Group Title: Bulletin New series
Title: Mushroom growing
Full Citation
Permanent Link:
 Material Information
Title: Mushroom growing
Series Title: Bulletin New series
Physical Description: 24 p. : ; 22 cm.
Language: English
Creator: Florida -- Dept. of Agriculture
Publisher: Dept. of Agriculture
Place of Publication: Tallahassee Fla. ;
Publication Date: 1938
Subject: Mushroom culture -- United States   ( lcsh )
Mushrooms -- United States   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
General Note: Cover title.
General Note: "August, 1938."
General Note: Reprint
General Note: Partially taken from circular 251, December, 1932, of the United States Department of Agriculture.
 Record Information
Bibliographic ID: UF00014588
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA7062
ltuf - AMF8375
oclc - 41435299
alephbibnum - 002453070

Full Text

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No. 49'


S DEC 2 197

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August, 1938


Mushroom Growing

The mushroom is a rapidly growing fungus of the class
"Basidiomycetes" and order "Agricales." There are edible,
non-edible, unpalatable and poisonous varieties. Millions of
pounds of edible mushrooms are consumed in this country
annually and a large per cent of them are imported from
The edible mushroom is classified botanically as "Agricus
campestris" or "Psalliota campestris." More than a thousand
species of mushrooms are known, but the common cultivated
variety above mentioned is the only species cultivated in the
United States, and most of the mushrooms consumed are of
this type. In Europe and Oriental markets several other kinds
of fleshy fungi are sold. The Cepes and Truffles of Europe,
the Kames of North Africa and the Shii-take and the Mastsii-
take of the Orient are examples.
The use of the word "toad stool" as applying to all
poisonous mushrooms is erroneous. The fact is there is no
line of demarkation that holds good when applied to all
varieties of this group of fungi. The list of edibles allowed
on the markets of some countries is not the same as the list
allowed in other countries. The varieties are of different
sizes from the almost microscopic to the large puff. They
differ also in texture and quality covering such a variety as
gelatinous, fleshy, cartilaginous, leathery, corky and woody.
The following is from the Encyclopedia Brittanica:
"The parts of a mushroom consist chiefly of stem and cap;
the stem has a clothy ring round its middle, and the cap
is furnished underneath with numerous radiating colored
plates lamellaee) called gills, free from the stem. The cap
is fleshy, firm and white within, never thin and watery;
externally it is pale brown, dry, often slightly silky or floccose,
never vacid.
"The cuticle of the mushroom readily peels away from the
thick flesh beneath. The cap has a narrow dependent margin
or frill; this originates in the rupture of a delicate wrapper,
which in the infancy of the mushroom entirely wraps the
young plant. The gills under the cap are at first white, then
rose-colored, finally brown-black. A point of great import-
ance is to be noted in the attachment of the gills near the
stem; the gills in the true mushroom are, however, more or
less free from the stem, they never run against it or down it.

When a mushroom is perfectly ripe and the gills are brown-
black, they throw down a thick dusty deposit of fine brown-
black or purple-black spores; it is essential to note the color.
The stem is firm, slightly pithy up the middle, but never
hollow; it bears a floccose ring near the middle; this originates
by the rupture of the thin general wrapper of the infant

The following is taken from circular 251, December, 1932,
of the United States Department of Agriculture:


Circular No. 251-1932-U. S. Department of Agriculture

The practice of cultivating the common mushroom orig-
inated in France in the latter part of the seventeenth century,
but it was not until toward the end of the nineteenth century
that it began to gain a foothold in the United States. At
first mushrooms were grown as a side line to market garden-
ing, but as the market developed specialization began.
Although at first abandoned limestone quarries, mines, and
sandstone caves were used, the structure that eventually
found most favor in this country was the specially con-
structed mushroom house filled with tiers of shelf beds. On
the average, better crops are obtained in these houses than
in caves, because favorable conditions can be more readily
obtained and diseases and insect pests are more easily con-
As a result of the widespread use of these special houses
and the development of better methods of making spawn,
the industry has made rapid strides in the last 30 years and
at present 15,000,000 to 20,000,000 pounds of mushrooms
are produced annually. About three-fourths of the industry
is located in eastern Pennsylvania and northern Delaware,
near the ready markets for mushrooms and supplies of
manure in New York City and Philadelphia, but there are
also important centers in other states.


