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Abstract: In early studies by HAYES, RÄNDLE, and LAST (1969), HAYES (1972) a significant link was demonstrated between the volatile by-products of mycelium growth in the compost and the occurrence of a group of bacteria in the casing soil which were shown to be associated with fruit-body formation in commercial white strains of Agaricus bisporus in pure culture. The bacteria, Pseudomonas putida and Pseudomonas Group IV strains were isolated following a pretreatment with a mixture of volatile organic com pounds which were known to be by-products of mushroom metabolism. In later work by HAYES (1972), HUME and HAYES (1972), and SMITH and HAYES (1972), laboratory techniques were employed in which the environmental conditions and media used were different to those used in the commercial system of culture, but nevertheless provided important leads which allowed us to postulate that activity was associated with the ability of these Pseudomonads to solubilise and reduce iron in the casing layer. From the beginning of this work progress has been hampered by the problem of technique, which perhaps is not surprising since fruiting in the commercially used white strains is governed by a complex of factors, some of which are not understood. Also, we have been aware from an early stage that there are a number of difficulties in translating results from pure culture studies on artificial media in the laboratory to the conditions as they exist in the casing soil of commercial crops. Of considerable importance, and a fact which is frequently overlooked, is the capacity of A. bisporus under the conditions of continuous laboratory culture, to adapt to these extreme and highly artificial conditions and progress from the mycelium stage to form initials and primordia without the involvement of the active group of Pseudomonads. These inherent adaptive mechanisms must therefore be taken into account in laboratory techniques, a fact which is emphasised by the behaviour of one A. bisporus strain used by us which is so adapted to laboratory conditions that it regularly forms primordia in pure culture, but when propagated on grain and inoculated as grain spawn onto compost and then managed according to accepted commercial practice, productivity is reduced by as much as 75% of its original potential. Also, it is known that pure cultures of A. bisporus can fruit if cased with activated charcoal (EGER, 1963), and COUVY has reported to this Congress on the use of charcoal in an experimental model for the study of substances involved in the initiation of fructification. Much valuable information can be obtained from these laboratory studies but extreme caution is required in the interpretation of results and its relationship to the methods employed in commerce, where both substrates and environment are different from the conditions employed in the laboratory. Furthermore the commercial method is a biologically complex mixed culture system involving many different groups of microorganisms interacting with the substrates and environment. This paper reports the results of experiments in investigations which are designed to understand more adequately the ecology of the casing layer especially as it relates to the fruiting process in white strains of A. bisporus.
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