Shiitake Mushroom Production: Inoculating Logs with Spawn (2024)

In nature, the shiitake fungus propagates and spreads from spores produced on the gills of the mushroom. While propagating mushrooms from spores is possible, spore germination may produce unpredictable strains. A technique more often used is propagation through spawn.

Obtaining Spawn

Spawn are actively growing mycelia that retain the genetics of the contributing fungus and are intended as inoculum for mushroom cultivation. Spawn are produced by inoculating media such as grains or sawdust with pieces of mycelium from a fungal culture. For log production, use sawdust, plug, or thimble spawn—avoid grain spawn, which is intended for indoor cultivation on sterilized bulk substrates. Because the quality of the crop can be no better than the spawn, growers must use viable shiitake spawn of a good strain in a pure culture that is free of weed fungi and bacteria.

Spawn Strain Selection

A culture contains a desirable strain, or cultivar, that produces mushrooms with certain characteristics. Different strains of shiitake perform differently, depending on the conditions they are grown in. For example, cold-weather strains develop well on sugar maples, while warm-weather strains, compared to the cold-weather strains, do better on red maples (Mudge et al. 2013). Initially, growers should experiment with different strains to ensure selection of strains that will be successful. Everyone’s selection will require fine-tuning during the production process. 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.

Cold-weather strains: These mushrooms fruit in the early spring and late fall and require a longer colonization period (known as a spawn run).

Warm-weather strains: Fruiting occurs in the summer and early fall months.

Wide-range strains: As the name suggests, these strains fruit at a wide range of temperatures. These mushrooms are quick to colonize and fruit compared to other strains.

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 (how long it takes a fungus to grow throughout a log and initiate growth of the first flush of mushrooms)
• ease of fruiting
• season of fruiting (cold-weather, warm-weather, and wide-range strains)
• ability to stimulate (force) fruiting
• required temperature for fruiting
• size, shape, color, and flavor of mushrooms
• mushroom storage characteristics

Sources of Spawn

There are numerous spawn suppliers throughout the United States. 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 cultivation; product catalogs can often be found on suppliers’ websites or requested. Fact sheet "Shiitake Mushroom Production: Steps to Cultivation and Considerations for Production"(F-0039) includes an up-to-date list of spawn suppliers and is available at ohioline.osu.edu, or you can request a copy from your county Extension office.

Forms of Spawn Used in Log Production

Shiitake spawn can be purchased on several types of media: sawdust, wood dowels, and thimble spawn. It is usually supplied in autoclaved plastic bags with filters that allow for gas exchange.

When spawn is received, it should be inspected for contaminants. Spawn should be moist, white (sometimes with a brown crust) and appear fuzzy. Good quality spawn should smell like mushrooms, not mildew or mold. Weed fungi and bacteria are controlled by not damaging or opening the spawn container until just prior to use of the contents. Once opened, the risk for contamination increases.

Spawn Storage

Spawn must be kept away from direct sunlight and temperature extremes. Spawn, depending on the type, should keep from one to six months and refrigerated at about 40 degrees Fahrenheit. Thimble spawn keeps for shorter periods, and dowel spawn keeps for longer periods—check the supplier’s website for strain-specific directions for storage. Spawn must not be stored in the freezer. Prior to inoculation, spawn can be left at room temperature following supplier’s instructions (usually two to five days in advance).

Obtaining and Preparing Logs for Inoculation

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. For more information on forest management, read fact sheet "Forest Management"(F-34), available at ohioline.osu.edu.

Suitable Tree Species

The hardwood tree family most recommended in the United States for shiitake cultivation is beech (fa*gaceae). The particular genus most successful in this family is Quercus (oak). Most oak trees can be used in shiitake production. The thicker bark of white oaks are more often used compared to red species that produce thinner bark. However, with the decline of white oak in Ohio and its increased timber value, other species may be preferred when grown as part of forest management plan. Sweet gum (Liquidambar styraciflua) and sycamore (Platanus occidentalis) did well in selected trials and may be good alternatives to oaks. Growers also report success with sugar maple (Acer saccharum), beech (fa*gus grandifolia), and American hornbeam (Carpinus caroliniana). That said, white oaks removed as part of forest management are ideal candidates for shiitake log production.

As a rule, thin-barked, low-density hardwood tree species provide relatively quick fruitings of mushrooms but only for a short period of time. Thin-barked logs tend to lose moisture quickly. Locust, poplar, red maple (except with warm-weather strains), ash, walnut, fruit wood, and all conifers are not suitable for shiitake cultivation.

Tree Quality

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

Optimum log size is 4–6 inches in diameter (although larger sizes may be used) and 3–4 feet in length (easy to transport); standard lengths make operations much more convenient. Logs with a thick sapwood layer and small heartwood area are preferred. Keep in mind that shiitake fungi grow best on logs with high-density wood.

Straight logs are easier to handle, but crooked logs can also be used. A smooth bark will make the inoculation 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. Cracks in the bark allow for moisture loss. Moisture is critical to colonization. Moisture loss can decrease the rates of successful inoculations.