Since the beginning of mushroom culture growers have
planted their beds with a specially prepared material filled
with living mushroom mycelium called spawn. In the early
day of mushroom growing the spawn was made from

mycelium found growing naturally in the fields or in the
caked horse manure of the mill tracks. The spawn maker
planted this mycelium in special beds which were later
broken up and sold to the growers. This type of spawn
was called French-flake spawn. The original wild mycelium
used to inoculate these beds was called virgin spawn or
mill-track spawn. Spawn made according to this method
was loose and easily injured by excessive heat and dry-
ing out. An improvement devised in England early in
the nineteenth century consisted of allowing the mycelium
from the bits of virgin or mill-track spawn to grow into
compressed bricks made of a mixture of horse manure, cow
manure, and partly rotted leaves. This product, known
as English brick spawn, was superior to French-flake spawn
in keeping and shipping qualities. But with both types
of spawn it was difficult to maintain pure varieties, and
often fungus diseases and insect pests were distributed with
the spawn.
Spawn was first made from pure cultures in France
shortly before 1900 and in the United States a few years
later. In France spawn was grown from germinated spores
while in the United States the mycelium was obtained by
simpler tissue-culture method, which consisted of sowing
in bottles of sterile manure bits of mushroom tissue extract-
ed under aseptic conditions from the caps of young mush-
rooms. After the original spawn was obtained in pure
culture by these methods it was used in the place of
the virgin spawn to inoculate the bricks. These methods
enabled the spawn maker to sell spawn of known varieties
and to start cultures comparatively free from disease. But
the spawn sold to the grower was not a pure culture and
there was still an opportunity for diseases and insect pests
to accumulate during the incubation of the bricks and to be
distributed with the bricks.
ALout 14 years ago the most successful spawn makers
in the United State. abandoned the tissue-culture method
for the spore-culture method and a few years later began
selling, direct to mushroom growers, quart bottles of pure-
culture spawn of the type formerly used to inoculate
bricks. Spawn of this kind has the obvious advantage over
the old brick spawn of being free from harmful fungi and
insects. At the present time nearly all the spawn used in
the United States is bottle spawn grown from germinated
spores. It gives the grower a practical approximation of
pure-culture spawn, pedigreed and free from diseases and
insect pests, the only disadvantage being the comparatively
poor keeping quality.


The methods used in manufacturing bottle spawn are
adaptations of ordinary pure-culture laboratory technic
to large-scale production. The principal operations are
the collection of spores, the germination of spores, and
the preparation, transferring, and incubation of spawn
In order to obtain pure cultures, mushroom spores are
collected and grown under aseptic conditions. Methods of
obtaining a pure collection of spores are based on the
fact that -the young gills of disease-free mushrooms
develop under approximately aseptic conditions in filtered
air before the veil is broken, and the mushroom will con-
tinue to expand and produce spores even after it has been
surface sterilized, peeled, or cut up. Different procedures
may be followed to prevent spores of molds on the sur-
face of the mushroom from contaminating the cultures.
Stems may be removed and the surface layer peeled off
with sterile forceps; the mushroom may be surfare steril-
ized by flaming it or soaking it in a disinfectant; or the
spores may be transferred directly from the gills, or from
spore "prints" made by suspending the treated mushroom
over a sterile surface in a sterile container.
The spores are transferred for germination either directly
to sterile washed compost or first to agar media to
make certain that the culture is free from contaminating
molds and bacteria. Ordinary potato-dextrose agar and
beer-wort agar are suitable media. If the culture is pure
it will be at least four days to two weeks before any
growth appears even at a favorable temperature, 75o F.
When agar is used the mycelium is usually allowed to
make a good growth on the agar before it is transferred
to spawn bottles.
The material most generally used for filling spawn
bottles is manure, short in straw and composted longer
than for use in beds, screened and partly dried. This
material is tamped firmly into imperial-quart milk bottles
and a hole is bored through the center for ventilation.
The bottles are then plugged with cotton and steam
sterilized. At a temperature of 700 to 800 F. the myce-
lium usually runs through the sterile manure in about 30
days. The spawn from these bottles is used to inoculate
compost in other bottles similarly prepared. As a rule,
repeated transferring is limited to three generations from
the original cultures to avoid so-called running out and
the danger of accumulating contamination. Bottles newly


"run" with mycelium are suitable for immediate use in
planting beds. If not used immediately they are usually
kept in cold storage.
The following special equipment is necessary for suc-
cessful spawn making: A microscope to discover the
nature of contaminating molds; a small autoclave for
the preliminary culture work; one or more large canners
retorts for sterilization; and apparatus for measuring the
reaction of media and manure. In addition, most spawn
makers use laundry centrifuges to dry their manure and
special motor-driven cutters to shred it. This apparatus
and machinery must be housed, special inoculating and
incubating rooms are necessary, and cold-storage facili-
ties must be provided for storing spawn. The establish-
ments of the leading spawn makers represent a total capi-
tal investment of from $20,000 to $50,000. Because of
this only a few growers in the United States have special-
ized in making spawn. These few, however, make a high
grade of pure-culture spawn originating from germinated
spores and, because of large-scale production and com-
petition, are able to sell spawn cheaper than the average
grower could produce it at home.
A list of established spawn makers will be furnished on
request by the Division of Mycology and Disease Survey,
Bureau of Plant Industry, United States Department of
Agriculture, Washington, D. C.


