Sphagnum peat mushroom casing soils: composition, function and microbiology.

In the mushroom growing process, Agaricus starter culture (spawn) is worked into the mushroom substrate (compost following pasteurization) and allowed to grow throughout the substrate for 14 days. After colonization of Agaricus mycelia in the substrate is complete, a 1.5-inch layer called casing, consisting of peat soil amended with calcium carbonate (to neutralize pH) and water (about 80 percent moisture), is applied on top of the compost bed. This layer is called the casing soil (Figure 1). The casing soil enhances the retention of irrigation water on the growing beds, and promotes mushroom fruit body formation.

[FIGURE 1 OMITTED]

Background and Composition of the Mushroom Casing Soil

The major component of casing soil used by mushroom growers in North America is sphagnum peat. Sphagnum peat is primarily decomposed sphagnum moss. Sphagnum moss typically grows in an aquatic bog environment. The bog water has a pH near 4.0, low levels of oxygen and few mineral nutrients. Each year, dead sphagnum moss accumulates in the bog to form peat. Due to conditions in the bog, sphagnum moss peat decomposes slowly. Over thousands of years, it can form layers up to 4-20 feet thick (PHI 2005).

Canada is the world's largest producer of horticultural peat. Currently, of Canada's 270,000,000 acres of peat wetlands, 40,000 acres are under harvest (PHI 2005). Peat bogs are composed of approximately 92 percent water. In preparation for peat harvest, the surface of the peat bog is cleaned by removing surface foliage, large root systems and debris. Drainage ditches are dug around and through the bog to drain the peat. The peat bog is harrowed to a depth of six inches, air-dried and the top two inches are removed with vacuum harvesters (CSPMA 2005).

Peat, with its high humus content and weed-free nature, is an ideal amendment for raised horticultural beds or small gardens. Soil amendment with peat improves the nutrient-and water-holding capacity of sandy soils, and improves the drainage and aeration capacity of clay soils. It is for these reasons peat is widely used by the greenhouse, and fruit and vegetable industries as a soil amendment.

Functions of the Mushroom Casing Soil

Peat is ideal for preparing mushroom casing soils because of the following:

1. Retention of irrigation water

2. Initiation of the sporophores Retention of irrigation water

Retention of irrigation water

Water is involved in several important functions in the growth of Agaricus bisporus (Lange) Imbach cells. Water makes up about 90-94 percent of the fruiting body mass. The growth of Agaricus mushrooms depends critically upon the ability of the cells to translocate water (Beyer et al. 2000). Water uptake by the mushroom has been suggested to be a limiting factor in yields (Kalberer 1991). Also, yield, size and quality of harvested mushrooms are correlated to the amount of moisture in the substrate or casing (Schroeder & Schisler 1981; Kalberer 1985; Kalberer 1987). Hence, a critical function of the mushroom growing medium (compost and casing) is to act as a continuous reservoir of water for the rapidly growing mushrooms. The high water retention capacity of peat based casing soils makes it an ideal growth substrate for mushroom growing.

Initiation of the sporophore 

An important process during mushroom growing is the pinning of mushrooms. Pinning is the initiation of the transition of Agaricus bisporus from the vegetative stage to the reproductive stage. Addition of casing soil onto the surface of Agaricus colonized compost stimulates pinning. One key factor to this transition, resulting in fruiting body (basidiome) initiation is the casing microflora (Eger 1972). The casing soil supports an active, aerobic bacterial flora (Hayes & Nair 1976), and the fluorescent Pseudomonad spp. represents up to 50 percent of the bacterial population in the casing layer (Samson 1986). Pseudomonas putida (Trevisan) Migula has been identified as an important species involved in basidiome initiation (Hayes etal. 1969; Rainey et al. 1990). The mechanism by which P. putida stimulates fruiting is not well understood, but the bacterium is thought to remove "self-inhibitory substances" produced by the vegetative mushroom mycelium (Eger 1972; Wood 1976).

It is a practice among some mushroom growers to vary the proportion of sphagnum peat and black peat. While sphagnum peat is the major component in casing soil, some commercial mushroom growers add up to 40 percent black peat in their casing layer. It is generally reported by growers that adding black peat to the casing layer regulates pinning over the period of the mushroom crop, thus avoiding peak production days and spreading the harvesting period over the duration of the flush. This cultural practice also helps the growers to harvest and sell mushrooms with symmetrical shape, equal size, consistent maturity, and equal solids content.