Tree Felling

Research suggests that winter and early spring prior to leaf out is the best time to fell trees for inoculation. Trees felled earlier in the winter months can be stored for several months and still produce acceptable yields. Most shiitake growers and researchers agree that trees should be felled sometime during the dormant season before spring sap movement and bud swell occurs.

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. Trees felled during the summer and fall months produce less dry weight of mushrooms at harvest compared to other times of the year (Mudge et al. 2013).

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, cutting them to final log size just 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 them with burlap, shade cloth, or plastic. If possible, logs should be stacked firewood-style in full shade under conifers, since conifers retain needles year-round and minimize the sunlight that reaches logs. Watering or soaking logs is recommended several days before inoculation if the moisture content of the logs drops below 35%. 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 be stored by supporting the butt of the log on the stump. Logs cut to length can be placed on pipe, concrete blocks, wood pallets, or other suitable material that keeps them off the ground. If logs are stacked in contact with the ground, select a well-drained site with good air circulation and use culled logs as supports.

When logs are cut to final length, plan all cuts to ensure you have the maximum number of suitable logs that are easily inoculated. All diseased and wounded sections, forks, crotches, and major kinks should be cut out. Small branches should be removed from the logs, leaving a stump of approximately two inches. Remember to coat wounds (except the log ends) in wax after inoculation to seal in moisture. A wire brush can be used to remove lichens and moss from the bark prior to inoculation, but for thin-barked trees, use care not to damage the bark.

Inoculation

Inoculation is the introduction of the live shiitake spawn into the log. A one-time inoculation will produce mushrooms after six months to two years and will continue to produce for three to four years, depending on the strains used and the conditions for the spawn run.

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 a day after the tree is felled, up to a maximum of three weeks. The exception is trees that are felled during the winter months, which can be stored longer. Winter and spring inoculations are ideal, and inoculation should always be done in a shaded area to avoid direct exposure of the spawn to sunlight, and to minimize moisture loss.

Logs should be watered if their internal moisture content drops below 35% prior to inoculation. Occasional thorough waterings are better than frequent light waterings. The former increases 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 people is suggested: one 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 workstation to the next.

In addition to the spawn, equipment and supplies are needed for inoculation:

• drill or angle grinder (preferably high speed if many logs are to be inoculated)
• bits and stop collars
• worktable 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)
• rubbing alcohol

Inoculation Procedure

Logs to be drilled should be secured in a saw buck or similar device to prevent the logs from moving. The log bark should be free of dirt and other possible contaminants. Dip the drill bit in isopropyl or rubbing alcohol after finishing each log (or even in between each row if possible) to minimize spread of weed fungi and bacteria.

The hole-drilling pattern varies from grower to grower. A general recommendation is to space holes 3–4 inches between rows and 3 inches apart moving down the rows, because the shiitake mycelium runs well with the grain but poorly across the grain. The closer the holes are, the quicker colonization occurs, and the quicker the shiitake fungus can outcompete weed fungi and bacteria. However, holes located further apart, which require less spawn, can also be used. Adjacent rows should be offset from one another to create a diamond pattern on the log.

Depth and diameter of the holes depends on the source as well as the form of spawn. Hole depth is approximately 1¼ inches with a hole diameter between ¼ and ½ inch. Most spawn suppliers 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 wiped with rubbing alcohol before handling spawn. With the log on a worktable or second saw buck, the 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. Each hole should be packed with spawn. 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 paraffin or cheese wax. Thimble spawn do not require sealing. Holes are sealed to prevent loss of moisture, limit contamination by undesirable microorganisms, and 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, which is similar to a turkey baster.

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 enables 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.

Knowing when to conduct a maintenance soak to prevent logs from becoming too dry is important. However, many moisture meters do not provide a reliable reading for shiitake logs. Manually measuring the moisture in a log takes time that many commercial growers do not have. Decisions on when to soak logs are based on the tree species used, log age, and site conditions including weather history. This information is often gained with experience in the field (Mudge et al. 2013).

Additional Resources

Frey, Gregory E., Tarik Durmus, Erin O. Sills, Fikret Isik, and Marcus M. Comer. 2020. “Potential Alternative Tree Species as Substrates for Forest Farming of Log-grown Shiitake Mushrooms in the Southeastern United States.” HortTechnology, 30 (6): 741–744.
journals.ashs.org/horttech/view/journals/horttech/30/6/article-p741.xml.

Hill, Deborah B., and Marcella Szymanski. 2010. “Kentucky Shiitake Production Workbook.” FOR-81. Lexington: University of Kentucky Cooperative Extension Service. PDF
ca.uky.edu/agcomm/pubs/for/for81/for81.pdf.

References

Mudge, Ken, Allen Matthews, Ben Waterman, Bridgett (Jamison) Hilshey, Steve Siergik, Nick Laskovski, Steve and Julia Rockcastle, and Steve Gabriel. 2013. Best Management Practices for Log-Based Shiitake Cultivation in the Northeastern United States. Northeast Sustainable Agriculture Research and Education. Burlington, VT: The University of Vermont.
projects.sare.org/wp-content/uploads/977907shiitake-bmp-3-11-final-for-print-hires.pdf.