Several distinct varieties of the common mushroom are
cultivated, but for convenience the trade has grouped
them according to the color of the cap, as white, brown,
and cream. The terms white and brown refer to distinct
varieties, but there are several cream varieties. The white
variety usually commands the highest price on the market,
owing to its color. It is prolific but is far from ideal
because of its tendency to stain when bruised by hand-
ling and to produce clusters on the beds and an excess
of button mushrooms. The brown variety is discriminated
against in most markets, but has advantages over the
white in better shipping quality, firmer flesh, and pro-
duction of fewer button mushrooms. Cream mushrooms
are intermediate in color and combine the other char-
acteristics of the white and brown in different degrees,
depending on the variety.
It has been customary to consider all cultivated mush-
rooms as varieties of "Agaricus campestris." However,

there are undoubtedly heritable differences in the color,
form, and growth habits of these so-called varieties which
breed true from spores, and in all probability some of
them will be considered distinct species when given critical
taxonomic consideration.


Mushrooms can be grown in any structure in which the
grower can control ventilation, maintain a moderately high
humidity, and a range of temperatures between 550 and
650 F. Contrary to popular belief, sunlight is not harm-
ful to mushrooms, and they are usually grown in window-
less sheds and in caves simply because it it easier and
cheaper to control temperature and humidity in struc-
tures without windows. Old barns are frequently used.
Occasionally waste space in greenhouses is planted to
mushrooms. A successful grower near Chicago has been
growing mushrooms in an old brewery for more than
a decade. Sandstone caves along the Mississippi River
near St. Paul, Minn., have been used by commercial mush-
room growers for more than 40 years. Along the Hudson
River near Albany, N. Y., mushrooms are grown exten-
sively in the old ice houses. Near Pittsburgh, Pa., they
are grown in partly abandoned mines, and near Akron,
N. Y., abandoned limestone quarries have been used for
30 years.
In the industry as a whole, however, the specially
constructed house has found most favor, and at least three-
fourths of the mushroom crop in the United States is
grown in these houses. The principal advantages of the
special mushroom house are: It can be suitably located
with relation to railroad facilities, markets, and manure
supply; ventilation, temperature, and humidity can be
easily controlled, the air temperature in the house can be
raised during final fermentation and the moisture and
temperature controlled during the spawn run and the
bearing period; separate houses may be handled as units
during fumigations, and prevent the migration of harmful
fungi and insects from old beds to new. The disadvan-
tages are the high initial cost of the house and the high cost
of cooling it for the summer crop.
The principal advantages in using caves or mines for
growing mushrooms are: The small initial investment,
the low temperatures for growing mushrooms in the sum-
mer; and the uniform temperature during the growing
period. The disadvantages are: The difficulty of disin-

fecting and ventilating, often a limiting factor in produc-
tion in these structures; the fact that the temperature around
the beds is not raised during the final fermentation; drip-
ping and excessive humidity in summer; the fact that bed
temperatures can not be controlled during the spawn run
and that distances from railroad facilities, markets, and
manure supply are frequently great.


Commercial mushroom houses are designed to simplify
and facilitate such operations as filling the beds, spawning,
picking, emptying the beds, disinfecting, fermentation in
the house, heating soil, heating, and ventilating. Experience
has shown that a house constructed along these general lines
meets these requirements.


In a standard house there are two tiers of beds, 5 to 10
beds high. A space of 6 inches to 1 foot is left beneath
the bottom bed to insure a good "heat" during the fer-
mentation of the compost in the bed. A space of 2 feet
is allowed between the bottom boards of the beds. These
bottom boards are laid in loose in order to facilitate fill-
ing and emptying the beds. The side boards are also
loose and are held in place by the compost in the bed.
The beds are usually 6 to 8 inches deep and 6 feet wide-
sometimes 5 feet-and run the full length of the house,
with the exception of the alleys at the ends of the house.
The tiers of beds are supported by 2 by 4 inch uprights
set at 4-foot intervals. The uprights are joined beneath the
bottom boards by 1 1-2 by 6 inch bed supports.


