MANGAGEMENT IN THE LAYING YARD

Much of the managment required for forest-cultivated shiitake production systems in the PNW relates to managing bolt moisture content levels, especially during spawn run. Spawn run is a critical time where you’re targeting having the shiitake fungus fully colonize the bolt as fast as possible. A fast spawn run optimizes your yield potential and effectively results in you being able to begin producing mushrooms sooner. An environment at 70-80°F with a relative humidity (RH) over 70% is optimal; the latter RH level helps keep optimal shiitake bolt moisture content within an optimal range. Shiitake growth is most optimal when the moisture content (MC) of the bolt is around 40%; when bolt moisture content drops to ≤24%, shiitake growth will effectively cease. The fungus may be able to recover if moisture goes back up, but spawn run completion is very likely to be delayed if this happens.

Both the climates of Japan and eastern North America will commonly provide conditions optimal for shiitake growth via regular rainfall and humidity throughout the summer months, while the shiitake fungus is most actively consuming wood. Conditions in the PNW summer are characteristically different though. Despite the PNW’s notorious reputation of perpetual rain west of Cascades, the summer period from July through September throughout the greater Puget Sound and Willamette Valley regions is often quite the opposite. While summer temperatures of 70-80°F are common in these regions, relative humidity levels are often below 70% at these times. This is an important understanding because for shiitake, most spawn run activity can occur during summer when temperatures are warm, rather than during the cool, rainy PNW fall, winter, or spring. Management steps taken during dry PNW summers which help retain adequate moisture for the shiitake fungus will help assure that bolts remain viable and produce as expected.

The effect of timber selection spawn run and bolt viability management:

The length of the spawn run period varies by shiitake strain, but it also varies by the species of wood it’s growing in. In addition to any nutritional factors that may be provided by a given wood species, spawn run appears to be strongly influenced by the 1) inherent initial moisture content of the bolt, 2) the bark’s ability to retain that moisture, and 3) the density of the wood. Spawn run time estimations and data related to different wood species’ influence on moisture availability during spawn run are shown in Table 1 below.

The significance of the factors shown in Table 1 to shiitake spawn run are:

• Initial moisture content (MC): This helps the shiitake fungus to successfully migrate into the wood of the bolt from the spawn before the spawn’s nutrients and moisture are exhausted. Most species of wood that we’ve evaluated have a favorable initial MC level for shiitake growth (with the exception of Oregon ash). High initial MC can critically help offset losses if the wood species is otherwise prone to drying (e.g. red alder). For species with a lower initial MC, bark integrity and/or resistance to log end-splitting are critical to to retaining enough moisture to support shiitake spawn run.

• MC in autumn of the spawn run year: Shiitake bolt wood species whose MC levels drop below ~24% by autumn of the spawn run year are at risk for a delayed spawn run. If early spawn run was vigorous though, spawn run may be able to recover over autumn > winter > spring with the onset of cooler temperatures and the return of rainy, cool weather.

• Proportion of initial MC lost by autumn of the spawn run year: This illustrates the magnitude of moisture that is typically lost from a given species of bolt wood by fall (i.e. throughout the PNW summer season). This metric is relative to the intitial MC of the wood as a freshly-cut, newly-inoculated bolt earlier that winter/spring. This estimate helps indicate the relative necessity of take management steps to keep MC above critical levels for spawn run; i.e., some wood species are more resilient/self-sufficient than other species in retaining their moisture (e.g. birches), and vice versa (e.g. maples).

• Average wood density: Dense wood can have smaller pore spaces, so denser wood may often contain a lower initial MC but constrain moisture migration to a degree. Wood density alone does not have as much influence overall on MC dynamics as the species used for a bolt. The fact that red alder and garry oak both have proven to be well-suited for shitake production illustrates this, as they are on opposite ends of the wood density spectrum. Dense wood can also mean that there is more carbon contained in the bolt overall for the shiitake to consume. This also means there is generally more woody matter there overall for shiitake to colonize (depending on how much of the total volume is heartwood), lengthening the spawn run timespan.

• Relative bark integrity: This is a subjective assessment of a wood species’ bark relative to bolt moisture retention for shiitake production. Robust bark integrity adequately sheathes, protects, and insulates against wood drying. The degree of bark integrity for shiitake production is dependent on the wood species’ bark thickness, tensile strength, shearing strength, and/or porosity. Each species has different degrees and combinations of these qualities affecting the bolt’s ability to guard against moisture loss.

• Relative propensity for log end-splitting: Each wood species has differing inherent tendencies to split lengthwise when the cut log/bolt end is exposed to drying conditions. When the bolt end splits, this effectively increases the wood surface area, increasing the area exposed to drying. For some species, resistance to end-splitting is aided by the tensile strength of the bark holding the bolt together (e.g. birches, sweet cherry). Heartwood is also more susceptible to splitting compared with sapwood, so species that typically produce a distinct heartwood portion can be more vulnerable to end splitting (e.g. cascara/buckthorn) than those that typically produce higher proportion of sapwood (e.g. alder, common hazel, birches).

