Mycelial growth of Agaricus bisporus on sterilized compost is strongly stimulated by preincubating the compost with the thermophilic fungus Scytalidium thermophilum. This stimulatory effect is not species specific, for either A . bisporus or S. thermophilum. Normal mushroom compost is almost completely colonized with S. thermophilum. In experimental composts a positive relation was found between the logarithm of mushroom yield of A . bisporus and the density of S. thermophilum. S. thermophilum provides for compost selectivity: it protects against negative effects of compost bacteria on mycelial growth of A . bisporus. S. thermophilum is inactivated by the growth of A . bisporus mycelium. INTRODUCTIONThe edible white button mushroom Agaricus bisporus (Lange) Imbach is cultivated in the western hemisphere on a substrate consisting of a composted mixture of horse manure, wheat straw, chicken manure and gypsum. Conventionally two composting phases are distinguished (Sinden & Hauser, 1950;Fermor et al., 1985). Phase I is an outdoor fermentation process during which NH3 and unpleasant-smelling compounds are emitted into the environment. Phase I1 is an indoor process of pasteurization followed by conditioning at approximately 45 "C. This can either be performed in limited quantities in mushroom houses or in bulk in 'tunnels'. In mushroom houses self-heating of the compost is controlled by ventilating air around layers of up to 30cm thick contained in trays or beds. In tunnels air is forced through the body of the compost, allowing a more accurate control, and layers up to 250 cm can be processed (Derks, 1973 ;Gerrits, 1988 a).If phase I were carried out indoors waste air could be treated before discharge, preventing the release of ammonia. A combined phase I and phase I1 indoor process as introduced by Lambert (1941) could prevent further environmental problems as preliminary results of our group P. J. L. Derikx, personal communication) indicate that the release of fetid volatiles is prevented in phase I1 carried out in tunnels. Recent experiments suggest that combined phase I and phase I1 indoor composting is feasible (Laborde et al., 1987;Perrin & Gaze, 1987;Gerrits, 1987). Perrin & Gaze (1987 and Gerrits (1987) processed raw moistened materials in tunnels and followed phase I1 procedures only. Good mushroom yields were obtained.The basic nutritional requirements of A. bisporus in vitro are already known (Wood & Fermor, 1985) and easily met by compost. However, successful colonization and use of compost by A. bisporus depends strongly on ecological properties, usually referred to as 'compost selectivity'. It is largely unknown how selectivity can be described in chemical or biological terms. Obviously the rate of substrate colonization during the vegetative phase of mushroom culture is important.

Twenty-two species of thermophilic fungi were isolated from mushroom compost. Scytalidium thermophilum was present in the compost ingredients, fresh straw, horse droppings, and drainage from compost and dominated the fungal biota of compost after preparation. Of 34 species of thermophilic fungi tested, 9 promoted mycelial growth of Agaricus bisporus on sterilized compost: Chaetomium thermophilum, an unidentified Chaetomium sp., Malbranchea sulfurea, Myriococcum thermophilum, S. thermophilum, Stilbella thermophila, Thielavia terrestris, and two unidentified basidiomycetes. These species will be considered for future experiments on inoculation and more controlled preparation of compost.

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Growth of Agaricus bisporus was studied on sterile compost and on sterile compost pre-grown with the thermophilic fungus Scytalidium thermophilum. Early growth was characterized by depressed, slowly growing hyphae. The mycelial radius extended exponentially in this stage. Mycelium on sterile compost switched to a fluffy growth type of radially orientated hyphae that extended at a linear rate, K,, of 5mmd-I. On compost pre-grown with S. thermophilum, early growth continued up to 30 mm, then extension became linear at K, = 7.2 mm d-l. If cultures were well ventilated, K, was higher. A new growth function was derived to combine both growth phases. The width of the peripheral growth zone, w, of A. bisporus mycelium was 3 mm. The exponential specific growth rate, p, was calculated from KJw = 2.4 d-l, which is much higher than a previous estimate of 0.19 d-l. In addition to growth-promoting effects, S. thermophilum had inhibitory effects; the period of early growth was reduced if S. thermophilum was inactivated in pre-grown compost. Under commercial conditions, maximum growth rates are not exploited. The introduction of selected strains of thermophilic fungi in compost and the development of a new kind of spawn inoculum of A. bisporus might lead to improvements in commercial practice.

Clostridium botulinum was not found (<l/100g) in fresh commercial mushrooms produced in the Netherlands. The log10 mean number of anaerobic spores in mushrooms was 3.19/g. In casing soil of mushroom production beds, however, C. botulinum was detected, and numbers varied from <1.3 to 1.6/100 g. Log10 counts of anaerobic spores present in this material amounted to 5.58/g. If it is assumed that the ratio of the number of anaerobic spores to the number of C. botulinum in mushrooms is the same as that in casing soil, then the estimated incidence of C. botulinum in mushrooms was between <0.08 to 0.16 organisms per 100 g of produce.