A space about 30 inches wide is usually provided be-
tween the tiers of beds to form a service alleyway which
runs the full length of the house. This alley is used in
filling and emptying the beds. An elevated runway is
built in this alley to make it easier to fill the upper beds.
Often the house is built lengthwise into a sidehill with the
high ground at the level of the elevated runway. An
18-inche alleyway between the outside walls and the tiers
of beds is also provided to facilitate picking.



The length of the house is usually determined as the
result of a compromise between the ideas of a long house
economical to build and a short house convenient and
economical to fill. Many successful growers consider 65
feet a suitable length. If the house is constructed with
two tiers of beds (six beds high, including the floor bed),
the width of the house is about 20 feet. Obviously the
height depends on the number of beds in a tier; usually
3 feet is allowed for working space between the top beds
and the ceiling. Often several houses are joined together
and built alongside each other. When this is done it is
customary to make one roof cover four tiers of beds. Two
houses joined under one roof in this way are called a
double house.


There are six doors in a mushroom house-three at
each end.


Mushroom houses must have ventilation systems which
provide gradual changes of air with the least possible
direct draft over the beds. Ventilation usually is accom-
plished by occasionally partly opening the doors and
opening the hinged vents in the ceiling. To facilitate air cir-
culation the ceiling is sloped upward from the side walls
to the ventilators over the center aisle and likewise from
one end of the house to the other. In some houses provision
is made for drawing the air off the floor and discharging
it outside, and the ventilators may be screened to prevent
the entrance of mushroom flies.


Several growers have installed small refrigeration plants
to assist them over the warm spells in the late spring
and early fall and a few have cooling plants extensive
enough to enable them to grow mushrooms in the summer.
The small refrigeration plants are usually based on the
washed-air principle and water pumped from deep wells
is used as the cooling agent. The larger plants use
mechanical refrigeration, in some cases supplemented
with water sprays. In either case the house must be well


insulated and the plant must be so designed that it can
be cooled without excessive humidity or circulation of air.

Mushroom growers usually use hot-water heat. The
radiation generally consists of four or five pipes running
around the house, hung on the inside of the walls, within
a few feet of the floor. In view of the recently adopted
practice of supplying auxiliary heat at the time of the
final fermentation, provision should be made for the occa-
sional use of steam.


The walls are made of any material having fair insu-
lating value, which will withstand dampness. Many
growers make their walls with a single layer of siding.
Others use a double wall filled with cork Some use cin-
der blocks, tile, etc. When the house is built with a
ceiling it is the general practice to cover the floor of the
loft with about 5 inches of loose shavings, for heat insula-
tion. The bottom boards and sideboards of the beds are
usually made of cypress, which resists rotting.


The average mushroom house, 65 by 20 feet, with beds
arranged in two tiers six beds high, contains 4,320 square
feet of bed space. Since it takes 1 ton of manure to fill
70 square feet of beds, such a house requires approxi-
mately 60 tons of manure. It is essential that the manure
capacity or bed space be large as compared to the air
space, for three reasons; namely, a large proportion of
manure to air space insures a better heat during the fer-
mentation in the house; a large capacity makes it easier to
maintain a high relative humidity; and it cuts down the
capital investment per square foot of bed space.


Although there is necessarily considerable difference in
the details of the procedure followed by growers under
different circumstances, the primary objectives are virtually
the same for all. They are the preparation of a suitable com-
post, the establishment of favorable conditions for growth
and reproduction, and the control of fungus and insect pests
by routine sanitation an disinfection.




The temperature at which the house is held largely
determines the length of the growing period and has con-
siderable influence on the quality of the mushrooms. If
the temperature is kept between 45o and 55o F. good beds
continue to bear mushrooms for five or six months,
whereas in a house held at 600 to 650 the beds exhaust
themselves in three months. In the former case, as a
rule, the mushrooms grow somewhat larger and are dis-
tinctly firmer and heavier than those grown at the higher
temperatures. The total yield is approximately the same
with perhaps a slight advantage in favor of the beds held
within the low temperature range. When two or more
crops are grown in one season, time is an important con-
sideration and the beds must be kept above 55o. The
practice of maintaining a uniform growing temperature
is widely favored, although many growers prefer to start
their crops at a low temperature and bring on successive
growths of mushrooms at slightly higher temperatures.
The temperature limits at which the common cultivated
varieties of mushrooms can be grown are 45o to 68.
Lower temperatures delay the crop but do not perma-
nently injure the beds. On the other hand, during the
time when the mushrooms are on the beds a period of
more than a few days with a temperature in the house over
700 will often injure the crop seriously.