The girth of a bolt has a negligible effect on its initial moisture content level. The species of wood and its other associated characteristics are far more influential. Bolt girth alone generally also has little to no effect on moisture retention for bolts that fall within the standard 4-6”-diameter range; i.e. there is no substantial advantage to selecting 6”-diameter bolts vs. selecting 4”-diameter ones for purposes of moisture retention. For bolts under the lower-diameter threshold of 4” though, there is incresing vulnerability to drying the smaller the girth is. Otherwise, generally, bolts within the smaller ~4-5” girth range with adequate moisture may be expected to complete spawn run sooner due to there being less wood volume for the shiitake fungus to colonize.

Recharging your bolt’s internal moisture:

There are a number of ways of irrigating that one could use to can try to recharge the internal moisture of your bolts. In other regions, overhead irrigation using sprinklers is commonly used to try and protect against dry weather conditions. Our research in the PNW though has illustrated that one of the simplest and effective methods is a managment strategy that is usually only used after spawn run is complete. This is a 24-hour immersion in water. In other regions, this strategy is typically only used later on to incite fruiting for your first harvest and harvests thereafter. The main reason why this alternative approach is being used in the PNW is because it provides relatively strong assurance that you get good penetration of water into the wood. Water needs to get through the bark and into the wood to be available for fungal growth. With overhead irrigation, much of the water applied will land on the bark and run off without penetrating into the wood; long periods of overhead irrigation are therefore needed to get effective penetration. Also, if there’s sub-par penetration of water into the wood and strong PNW summer drying weather conditions are prevalent while irrigating, much of the water applied can be easily lost to evaporation. Moreover, if overhead irrigation is used frequently this can incite unwanted fungal growth on the bolt’s bark, consequently hastening its breakdown.

Soaking you bolts sometime between the latter half of June and early July of the spawn run year (a few months after inoculation) for moisture recharge is a timing that is both logistical and effective. This timing coincides with when you may already be soaking bolts inoculated in previous years for a harvest; any new bolts starting spawn run can then be soaked afterwards and even use the same water. This soaking timing will also help assure that bolt moisture gets a boost before going into the drier season that usually begins in July. This timing was also informed by datasets that suggested that a strong early spawn run may be more important than whether the log gets dry later on. See the “Harvest” page for further detail on best managment practices for soaking your bolts. The spawn run soaking process is virtually identical to the forced fruiting process that will be used the following year, although for the spawn run year soaking, the temperature of the water does not matter since you are not trying to incite fruiting at this time. Until there are data to suggest otherwise, one soaking appears to be sufficient to help assure adequate spawn run in the PNW, as any further soakings in the spawn run year would need to consider whether the effort substantially leads to reliably earlier shiitake yields.

Retaining your bolt’s internal moisture:

The second core management strategy developed for PNW forest-cultivated shiitake focuses on retaining your bolts’ moisture by reducing airflow over the bolt surface and keeping the ambient relative humidity around the bolts elevated. To do this, crib stacks are typically covered with large fabric covers throughout the summer months (Figure 1.). The fabric used is typically a spun polyester type of fabric used in vegetable production and gerdening that is usually referred to as “floating row cover”, “frost blankets”, or by the trade name “Reemay®”. For shiitake production, at least 1.5 oz/50% light transmission fabric is needed, as lighter weight fabric is too vulnerable to tearing, and is a less effictive moisture barrier. Other types of fabric may also suffice for covering, but have not been researched. Some PNW growers have used fitted bedsheets (which are easy to source affordably at thrift stores) to cover crib stacks. In Japan, sometimes leaf litter and sticks are piled on top of bolts to help protect them from drying sun and wind. Whatever cover material is ultimately used, it is important that it is breathable. Covers that are impermeable (such as a standard poly tarp) will develop condensation on the underside which will lead to excessive unwanted mold growth on the bark of the bolts.

A 14x16’ peice of fabric is the standard size used for covering bolts in these systems, as that size will sufficiently cover two adjacent crib stacks, and is also the right size to later serve as the fabric used for “fruiting tents” described in on the “Harvest” page. Each cover is placed over the bolts immediately after the spawn run year soaking is complete and the bolts have been put back into the crib stack formation. The covers are held down over the crib stacks with the sides against the ground; bricks or smooth rocks work very well for holding the fabric against the ground. Check the covers periodically throughout the summer to make sure none have been lifted by wind, or have been damaged by animals or tree branches falling from overhead. Depending on well your laying yard is be protected from sun and wind, you may opt to hang shade cloth or tarps strategically to provide additional protection from drying weather.