Microbiology of Casing Soil

The casing layer on which the mushroom fruiting bodies develop is a significant reservoir for the microflora of fresh mushrooms (Doores et al. 1986). Doores et al. found that aerobic bacterial populations from casing material ranged between 8.2 and 8.5 log CFU/g. Samson 1986 demonstrated that fluorescent pseudomonads can represent up to 50 percent of the total bacteria in casing samples, whereas Doores et al indicated that they represented 2 percent of the total casing bacteria only. Miller et al. 1995 demonstrated that the populations of casing bacteria changed over the Agaricus growth cycle. The casing soil harbored 8.7 to 9.7 log aerobic bacteria per gram of casing soil. The proportion of fluorescent pseudomonads in casing was shown to fluctuate between 14 to 41 percent of the total bacteria present, increases coinciding with the onset of fruiting. Studies in our laboratory have also demonstrated that the casing layer on which the mushroom fruit body develops is high in microbial populations. Total aerobic bacterial populations range from 8.0 to 8.5 log CFU/gm of casing material. The major bacterial genera present in casing soil were the Pantoea genus (8.2 log CFU/gm) and the Pseudomonas genus (7.7 log/gm casing soil).

Casing soil also contains a significant population of yeasts, molds, and actinomycetes. Sphagnum peat, the major component of casing soil, is known to contain Trkhoderma and Streptomyces (Tahvonen 1993). Studies in our laboratory have shown that casing soil in the production environment harbor approximately 5.2 log CFU of molds and 6.7 log CFU of native yeast per gram. Penicillium is the predominant genera of mold present in casing soil. Species level identification based on macro-and micro-morphological features determined that the following species of Penicillium were the predominant ones: P. decumbens, P. chrysogenum, P. glabrum, P. citreonigrum, and P. digitatum. Aspergillus niger was occasionally isolated from the casing soil.

Waksman and Purvis 1932 conducted a study to characterize microbial populations of an undrained peat bog in Florida (Table 1). The study was conducted by obtaining samples from different depths of the peat bog. The upper layer of the peat bog was abundant in aerobic bacterial populations, actinomycetes, and fungi. The depth of the bog significantly influenced microbial populations. With increasing depth, populations of aerobic bacteria, actinomycetes, and fungi decreased and the number of anaerobic bacteria increased.

Table 1: Microbiological population of an undroined peat bog in
Florida, adapted from Waksman and Purvis 1932.

           Numbers In thousands/gram of moist peat

Depth of  Aerobic   Actinomycetes  Fungi  Anaerobic
Peat, cm  Bacteria                        Bacteria

2-20        890          370.0      20.0      120
23          960          290.0      10.0      180
45          410          100.0       7.0      180
75           18           13.0       0.3       16
120          30            0.3       0.0       75
165         235            3.3       0.0      380

* Moisture content of peat varied from 80.1 to 87.4 percent

While soil type significantly influences indigenous microflora, peat soils are generally abundant in actinomycetes (Table 2) (Rao& subrahmanyan 1929). Also, the preparation of casing soil involves the neutralization of peat PH by the addition of calcium carbonate. In his book, The Actinomycetes: Asummary of current knowledge (Waksman 1967), Dr. Waksman indicates that the draining of soil and subsequent addition of calcium carbonate to soil favors the development of actinomycetes.

Table 2: Numbers of actinomycetes in Indian Soils, adapted from Rao
and Subrahmanyan 1929.

Soil Type         Crop Raised  Actinomycetes, thousands
                                   per gram of soil

Black Cotton                             3,340
Alkaline          Wheat                  2,540
Peaty             Paddy                  2,340
Alluvial          Paddy                  1,000
Raddish Laterite  Tea                      250
Kalar Soil        Fruit                    680
Red Sandy Loam    Coconut                   40

Disease and Foodborne Pathogen Suppressive Characteristics of Sphagnum Peat and Sphagnum Peat Based Mushroom Casing Soil

Sphagnum peat, the major component of casing soil, is known to contain Trichoderma and Streptomyces, which are effective at suppressing certain root disease organisms (Tahvonen 1993). Their presence in sphagnum peat has been found to suppress the plant pathogens Fusarium, Rhizoctonia solani, Phythium (Tahvonen 1993; Chen & Avnimelech 1986), and Alternaria brassicicola (Tahvonen 1993). In a recent research study (Chikthimmah et al. 2006) we established that foodborne pathogens do not have the ability to survive in sphagnum peat based mushroom casing soils (Chikthimmah et al. 2006). In that study, batches of casing soils were either untreated or autoclaved at 121[degrees]C for 90 min to destroy populations of native casing microflora. The casing soils were inoculated with Listeria monocytogenes and/or Salmonella sp., maintained under simulated mushroom-growing conditions (80 percent moisture, 23[degrees]C), and periodically sampled for enumerating populations of the foodborne pathogens. Inoculated population levels of L. monocytogenes and Salmonella sp. remained largely unchanged in autoclaved (sterile) casing soil over the sampling period (35 days). However, populations of the foodborne pathogens rapidly declined in untreated casing soil. A 5.7-log population of L. monocytogenes was reduced to undetectable levels within 14 days of introduction into the untreated (unsterile) casing soil. Results demonstrate that commercial sphagnum peat casing soils (that are not subjected to pasteurization or heat treatments) are effective in destroying introduced foodborne pathogens. The results suggested that the native sphagnum peat casing microflora are beneficial for food safety. Cultural practices such as thermal pasteurization may negatively affect the native sphagnum-casing microflora and hence compromise food safety. Hence thermal pasteurization of sphagnum-casing soils is not recommended at this time. Also, the survival of the foodborne pathogens in black peat is currently not known. Since some growers add black peat in their casing layer, further studies on the food safety characteristics of black peat are recommended.