The relative humidity in the average mushroom house
during the bearing period ranges from 70 to 80 per cent.
This condition is easier to maintain in some houses than in
others because of the difference in the exchange of air
through cracks and crevices and differences in the propor-
tion of air space to bed space. When the humidity is
allowed to drop much below 70 per cent the casing soil
has a tendency to dry out too quickly and the surface of
the mushrooms becomes tough and under extreme condi-
tions cracked and seamed. Conversely, if too high a relative
humidity is maintained, the mushroom disease known as
"spot" will be aggravated by the reduced rate of evapora-
tion of the contaminated water spattered on the mushrooms
during the watering of the beds.



Procedure in watering mushroom beds is governed
largely by two objectives: Maintaining in the soil the
proper moisture content to induce therein an abundant
growth of healthy mushroom strands or rhizomorphs;
and minimizing the number of spots and blemishes on the
surface of the mushrooms caused by water spattered on
growing mushroom caps. Usually water is first applied
to the beds shortly after they are cased. At this time the
beds are watered lightly every day until there is just
sufficient moisture in the soil to cause normal strand for-
mation throughout the soil layer. This moderate moisture
content is maintained until the mushrooms begin to
appear. Care should be taken to avoid an excess of
water in the soil at this time, as it may prevent normal
strand formation and seriously reduce the subsequent
yield of mushrooms. The amount of water necessary to
maintain the proper moisture content is quite different for
different soils and in different localities and seasons. It
depends on the relative humidity in the house, the mois-
ture content of the compost, and the water-holding
capacity of the soil. In general, several light waterings
are preferable to a few heavy ones because of the dan-
ger of excess water percolating through the soil and causing
the formation of a wet layer of manure under the soil.
Such a layer may prevent the healthy mushoom myce-
lium lower in the bed from growing up to the soil.
Puddles of water on the bed are also objectionable be-
cause they tend to stimulate the development in the soil
of a harmful green mold and to cause submerged pinhead
mushrooms to turn brown and die. On the other hand, if
the soil is too dry or if only the upper layer is moist, fewer
mushrooms will develop, and the first mushrooms to come
up will have a tendency to form beneath the soil layer rather
than on the upper surface.
After the mushrooms begin to appear they usually
develop in sudden outbreaks at intervals of about a week.
These outbreaks are called flushess" or "breaks" and are
followed by periods during which there are only a few
mushrooms on the beds. There are several systems of
watering in relation to these breaks. Some growers water
only between breaks so as to avoid wetting the mush-
rooms. Others water lightly two or three times a week
regardless of the breaks. Many follow a compromise
system of watering regardless of the breaks during the
first three breaks. During this period spotting is not


serious because the fresh mycelium is vigorous and harm-
ful fungi and bacteria in the soil are comparatively
scarce. Thereafter, watering is done only between breaks.
In any case, when water is applied to the beds while mush-
rooms are growing a gentle shower is used to avoid spatter-
ing soil on to the caps of the mushrooms, and ventilation
is increased after watering until the droplets on the mush-
room caps are evaporated.


Considerable ventilation is necessary in growing a good
crop of mushrooms, and it is advisable to give as much
ventilation as possible without interfering with tempera-
ture and humidity control or causing excessive evaporation
from the beds by cross drafts. Usually it is easier to venti-
late, without interfering with these factors, in the spring
and fall months. When the temperature outside the
mushroom house is higher than that inside, the fresh air
will become damp upon entering the house. Conversely,
cold air brought into a warm mushroom house absorbs
moisture and will have a tendency to lower the relative
humidity inside the house.


As congested centers of mushroom growing develop it
is becoming more and more apparent that cumulative
losses caused by fungi and insect posts can scarcely, be
avoided unless a carefully planned program of disinfec-
tion and sanitation is made a part of the routine of cul-
tural practice. It is advisable to thoroughly disinfect the
composting grounds and the mushroom house between
crops and to take special precautions to prevent the com-
tamination of casing soil and water.
A few growers have concrete composting surfaces, but
in most cases the manure is composted on the bare ground
and a sanitary condition is maintained by keeping the
ground free from old manure and standing water between
crops and by thoroughly drenching the soil with a disin-
fectant a few weeks before assembling the manure. A
solution of formaldehyde made by dissolving 1 pint of
fresh formalin in 15 gallons of water has satisfactory
germicidal properties for this purpose and in addition has
the advantage of being non-corrosive to metals and of re-
maining only temporarily in the soil.


