Introduction to Fungi, Third Edition

534 HOMOBASIDIOMYCETES
2 million tonnes (Moore & Chiu, 2001). In areas
with a warmer climate, e.g. in Southern Europe,
A. bitorquis is also grown.
Mushrooms were originally cultivated in
caves, but today most are grown in specially
constructed sheds. The basic substratum is a
composted straw/manure mixture (Straatsma
et al., 1995). The heap of compost is kept outdoors
where it heats up naturally to 80°C over a period
of 2 weeks, with occasional turning. The heat
produced during composting selects for the
growth of specialized thermophilic microbes,
the most important of which for mushroom
production is the anamorphic mould Scytalidium
thermophilum (syn. Humicola insolens) (Straatsma &
Samson, 1993). The yield of mushrooms from
a compost inoculated with S. thermophilum is
doubled as compared with pasteurized controls
(Straatsma et al., 1994a,b) because the A. bisporus
mycelium can grow partly on the residues
associated with the activities of S. thermophilum
and partly on its living mycelium and conidia
(Bilay & Lelley, 1997). The heat produced during
composting destroys many competing microbes,
and further destruction is achieved by a pasteurization
process in which the compost, now
moved into a shed, is heated by steam to a
temperature of 60°C for 8 h which minimizes
diseases caused later by other fungi.
After cooling, the compost is inoculated with
mushroom spawn pre-grown on sterilized cereal
grains. The inoculated compost is incubated at
25°C for 2 weeks, and fruiting is induced by
covering the colonized spawn with a 3 5cm
deep layer of casing soil or a special mixture of
moist peat and chalk. The mycelium grows up
through the casing layer and forms anastomosing
mycelial cords from which ‘pinheads’
develop at the surface, some of which expand
to form mature basidiocarps. In addition to any
physical or chemical effect associated with the
casing layer, there is also a biological component.
This is shown by the fact that sterilized
casing soil is far less effective than natural soil in
encouraging fruiting. Re-inoculation of bacteria
isolated from casing soil into sterile casing has
shown that Pseudomonas spp. and especially
P. putida are effective in inducing fruiting
(Hayes et al., 1969). The growth of P. putida,
in turn, is selectively stimulated by volatile
substances such as ethanol emanating from
the mushroom compost. Bacteria accumulate
around Agaricus hyphae in a zone sometimes
termed the hyphosphere.
Bacteria which are active in inducing basidiocarp
formation might achieve their effect
in two ways. Possibly, they induce starvation
of A. bisporus hyphae in a manner similar to
fungistasis in soil, where the depletion of
nutrients by competing microbes inhibits
fungal spore germination. An alternative idea
is that bacteria in the casing layer
absorb substances which retard the fruiting of
A. bisporus. This idea is supported by observations
that fruiting can be induced by replacing the
casing layer with activated charcoal, believed to
adsorb inhibitory substances (Long & Jacobs,
1974; Flegg & Wood, 1985; Noble et al., 2003).
The isolation of putative inhibitors is a difficult
task, but one substance, 1-octen-3-ol, has been
shown to inhibit primordium formation in plate
culture.
Two weeks after application of the casing
layer, fruiting is stimulated by forced
air ventilation which lowers the temperature
to 20°C and the CO 2 concentration to
300 1000 ppm. Cropping occurs after 10 days,
followed by 2 3 more flushes at weekly
intervals. The whole cycle from compost preparation
to cropping is completed in 10 weeks.
Morphogenesis of A. bisporus basidiocarps
The pinheads which form at the surface of the
casing soil attached to mycelial cords are
basidiocarp primordia, consisting at first of
loose aggregates of hyphae. Basidiocarp development
has been followed by several workers
(see Bonner et al., 1956; Umar & van Griensven,
1997). The hyphal aggregates become compact
and enlarge to form flattened ‘buttons’. When
these are about 2 mm high, there are signs of
reorientation of hyphae in the region of the gills,
and a gap develops beneath the gill region as a
result of programmed cell death (apoptosis)
to form the hymenial chamber, enclosed below
by the partial veil. At a height of about 5 mm
the stipe becomes recognizable by the parallel
arrangement of hyphae and, by the time the