Mushroom Casing Layer: Impact on Yield

The document opens by stating that a proper casing layer is essential for mushroom cultivation; its absence results in poor or no mushroom production (Pizer and Leaver, 1947). The casing layer's nature is extremely complex, influenced by a multitude of interacting physical, chemical, and biological factors. Despite significant research, a complete understanding of these interactions and their impact on sporophore initiation and mushroom production remains elusive. The mushroom industry's main goal is to cultivate mushrooms with greater precision and control (Hayes and Nair, 1976), which requires a deeper understanding of the casing layer's function.

While peat is the most widely used casing material, owing to its seemingly suitable properties (Yeo and Hayes, 1979), the research highlights the successful use of alternative materials by mushroom growers worldwide. Examples include a soil, weathered spent compost, and ground tuff mixture in Switzerland; ground marl in France; loam topsoil in the USA; pumice soil and granulated bark in New Zealand; and decomposed cow manure in remote Himalayan regions (Nair and Bradley, 1981; Hayes, 1981). New Zealand growers, with access to domestic peat reserves in Southland and Hauraki, avoid the problems faced by growers importing European peat. However, they encounter challenges due to the young age of their peat reserves, leading to variability in the material and management difficulties (Bates, 1974).

New Zealand's relatively young peat reserves, unlike older German or Irish peats, contain many organisms. Some, like certain pseudomonads, benefit A. bisporus growth (Hayes et al., 1969), while others, such as nematodes, phorids, and certain bacteria and fungi, are harmful pests. Steam sterilization, once common, is rarely used now due to its toxicity. The variability of New Zealand peat, stemming from random harvest site selection by commercial peat harvesters and the peat's youth, poses significant management problems for growers (Bates, 1974). The discovery of a uniform casing material is seen as crucial to addressing these challenges and improving the mushroom industry's efficiency. While requirements for productive casing materials (adequate structure, water-holding capacity, texture, and pH) have been outlined (Atkins, 1974; Hayes, 1979), the difficulty in measuring productivity-relevant factors often leads to vague and general statements. The importance of biological factors, particularly bacteria, is now widely acknowledged (Eger, 1961; Mathew, 1961; Hayes et al., 1969; Park and Agnihotri, 1969a; Hume and Hayes, 1972; Hayes and Nair).

Experiments with peat and sand mixtures showed that freely available water (low moisture tension) correlated with higher mushroom yields and earlier cropping. A large water-holding capacity, combined with a structure that withstands repeated watering without collapse, is beneficial (Flegg; Reeve et al., 1959). Trials with alternative casing materials, including spent mushroom compost (Stoller, 1979) and Hygromull (Visscher, 1982), were conducted, highlighting trade-offs between productivity and cost or practicality. Analysis of physical properties revealed that casing layer structure is crucial for fruiting, with air-filled pore volume being more significant than water-filled pore volume. Lime's impact is beneficial for open structures but detrimental to dense ones, offering a potential low-cost method for manipulating casing layer structure. The study also highlighted that a productive casing material does not necessarily require a large water-retaining capacity.

Eger's 'Halbschalentest' (1961) demonstrated the crucial role of microbes in sporophore initiation. The experiment showed that sporophore initiation was suppressed when autoclaved (sterile) casing materials were used, highlighting the importance of microorganisms in this process. Eger also found that activated charcoal could substitute for the effects of microorganisms, stimulating fructification. This finding was later corroborated by other researchers (Long and Jacobs, 1974; Couvy, 1976; Peerally, 1979). Eger hypothesized that bacteria control sporophore initiation by removing metabolites released from the hyphal tips of A. bisporus.

Hayes et al. (1969) built upon Eger's work, using known volatile metabolites of A. bisporus to select bacterial populations that stimulated sporophore initiation. These stimulatory bacteria were found to be related to Pseudomonas putida. Other studies also indicated that yeasts and microalgae could enhance sporophore production and mycelial density (Curto and Favelli, 1972). While Park and Agnihotri (1969a, 1969b) reported the stimulatory effects of various soil bacteria and their culture filtrates, as well as chemicals like biotin and oxalic acid, some researchers questioned these results (Eger, 1972; Wood). Hayes (1972) suggested that close contact between bacteria and mycelium is essential for fruiting, possibly downplaying the role of volatile substances (other than CO2).

