away from direct sunlight and temperature extremes. Storage for a month or more should be in a cool (34-38 F) location away from direct sunlight. Spawn must not be frozen. Prior to inoculation spawn should be warmed to room temperature (70 F) for two to five days.

Obtaining and Preparing the Logs

Trees cut for shiitake mushroom production should be harvested as part of an overall forest management plan. Individuals interested in producing shiitake mushrooms from their woodlot should contact a forester for assistance in selecting the appropriate trees.

Suitable Tree Species

The hardwood tree family most recommended in the United States for shiitake cultivation is the beech family (Fagaceae). The particular genus most successful in this family is Quercus (oak). All oak trees can be used with the possible exception of live oak. The thicker bark oaks such as white and chestnut oak are often preferred over the thinner bark red, scarlet and pin oak.

Beech, birch, chestnut, chinkapin, alder, maple, cottonwood, willow, aspen, poplar, elm and hophornbeam are suitable species but may have commercial limitations. As a rule, the thin-barked low-density species provide relatively quick mushroom production but only for a short time period. Locust, walnut and all conifers are not suitable for shiitake cultivation.

Sweet gum (Liquidambar styraciflua) and sycamore (Platanus occidentalis) have outperformed oak species in selected trials in North Carolina. Other species may prove to be excellent for shiitake cultivation once testing is completed.

Tree Quality

Logs used for shiitake production must be cut from live, healthy trees. Living trees with obvious insect or disease damage should not be used.

Optimum log size is 4-8 inches in diameter and 3-4 feet in length; standard lengths make operations much more convenient. Logs with a thick sapwood layer and small heartwood area are preferred. Logs can be cut from young hardwood trees or branches of older trees.

Logs should be straight for ease in handling but crooked logs can be used. A smooth bark will make the inoculation (seeding) process easier but thin bark tends to crack and peel sooner than thick bark. Regardless of thickness, the bark must be intact on the log.

Tree Felling

Some authors suggest the optimum felling time is when 30 to 80 percent of the tree leaves on the chosen species have changed color. Others suggest tree felling during the coldest time of winter. One reason for the different opinions on tree felling is due to the different theories on the best time to inoculate (seed the logs). Most shiitake growers and researchers agree, however, that trees should be felled sometime during the dormant season (mid-autumn to late winter), before spring sap movement and bud swell.

Logs should not be used for shiitake production if they were cut from felled trees that seasoned during the summer months. For example, trees cut prior to the summer months (even if cut while dormant) should not be used after being exposed to the warm weather conditions of summer.

Log Preparation

After tree felling, logs need to be prepared for inoculation. Regardless of the method of log preparation, two areas need careful attention: the moisture content of the log needs to be maintained above 35 percent and potential log contaminants must be minimized.

One method of reducing moisture loss is to keep logs in whole tree lengths and cut to final log size prior to inoculation. Rain and snowfall can be permitted to wet the logs. If trees must be immediately cut to final log length, logs should be protected from drying winds and direct sunlight by covering with burlap, shade cloth or plastic. If possible, logs should be stacked firewood-style in full shade under conifers. Watering or soaking logs is recommended several days before inoculation if the moisture content drops below 35 percent. Log surfaces should be allowed to dry prior to inoculation.

Log contaminants (insects, diseases, etc.) can be reduced by storing the logs off the ground. Tree length logs can have the butt supported on the stump. Logs cut to length can be placed on pipe, concrete blocks or other suitable material that will keep them off the ground. If stacking logs in contact with the ground, select a well-drained site with good air circulation and use cull logs as supports.

When logs are cut to final length, plan all cuts to give the most good logs. All diseased and wounded sections, forks, crotches and major kinks should be cut out. Small branches should be removed from logs, leaving a stump of approximately two inches. A wire brush can be used to remove lichens and moss from the bark prior to inoculation.

Inoculation

Inoculation is the introduction of the live shiitake fungus into the log. A one-time inoculation will produce mushrooms after 6-18 months and will continue to produce for 3-6 years.

In the past, logs were normally cured after felling for at least two weeks before inoculation. However, many researchers and spawn suppliers are now recommending inoculation as soon as possible after felling. Also, early spring inoculation is now being replaced by fall and spring inoculation. Inoculation should always be done in a shaded area to avoid direct exposure of the spawn to sunlight.

