Growing shiitakes, whether for home or commercia use, requires problem-solving. Some problems are strictly environmental; others include pests and pathogens. Many pest and pathogen issues can be traced back to inoculating logs that are not freshly cut. The more time logs sit prior to inoculation, the greater the risk of contamination from weed fungi that can outcompete the inoculated species during colonization. Using fresh-cut logs from the appropriate tree species can help mitigate problems with colonization and fruiting. Refer to Ohioline fact sheets F-0039 Shiitake Mushroom Production: Steps to Cultivation and Considerations for Production, F-0040-Shiitake Mushroom Production: Inoculating Logs with Spawn, F-0041-Shiitake Mushroom Production: Logs and Laying Yards, and F-0042-Shiitake Mushroom Production: Fruiting, Harvesting, and Storage that cover the best practices for growing shiitakes.
Shiitake growers encounter many common issues:
- poor-quality mushrooms
- malformed mushrooms
- low mushroom yields
- no mushrooms produced on logs
- growth of mushrooms other than shiitake on inoculated logs
- damage from insects and other wildlife
Development of Poor-Quality or Malformed Mushrooms
A variety of factors can contribute to poor-quality or malformed mushrooms. Often these can be tied to environmental conditions during fungus growth and development, such as moisture management, access to sunlight, or air exchange.
Did It Rain During Fruiting?
Overwatering or exposure to excess rainfall during fruiting can lead to a reduction in the quality of shiitake mushrooms because the fungal mycelium is spongelike and soaks up water. These waterlogged mushrooms have a reduced market value or can become unmarketable. Protect logs from the elements once fruiting begins by covering them with tarps or fruiting blankets.
Are Air Circulation and Light Adequate?
Malformed mushrooms with large or elongated stems and small caps can result from poor air exchange and/or from too much shade. Arrange log yards for adequate air circulation to reduce carbon dioxide buildup where the mushrooms form. If the overhead canopy creates too much shade, consider removing several trees using crop tree release (CTR). CTR opens the canopy by thinning and/or removing the crown competition from a small number of selected trees to benefit both the log yard and the overall structure of the woodlands. See fact sheet F-50 Crop Tree Management: A Tool to Help You Achieve Your Woodland Goals for more information on CTR.
How Long Were Mushrooms Left on the Log Prior to Harvest?
Wide and flattened caps on shiitake mushrooms occur when logs become overmature. Harvest mushrooms in a timely manner when the caps are slightly rolled under along the margins.
Were There Any Major Temperature or Humidity Fluctuations from Pinning to Maturity?
Cracked capped mushrooms, or flower shiitakes, may occur when the conditions following pinning are dry or when humidity and temperature fluctuations occur between day and night. Growers may purposely induce cracking in mushroom caps since these mushrooms can have greater value in some markets.
Low Mushroom Yields
When a log does not produce many shiitake mushrooms, causes are often related to competition from other microorganisms in the log, contamination of the spawn, or the compatibility and characteristics of the tree species inoculated.
What Was the Condition of the Harvested Tree?
Only inoculate logs or bolts recently cut from a healthy tree. Avoid using logs from trees with symptoms of heart rot, such as hollow centers or loose bark. The wood from these trees harbors microbial competitors. These competitors reduce space, decrease nutrients and utilize shiitake fungus food sources. Do not leave logs in direct contact with soil—this can introduce competitors that reduce shiitake yields.
Is Your Spawn Contaminated?
Microbial competition may be present before inoculation. Always check spawn when it arrives. Make sure there are no contaminants that can cause issues later. Shiitake spawn should smell earthy and should have no pink or green coloration. Some contaminates may not be visible on the surface of the spawn. Contamination can be especially difficult to identify on sawdust spawn. If you suspect contamination, do not use the spawn for inoculation as it can compete with the shiitake fungus during colonization and inhibit its growth.
Was the Tree Species Used Recommended for Shiitake Production and Did It Have a Thick Sapwood Layer?
It’s important to consider when the log was harvested and the thickness of its sapwood. Using logs or bolts from tree species that are not considered optimal for shiitake production can diminish mushroom yields. Research at University of Missouri, Cornell University, and University of Vermont found that oak, sweetgum, and sugar maple work very well for shiitake production, but wood from coniferous and fruit trees is not suitable. Shiitake grow best on logs that also have a thick layer of sapwood. Trees used for shiitake production should be dormant when harvested to ensure that a high amount of carbohydrates is stored in their sapwood.