It is almost a universal practice to thoroughly disinfect
the inside of the mushroom house a few weeks before fill-
ing time either by burning sulphur or releasing formal-
dehyde gas. Effective fumigation may be obtained by
either method, especially under warm, damp conditions,
but both sulphur fumes and formaldehyde gas are in-
jurious to growing mushrooms and special precautions
are necessary when fumigating a house adjacent to one in
which a crop is growing.
When sulphur is used it is usually burned at the rate
of 5 pounds per 1,000 cubic feet of air space. Either
crude sulphur or flowers of sulphur will burn readily with
the aid of rag wicks soaked in kerosene. Deep containers
must be used and air pockets in the sulphur heap must be
avoided in order to minimize the fire hazard of running
molten sulphur and burning sulphur spattering over the
bed boards. It is not advisible to burn sulphur directly
on the cement floor of the alley, since the heat generated
may cause the cement to buckle and throw out particles
of burning sulphur. Some growers burn the sulphur out-
side the house and use a forced draft to blow the fumes
into the house.
Fumigation with formaldehyde is accomplished by
vaporizing commercial formalin-40 per cent formalde-
hyde solution in water-at the rate of 1 quart to 1,000
cubic feet of air space. The formalin is usually placed in
pails or tubs along the alley and vaporized by adding
crystals of potassium permanganate at the rate of 1
pound per quart of formalin. As in fumigating with
sulphur, all preparations for quickly leaving and closing the
house should be made before the gas is released and ex-
posed lights should not be used in house, since formalde-
hyde gas is explosive under certain conditions.
While the crop is being picked all mushrooms affected
with bubbles should be carefully removed from the house
and burned to prevent the spread of the disease. After
doing this work the men should thoroughly disinfect their
hands. It is also advisable to burn all mushroom refuse.
After each crop all traces of spent manure should be re-
moved and disposed of so that none will be used on fields
near the mushroom house or where it can possibly contam-
inate prospective casing soil.
The water supply also should be carefully guarded
against contamination with fungus spores or any traces
of grease or oil which might cause diseased or deformed


For general disinfecting around the packing house, or
disinfecting workmen's hands or diseased areas on a bed,
commercial preparations having carbolic acid, creosote,
hypochlorite, or mercury as active ingredients are widely


There as three general classes of mushroom diseases:
Those caused by parasitic fungi or bacteria; those caused
by fungi which make conditions unfavorable for the mush-
room by growing like weeds in the bed; and those that
cause malformation of the mushroom apparently stimulated
by nonliving irritants. The bubbles and spot are examples
of the parasitic class, plaster mold and truffles are examples
of the weed type, and rose comb is an example of the irri-
tant type. Certain of the more important of these are de-
scribed on the following pages.

Bubbles is the most destructive disease of cultivated
mushrooms. It is caused by the fungus "Mycogone
perniciosa" Magn., which grows into the mushroom and
transforms it into a distorted, putrid mass. Soon after
the mushroom is attacked the parasite produces a layer
of white or brown spores over the surface. These spores
may be spread by various agencies and are able to live
through a long rest period under unfavorable conditions.
The recurrence or accumulation of the disease from one
crop to another indicates that the fungus either is remain-
ing alive inside the house from one crop to another or is
being carried into the house during one of the cultural
operations. There are several possible methods of intro-
ducing the fungus into the house-in the air or on water,
entering through doors or ventilators, or in the water,
spawn, compost, or soil, or on the clothing or hands of
If the house is thoroughly disinfected with either sul-
phur or formaldehyde there is practically no chance for
inoculum to remain in the house from one crop to another.
Likewise, disinfetion of the area surrounding the house
and sanitary disposal of mushroom refuse will materially
reduce the danger from wind-blown or insect-carried
spores. If an open well is used it may be necessary to
disinfect it occasionally. There is little chance for Myco-
gone to be spread in bottle spawn that has been made under
aseptic conditions.


Mycogone is killed by long exposure to moderately
high temperatures. All the evidence at hand indicates
that a temperature of 1200 F. for 48 hours in a mushroom
house will eradicate the fungus from the house, and the
manure in a house which has beenthrough a good heat should
be free from Mycogone. This and other circumstantial
evidence indicates that most severe outbreaks of bubbles
in commercial houses are due to carelessness in disinfecting
the house or to infested casing soil.
Losses from infested casing soil can be eliminated by
taking precautions to prevent the contamination of the
soil. To determine whether soil is contaminated, small
test beds may be cased with soil samples taken from
fields that are to be used as sources of soil for subsequent
crops. If soil infestation becomes general and there is no
Mycogone-free soil available, the fungus can be eradi-
cated by heating the soil to at least 1200 F. for 48
hours. This can be done in specially equipped rooms, or by
placing the soil in trays near the top of the house during the
heat, if the heating plant is large enough to raile the
temperature artificially in the event that the manure does
not generate sufficient heat. When the soil is heated to
more than 150, Mycogone spores are killed in less than
an hour. Some growers find partial sterilization with live
steam, as described in Farmers' Bulletin 1629, quite satis-
factory. Others complain of a loss of water-holding
capacity, and molding of the steamed soil. The injurious
effect of steaming apparently varies with different types
of soil and in many cases is temporary and can be elimi-
nated by steaming some time prior to casing, or by aera-
tion of the steamed soil.
After the disease has become established in a house strict
sanitary measures are necessary to prevent workmen
from spreading it. The loss may be reduced somewhat by
growing the crop around 500.
The measures outlined above apply particularly to the
prevention of the disease in conventional mushroom
houses, but the principles may be applied to most situa-