REFERENCES

Beyer, D. M., Lomax, K. M., and Beelman, R. B., The use of time domain reflectometry to monitor water relations in mushroom substrate and casing, Mushroom Science XV., I, 341-348, 2000.

Chen, Y. and Avnimelech, Y., The role of organic matter in modern agriculture, Martinus Nijhoff Publishers, The Netherlands, 1986.

Chikthimmah, N., Beelman, R. B., and LaBorde, L.F., Sphagnum peat casing soils do not permit the survival of Listeria monocytogenes and Salmonella sp., Mushroom News September 2006, 6-13, 2006.

CSPMA, Canadian Sphagnum Peat Moss Association, Harvesting Peat, http://www.peatmoss.com/pm-har-vest.html, 2005.

Doores, S., Kramer, M., and Beefman, R., Evaluation and bacterial populations associated with fresh mushrooms (Agaricus bisporus), in Developments in Crop Science (10): Proceeding of the International Symposium on Technical Aspects of Cultivating Edible Fungi West, P.J., Royse, D.J., and Beelman, R. B. (eds.), University. University Park, Pennsylvania, 283-294, 1986.

Eger, G., Experiments and comments on the action of bacteria on sporophore initiation in A. bisporus, Mushroom Science 8, 719-725, 1972.

Hayes, W. A., Randle, P. E., and Last, F.T., The nature of the microbial stimulus affecting sporophore formation in Agaricus bisporus (Lange) Sing., Ann. Appl. Biol., 64, 177-187, 1969.

Hayes, W. A. and Nair, N. G., Effects of volatile metabolic by-products of mushroom mycelium on the ecology of the casing layer, Mushroom Science, ix, 259-268, 1976.

Kalberer, P. P., Influence of the depth of the casing layer on the water extraction from casing soil and substrate by the sporophores, on the yield and on the dry matter content of the fruit bodies of the first three flushes of the cultivated mushroom Agaricus bisporus, Sci. Hort., 27, 33-43, 1985.

Kalberer, P. P., Water potentials of casing and substrate and osmotic potentials of fruit bodies of Agaricus bisporus, Sci. Hort., 32, 175-182, 1987.

Kalberer, P. P., Water relations of the mushroom culture (Agaricus bisporus): Influence on the crop yield and on dry matter content of the fruit bodies., Mushroom Science, 13, 269-274, 1991.

Miller, N., Gillespie, J.B., and Doyle, O. P. E., The involvement of microbiological components of peat based casing materials in fructification of Agaricus bisporus, Mushroom Sci., 14 (1), 313-321, 1995.

PHI, Premier Horticulture Inc, Sphagnum Peat Moss, www.premierhorticulture.com, 2005.

Rainey, P. B., Cole, A. L.J., Fermor, T.R., and Wood, D. A., A model system for examining involvement of bacteria in basidiome initiation of Agaricus bisporus, Mycol. Res., 94, 191-195, 1990.

Rao, M. G. and Subrahmanyan, V., Studies on soil actinomycetes. Par III. Standardization of a plate method of counting soil actinomycetes., Journal of Indian Institute of Science, xii, (A), 57-68, 253-273, 1929.

Samson, R., Variability of fluorescent Pseudomonas populations in composts and casing soils used for mushroom cultures, in Proceedings of Intl. Symp. Scientific. Technical. Aspects of Cultivating Edible Fungi, Wuest, P.J., Royse, D. J., and Beelman, R. B. (eds.), University Park, PA, 1986, pp. 19-25.

Schroeder, G. M. and Schisler, L.C., Effect of supplementation, substrate moisture and casing moisture on size, yield, and dry weight of mushrooms, Mushroom Science, 11, 511-521, 1981.

Tahvonen, R., The disease suppressiveness of light colored sphagnum peat and biocontrol of plant diseases with Streptomyces sp., Acta Horticulturae., 342, 37-42, 1993.

Waksman, S. A. and Purvis, E. R., The microbiological population of peat, Soil Science, 34, 95-113, 1932.

Waksman, S., A., The Actinomycetes: A summary of current knowledge, The Ronald Press Company, New York, 1967.

Wood, D. A., Primordium formation in axenic cultures of Agaricus bisporus (Lange) Sing., J. Gen. Microbiol., 95, 313-323, 1976.

Naveen Chikthimmah, Robert Beelman, Luke LaBorde

Dept. of Food Science The Pennsylvania State University