No Mushrooms Produced From the Log
What Type of Spawn Was Used?
Consider which spawn strain was purchased. Each strain has pros and cons as well as its own fruiting period. Cool-season spawn fruits in the spring and fall. Warm-season spawn fruits in the summer. Force fruiting logs will not change the spawn’s fruiting season. Furthermore, some strains, such as cold-weather strains, do not respond to forcing. Experiment with different strains to see which ones work best.
Was the Appropriate Log Used?
The substrate used for inoculation also plays a role in growing shiitake mushrooms. Were the recommended tree species used for inoculation? Were the logs inoculated within the required time from when they were harvested? Logs left sitting more than three months prior to inoculation are more likely to result in failure to fruit.
Was Log Moisture Maintained?
Logs should be kept in a shady area, not in direct sunlight or at windy locations. Logs placed under evergreen trees ensures year-round shade. Keeping logs at the right moisture level is critical for shiitake cultivation. Logs that drop below 25% in moisture result in death of the shiitake fungus. If the mycelium is dead, white shiitake mycelium will not grow from either end of the log. Freshly cut logs contain the most moisture. Weighing logs just prior to inoculation and monitoring for decreases in weight during dry periods can give a rough estimate of moisture content. Research from University of Vermont indicated that moisture meters do not provide reliable readings from shiitake logs.
What Was the Colonization Time?
Sometimes a little patience goes a long way. Fruiting times can depend on the type of tree species used as well as the log size. Larger logs take longer to fully colonize and produce mushrooms. Fruiting will be triggered only after a substrate is fully colonized. Also, spawn applied under the recommended rates and holes drilled farther apart than recommended can take longer to colonize. These factors may lead to increased competition from other wood-decay fungi. Not all fungi competing with shiitake will be observed since much of the growth occurs within the wood. Often, weed fungi are noticed only when they produce fruiting structures.
Following supplier instructions will often increase fruiting success.
Other Mushrooms Are Growing on the Log
Different species of white rot fungi can inhibit shiitake growth through competition for similar resources. The term white rot fungus refers to some species of fungi that digest or break down lignin. Examples of white rot fungi include black mold (Annulohypoxylon spp.), turkey tails (Trametes versicolor) and false turkey tails (Stereum ostrea), split gill mushrooms (Schizophyllum commune), white cheese polypores (Tyromyces chioneus), and violet-toothed polypores (Pallidohirschioporus biformis) (Figures 1A, 1B, 1C, and 1D).
Always verify the species of a mushroom before consuming it. Shiitake mushrooms may not be the only mushroom to appear, especially if there was a delay in inoculating logs after they were cut. Shiitake characteristics include a light to brown cap with whitish gills that turn brown with age, a white-spore print, and a white to light-brown stalk with an often fuzzy texture (Fig. 2). Shiitakes only grow on hardwood logs. Some mushrooms are similar in appearance and grow in similar environments as shiitakes, including Galerina marginata, Armillaria spp., Kuehneromyces mutabilis, Hypholoma capnoides, and other small brown mushrooms (Fig. 3). Some of these are poisonous.
Logs not used in a timely manner can dry out or become contaminated with white rot fungi quickly. Logs sitting for more than 90 days after cutting should be avoided.
Insect and Other Macroinvertebrate Feeding
Are the following invertebrates on your logs or mushrooms?
Of all the invertebrates, slugs cause the most preharvest damage to shiitakes. Slugs thrive under humid or wet conditions and will feed on a wide variety of mushrooms, not just shiitakes (Fig. 4).
Early detection is important for slug control. Regularly scout log yards for symptoms of slug damage. Slugs feed on crops primarily during the evening hours. Wet cardboard or plywood provides shelter that attracts slugs over the course of several days. Check traps each morning and kill any slugs found. Beer traps can also be used but the range of these traps extends to only a few feet.
To prevent slugs from reaching shiitakes, growers can apply lime and wood ash around logs. Practice good sanitation by removing organic debris and ensuring that conditions are not too wet. Keep areas clean by adding a base layer of gravel under the logs.
Natural enemies of slugs include arthropods such as ground and rove beetles, centipedes, and some species of birds and amphibians. Attract these predators to log yards by increasing plant diversity and using only necessary insecticides.