In autumn, once weather has cooled and the the rainy season has begun, the covers need to be pulled off the stacks and put into rodent-proof storage over the winter. This importantly allows more winter rain to get to the bolts so a degree of moisture recharge can occur. Even though the covers are permeable, they are not permeable enough to let water flow freely and evenly through them. Research in the PNW demonstrated that when covers are left on over winter, any moisture retained by covering over the summer was negated by the covers inhibiting the flow of rain water into the stack over winter. In another related trial, 23% of red alder and garry oak bolts that were not soaked and left in open crib stacks dropped below the critical level of 24% MC by fall, compared to 0% of bolts that were soaked and covered. In this same trial, the soaked and covered bolts retained 30.5% MC compared to 26.6% MC retained by the bolts that were not soaked and left uncovered throughout the spawn run year. These MC levels translated to only 33.3% of bolts with a delayed spawn run when soaked and covered, compared to 69.0% of bolts with a delayed spawn run for bolts that had not been soaked and covered.

Managing bolt moisture the year after spawn run and thereafter:

Once spawn run is complete and your bolts have begun to produce mushrooms, bolt moisture management is largely self-tending. This is because bolts will be getting soaked three times per year for mushroom harvests. This cycle of soaking will help keep bolts adequately hydrated to keep your shiitake fungus viable. Keeping bolts covered every throughout each summer of their lifespan will further aid in keeping bolt hydration consistenly elevated, and your shiitake fungus consuming wood.

Laying yard pests:

There are some pests that affect resting bolts in the laying yard. The most notable are rodents and birds, although sometimes wood boring beetles and dampwood termites are observed. Dampwood termites are more likely if the bolt’s wood is against the ground. These insect pests do not appear to have a significant impact on bolt viability, though. Rodent damage, on the other hand can significantly diminish the viability of the bolt. Rodents will chew bark off the bolts, and appear to eat the shiitake mycelial mat under the bark (Figure 4), largely over winter. Usually the top layer of bolts in a stack may be affected, but sometimes lower layers are also affected. Rodent damage has been observed on virtually all wood species that have been researched so far, but seems to be more revalent species with furrowed bark, such as oak (see figure 5A) and bigleaf maple. Alternatively, sometimes when a sheathing type of bark (e.g. birch, common hazel etc.) is damaged by rodents it will incite a cascade of bark loss. Largely rodent-damaged areas of the bolts will not produce mushrooms, and the longevity of the bolt is significantly compromised. Birds damage has also been observed, where the spawn is pecked out of their holes with the holes left bare, presumably affecting spawn run and moisture retention. This kind of damage is expected to be most compromising early on before spawn run is complete. In areas with a coyote population, damage to water lines has been observed. This occurs when the weather becomes hot and dry, and they are seeking water.

Animal pest damage managment has not been researched yet. Possible future approaches that may be trialed are covering the crib stacks with shade cloth material (≤ 50% shade rating), mesh window screen material, or hardware cloth. Any cover placed over the stacks during winter should be very porous to allow rain water to flow freely and evenly through it. Elevating water lines off the ground and into trees or burying them may help keep them more out of reach to coyotes.

There is also a diverse array of feral (“weedy”) fungi that are likely to appear on your bolts. By in large, feral fungi is not easily managed, and is often impractical in an oudoor production system. They are instead are largely managed premptively through 1) facilitating a strong shiitake spawn run, and 2) not causing bolt bark surfaces be wet for extended perids of time during warm weather. Fortunately, the shiitake fungus appears to be very competitive once established. To date, only one type of feral fungus has been observed to be overtly competitive and compromising with shiitake. This is a common bracket fungus, likely a Trametes (“turkey tail”) species (Figure 6A-C). It appeared to be already present in the bolts affected (stemming from diseased spots and branch stubs on the logs that were originally overlooked, see Figure 6A and B). Once these bolts were soaked for their third forced fruiting (~16 mo after inoculation), the Trametes fungus proliferated readily, at a rate comparable to shiitake. Trametes fungi have also been observed to outcompete shiitake in bolts that had an overly slow shiitake spawn run (due to prolonged bolt dryness and/or strain) particularly on sweet cherry (Figure 6C). These bolts are unlikely to continue producing shiitake and can be culled.

Species of Trichoderma fungi (“green mold”) are also very common in the PNW and will also commonly appear on log ends and other parts of a bolt where shiitake mycelium is exposed (Figure 6D). Trichoderma fungi are known to be of the most serious, impactiful pests of shiitake and other mushrooms in indoor production systems, but they appear to be less impactful in forest-cultivated shiitake, where conditions and circumstances are comparatively less ideal for Trichoderma fungus to proliferate. In the PNW, Trichoderma is commonly observed on bolts actively producing shiitake, and that continue to produce shiitake. Trichoderma may impact PNW forest-cultivated shiitake yield in ways that have yet to be quantified, but Trichoderma does not yet appear to completely compromise a bolt in the way that Trametes has been observed to.