Wood (1976), considering Neilands' (1974) findings, investigated Hayes' (1972) theory but found no evidence that P. putida strains stimulate primordium formation through iron-binding compounds. However, Hayes' later work (1981) tended to confirm his earlier findings (1972, 1974) regarding the role of bacteria in the casing layer and iron levels. Under axenic (sterile) conditions, water-soluble iron increased over time, while under non-axenic conditions, it remained at a constant low level. This suggested that bacteria fix water-soluble iron (possibly inhibitory to A. bisporus growth at certain concentrations) into an insoluble form, maintaining favorable iron concentrations for mushroom growth and fruiting.

The dominance of Pseudomonas species in peat-based casing material 10 days after casing was demonstrated (Hayes and Nair, 1976). These researchers showed that enriching the atmosphere with volatile compounds from mushroom mycelium increased the number of a Pseudomonas isolate in pure culture. The diverse characteristics of pseudomonads, including disease causation, material degradation, and antibiotic production, are often controlled by plasmids (Chakrabarty, 1976; Timmis and Puhler, 1979). The possibility of plasmid-borne genes being responsible for P. putida's role in sporophore initiation was investigated. The study employed various methods to assess bacterial interactions including the single-phase technique of Hume and Hayes (1972) and Peerally's (1979) modification of Eger's 'Halbschalentest'. These methods involved inoculating mushroom strains with bacterial suspensions and observing their effects on growth and sporophore initiation.

The study involved analyzing the physical and chemical properties of nine different casing materials, both with and without lime. Due to time constraints, replication was limited to two repetitions in most cases. If results varied by more than 5%, the experiment was repeated until consistency was achieved. The uniformity of the materials meant that, in most instances, duplication was sufficient. Larger samples were used where necessary to reduce variability. Parameters measured included bulk density, total exchangeable bases (TEB), and cation exchange capacity (CEC). The method for extracting and analyzing exchangeable cations is described, noting that glass apparatus was used due to a lack of suitable plastic equipment for sodium analysis. TEB was estimated following Metson's (1956) recommendation for calcareous materials, assuming it was equal to CEC. A method for measuring mycelial mass is also outlined, acknowledging that some casing material often remained attached to the mycelial strands after cleaning.

Bulk density measurements were categorized as very low (<0.2 gcm-3), typical of peats, and low (0.2-0.8 gcm-3), typical of yellow-brown pumice soils. The addition of lime approximately doubled the bulk density of most casing materials, except for one material (Pu) which showed only a slight increase. This suggested that Pu contained a large proportion of fine particles similar in size to lime, resulting in little change in bulk density when lime was added. The study also considered the relationship between mycelial mass and mushroom yield. It was noted that mycelial strand diameter and mass varied between treatments. Denser casing materials (like Pu) appeared to impede mycelial penetration, suggesting that mycelial strand development, rather than simply mass, may be more significantly correlated with yield. Although not statistically significant, the correlation between mycelial mass and bulk density is noted.

Statistical analysis (Duncan's New Multiple Range Test) revealed significant differences in yield between treatments. The significant correlation between yield, mean mushroom weight, and CEC suggests available nutrient ions in the casing layer affect A. bisporus growth. Interestingly, CEC had a positive effect on yield, while soluble salts had a negative influence, indicating that not all ions are beneficial. Due to the complexity of interactions within the casing layer, further speculation on the role of exchangeable cations is deemed inappropriate, emphasizing the need for more detailed research into the casing layer's role as a nutrient source. Analysis of physical properties revealed the importance of casing layer structure. Air-filled pore volume was found to be a significant factor influencing fruiting, while the volume of water-filled pores seemed unrelated to yield. Lime played a significant role, showing beneficial effects in open-structured materials but detrimental effects in dense ones. This points to the potential of using different lime types to manipulate casing layer structure as a low-cost management strategy.