Slug baits have been used with some success. However, these are often expensive and may wash away after rain. The active ingredient in many of these baits is metaldehyde which causes significant water loss that results in death of the slug. Keep in mind that these products are toxic and cannot be applied directly to edible crops. Less toxic options contain iron phosphate as the active ingredient (e.g., Sluggo®) and have been found to be just as effective as products containing metaldehyde. Always read and follow the product label instructions.
Termites have been known to cause issues with shiitake logs in Ohio (Fig. 5). These insects do not feed on the mushrooms but will destroy the log. Keep logs off the ground when possible and avoid placing shiitake logs suspected of containing termites next to buildings. Use only freshly cut logs for shiitake production.
Some tree species are more vulnerable to boring insects. For example, some types of hickory attract more long-horned beetles. A sign of boring insects is the presence of frass, or a sawdust-like excrement, around a tree or log. Covering inoculated logs can impede adult beetles from laying eggs.
Pleasing Fungus Beetles
Some species of pleasing fungus beetles feed on fungi, although a few species bore into and feed on surrounding wood (Fig. 6). Dacne fungorum is a species of pleasing fungus beetle that is known to feed specifically on shiitake. Harvest mushrooms in a timely manner to avoid competition with fungivores and remove infested logs.
Thrips are tiny insects known to feed on the spores of shiitakes but do not cause damage to the mushroom. Gently tapping the cap will knock thrips out of the gills.
Mammals and some bird species are known to feed on shiitakes, with squirrels being some of the most notorious. Identify the species causing the problem to form a management plan. Harvesting mushrooms in a timely manner can reduce losses due to nuisance wildlife. Other potential management options include repellants, fencing, and other physical barriers.
In Ohio, it is illegal to relocate certain animals, such as raccoons. The Ohio Department of Natural Resources, Division of Wildlife maintains a list of certified nuisance wildlife trappers who are licensed. This list can be accessed at ohiodnr.gov/discover-and-learn/safety-conservation/wildlife-management/nuisance-wildlife/contact-nuisance-trapper.
Deer have been known to feed on shiitake crops, although this is not common. Establishing deer fences greater than 8 feet around the laying yard and applying repellents such as blood meal or putrefied egg solids can keep deer out of shiitake laying yards.
Using only fresh-cut logs harvested from healthy trees, inoculating with contaminant-free spawn, and harvesting mushrooms just as they reach maturity can save a shiitake grower a lot of headaches. Once the cause of an issue is found, there is often not much to be done with the current batch to correct the problem other than disposing of the affected logs. Successfully identifying issues with shiitake logs and implementing effective management strategies will come with experience.
For more information, check out woodlandstewards.osu.edu.
Learn more about shiitake production and how to troubleshoot issues with the following resources:
Hill, D. B. & Szymanski, M. (2010). Kentucky shiitake production workbook, inoculation (FOR-81). University of Kentucky College of Agriculture.
Kobayashi, T., Oguro, M., Okiba, M., Taki, H., Kitajima, H., & Ishihara, H. (2020). Mushroom yield of cultivated shiitake (Lentinula edodes) and fungal communities in logs. Journal of Forest Research 25(4), 269–275.
Laskovski, N., & Jamison, B. (2014). Maximizing log based shiitake mushroom production by determining optimal fruiting conditions (FNE11-720). Sustainable Agriculture Research & Education.
Lyon, E., Amante, M., Londo, A., Toland, K., & Bratkovich, S. M. (2022). Shiitake mushroom production: fruiting, harvesting, and storage [Fact sheet]. Ohioline.
Lyon, E., Amante, M., Londo, A., & Toland, K. (2022). Shiitake mushroom production: inoculating logs with spawn [Fact sheet]. Ohioline.
Lyon, E., Amante, M., Londo, A., Toland, K., & Bratkovich, S. M. (2022). Shiitake mushroom production: logs and laying yards [Fact sheet]. Ohioline.
Mudge, K, Matthews, A., Waterman, B., Hilshey, B., Siergk, S., Laskovski, N., Rockcastle, J., Rockcastle, S, & Gabriel, S. (2013). Best management practices for log-based shiitake cultivation in the Northeastern United States. Northeast Sustainable Agriculture Research & Education.
Penn State University Extension. (2023, June 19). Slugs and their control [Online article].