Logs should be watered if the internal log moisture content drops below 35 percent prior to inoculation. Occasional thorough waterings are better than frequent light waterings. The former will increase the internal log moisture content while the latter often just wets the bark surface.

Personnel, Equipment and Supplies Needed

While the entire inoculation process can be done by one person, a minimum of three is suggested: one person to drill holes, one to place the spawn in the holes and one to seal the inoculation sites. A fourth person can be useful in moving logs from one work station to the next.

In addition to the spawn, equipment and supplies needed for inoculation are: drill (preferably high speed if many logs are to be inoculated), bits, work table or saw buck, yard stick or measuring tape, hammer (for dowel spawn), inoculation tool (optional, for sawdust spawn), paraffin/cheese wax/plastic foam (for sealing inoculation sites), heat source (for melting wax), wax dropper or brush (for applying wax), and rubbing alcohol.

Inoculation Procedure

Logs to be drilled should be secured in a saw buck or similar arrangement to prevent the logs from moving (Figure 1). The log bark should be free of dirt and other possible contaminants. Dip the drill bit in rubbing alcohol after finishing each log as a precaution.

The hole-drilling pattern will vary from grower to grower. A general recommendation is to space holes 6-16 inches within rows and 2-4 inches between rows since the shiitake mycelium runs well with the grain but poorly across the grain. Adjacent rows should be offset from one another to create a diamond pattern on the log (Figure 2).

Depth and diameter of the holes will depend on the source as well as the form of spawn. Hole depth generally ranges between 3/4 and 11/4 inches with hole diameter between 1/4 and 1/2 inch. Most spawn suppliers will recommend dimensions for hole size. After a log has been drilled, holes should be filled immediately so as not to lose moisture or permit entry of airborne spores.

Hands should be washed and then wiped with rubbing alcohol before handling spawn. On a work table or second saw buck, dowel spawn can be placed into the holes and then gently pounded in with a hammer. A convenient method is to initially hold the dowels with forceps. Sawdust spawn can either be inserted manually or with an inoculation tool available from many spawn suppliers. Disagreement exists as to whether the sawdust spawn should be packed tightly in the hole or just lightly tamped in. Growers are advised to follow the spawn supplier's recommendations.

The final step in the inoculation process is to seal the spawn-filled hole with either a paraffin or cheese wax or styrofoam plug. Holes are sealed to prevent loss of moisture, to prevent contamination by undesirable microorganisms and to allow the spawn to grow within the confines of the log. Hot wax also tends to disinfect the inoculation surface. Melted wax can be applied by brush or wax dropper (similar to a turkey baster). Styrofoam plugs are placed on top of the spawn, flush with the surface of the bark.

Suggested Practices After Inoculation

All inoculated logs should be coded to record important information such as spawn strain, tree species, etc. Small aluminum tags fastened to log ends with a staple work well for this purpose. Good record keeping will enable growers to duplicate successful practices by learning from past experiences.

Inoculated logs may be dead piled (firewood style) and shaded with plastic immediately following inoculation. If the log moisture content is low, burlap or similar material should be used to allow rain to reach the logs. If surface molds develop, logs should be moved from temporary to a permanent laying position.

References

Cotter, V. T., et al. (1987). Shiitake farming in Virginia (Pub. 438-012). Blacksburg, VA: Virginia Tech and Virginia State University, Virginia Cooperative Extension Service.

Haney, A. (1989). Mushrooming forest profits. In Shiitake Mushrooms: Proceedings of a National Symposium and Trade Show (pp. 57-61). St. Paul, MN: University of Minnesota, Educational Development System, 405 Coffey Hall.

Harris, B. (1986). Growing shiitake commercially. Madison, WI: Science Tech Publishers.

Kerrigan, R. (1982). Is shiitake farming for you? South San Francisco, CA: Far West Fungi.

Kuo, D., and Kuo, M. (1983). How to grow forest mushroom (shiitake). Naperville, IL: Mushroom Technology Corp.

Leatham, G. F. (1982). Cultivation of shiitake, the Japanese forest mushroom, on logs: A potential industry for the United States. Forest Products Journal, 32(8), 29-35.

Figure 1. Log drilling in a saw buck. Ohio State University Extension

Figure 2. Pattern to guide the placement of holes (inoculation sites) across the log surface.


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