The spot disease of mushrooms caused by "Bacterium
tolaasi" (Paine) Elliott and other soil organisms is found
wherever mushrooms are grown commercially. It is the
most troublesome disease in caves and is oten quite
serious in standard houses, especially toward the end of a


crop period or in growing summer crops in artificially
cooled houses. It is characterized by brown, almost
black, spots over the surface of the mushroom cap. These
spots are often covered with a bacterial ooze. Usually
they do not penetrate deeply into the mushroom flesh,
but at times the bacteria follow along the tunnels of fly
larvae and blacken the entire stem and parts of the cap.
Infection is known to be favored by high humidity and
prolonged wetting of the mushroom caps while watering
the beds. Circumstantial evidence indicates that the
inoculum comes principally from the soil. Growers should
endeavor to avoid spattering water from the soil on to the
mushrooms, take every precaution to prevent the house from
becoming too damp, and, weather permitting, attempt to
dry off the mushrooms rapidly after watering by opening
the doors and ventilators for a few minutes. Mushrooms
on old beds seem to be more subject to spotting than do
new breaks and at times poor ventilation seems to be an
aggravating factor.


The "green mold" disease spreads in patches from the
masses of mushroom tissue which are sometimes left in
the soil after picking. Once a patch of the fungus caus-
ing this disease becomes established in the soil, more
button mushrooms are rarely formed in that area. Its
development can be retarded by removing all of the solid
fungus tissue at the base of the mushrooms when pick-
ing them, and filling the holes with fresh soil. Excessive
dampness should also be avoided. Liming seems to be
somewhat beneficial as a preventive but not as a control
The presence on the bed of a large proportion of de-
formed mushrooms with superfluous gills over the upper
surface of the cap resembling the rose comb of poultry
and sometimes deeply seamed and cracked has been
traced in many cases to mineral oil or oil products. In
some cases abnormalities were apparently due to the use
of kerosene in smudges, disinfectants, and insect sprays.
In others they were due to accidental contamination of the
water supply with oil or grease.
Among the diseases caused by competitive fungi in the
mushroom bed the white plaster mold (sometimes called
"flour mold") and truffles are the most troublesome. The
former is caused by a fungus, "Monilia fimicola" Cost.
and Matr., which produces a grayish spore dust through-


out the interior of the compost. When this fungus is
abundant in the beds the mushroom mycelium rarely grows
more than a few inches from the spawn piece and a total
crop failure often results. In some cases the origin of serious
infestation with this mold has traced to contaminated brick
spawn. Usually severe outbreaks are due to insanitary con-
ditions around the mushroom house or to a wet, soggy
condition of the manure at filling time.
"The truffles disease is characterized by the development of cream-
colored wefts of fungus mycelium which appear under the side-
boards and in the manure at about the time of casing. Unlike the
plaster mold, the truffles fungus 'Pseudobalsamia microspora' Diehl
and Lambert seems to stimulate rather than prevent the run of
spawn in the early stages of the crop. A few weeks later, however,
when the truffles fungus matures and forms wrinkled fungus bodies
in the manure and on the soil, the parts of the beds infested with
truffles become barren and the mushroom mycelium almost com-
pletely disappears. The source of this fungus is not known, since
it has not yet been found outside of mushroom houses. However,
the nature of the fungus and the history of the disease suggest that
it lives in the soil and is carried into the house in the compost.
There is also some evidence that it may remain from one crop to
another in the bed boards and that high temperatures and an over-
wet condition of the manure favor its development. The spores are
probably distributed principally at the time of emptying the beds."


The chief pests causing commercial damage to mushrooms
are the fungus gnats, mites, and springtails.
In general, the fungus gnats of the genus Sciara cause
the most injury to the mushroom industry. They are
prevalent in almost every type of mushroom house or
cave, since they usually enter in the compost when it is
taken into the houses. The larvae or maggots of these
flies cause injury both by destroying the mycelium in the
beds and by feeding on the small mushrooms, which they
completely devour in many instances. These maggots
can also render the large mushrooms unfit for market by
tunneling upward through the stem and cap. The adult
flies often transport injurious mites which attach them-
selves to the bodies of the flies from one mushroom house
to another and they also aid in disseminating some mush-
room diseases.
The mites, while not generally so prevalent as the
fungus gnats, are capable of causing serious losses once
they become established in mushroom houses. The mush-
room mite proper, "Tyroglyphus lintneri" Osb., feeds on
Prepared by O. E. Gahm, formerly assistant entomologist, Bureau of Ento-
mology, U. S. Department of Agriculture.


the mushroom, producing dark pits which result in decay,
destroys the mycelium in the beds, and cuts off the feeder
root system so that the mushrooms do not mature, and
decreased yields result.
It is much more widely distributed, apparently, than the
mite, "Linopodes antennaepes" Banks, which has also been
found causing commercial damage to mushrooms in several
Springtails cause very little damage to mushrooms in
the East, but are one of the most serious pests with which
the grower operating in the sandstone caves of the
Northwest have to contend. While the species found in
the sandstone caves has never been described in the
United States and apparently is not present in the East,
it is doubtful whether it would cause a great amount of
damage in the modern eastern houses, since they provide
atmospheric conditions unfavorable to the insects' rapid
development and reproduction.


Because the mushroom mycelium, as well as the mush-
room itself, is extremely sensitive to most fumigants, it is
necessary to take certain precautionary measures before
placing the spawn in the beds, in order to prevent heavy
infestations by insect pests and subsequent damage.
In the course of the heat in the beds the temperature will
rise high enough, if forced air circulation is employed, to
either kill the insects in the compost or drive them to the
surface where fumigants can be used effectively. Electric
fans provide the necessary forced circulation of air during
the heat.
Two 16-inch fans will equalize the air temperature all
over the house and make the temperature in the compost
fairly even in all the beds. The various pests can then be
killed by fumigation while the temperatures are highest.
Calcium cyanide, scattered on the floor in the alley-
ways at the rate of 1 pound per 1,000 cubic feet of air
space has heretofore been most widely used, but the burn-
ing of sulphur, which is cheap and has a double role as a
fungicide and insecticide, is gradually replacing the use of
cyanide. Burning sulphur at the rate of 2 pounds per
1,000 cubic of air feet space while the compost is at its
greatest heat in the beds, and leaving the house closed for
five hours after all the sulphur has burned, has proven
very effective against any pests in the house at the time.


Results of yield tests indicate that this process has not injured
the compost for subsequent mushroom culture.
Results of hydrogen-ion determinations have shown con-
clusively that the sulphur fumes do not penetrate much more
than 1 inch into the uncased compost and that the surface
compost is rendered slightly more acid than it was before
being fumigated. Hydrocyanic acid gas penetrates the com-
post to the same depth.
To prevent possible infestation of the houses after the
compost has gone through its heat and has been fumi-
gated, the doors and ventilators should be screened with
30-mesh copper-wire cloth. To prevent rapid develop-
ment and multiplication of insects and mites the tempera-
ture of the house should not go above 55o F. while crop-
A dust consisting of 60 per cent pyrethrum powder and
40 per cent finely ground clay, used at the rate of 2 1-2
ounces per 1,000 cubic feet of air space, has proven very
satisfactory for control of adult flies and does not injure the


Comparatively little skill is required in picking mush-
rooms. They are usually gathered at a stage of growth
about 12 hours before the veil would normally rupture.
Mushrooms in the same stage of growth often range from
1 inch to 3 inches in diameter, so the principal considera-
tion is not the size of the mushroom but whether it has
finished growing in the closed form. Mushrooms are
pulled rather than cut. After a mushoom or clump of mush-
rooms is picked, the fleshy stump is carefully removed and
the hole is usually filled with fresh soil. The removal of these
stumps is important, since their presence in the bed favors
the development of green mold in the soil and the
green mold prevents the formation of new mushrooms
in the moldy areas. Large numbers of button mushrooms
from one-eighth to three-eighths inch in diameter die
off, even on normal beds, presumably because of the
crowding out or breaking of the mycelium strands con-
necting the young mushrooms with their supply of nutrition
in the compost. With a little practice these mushrooms are
easily distinguished from healthy buttons and they should be
removed from the bed for the same reason that the dead
stumps are removed.
Most of the mushrooms sold in the United States are
marketed fresh, although a well-established canning in-


dustry has developed in the last 15 years to take care of
the demand for canned mushrooms. Fresh mushrooms
are sold on the basis of weight, in the East in 3-pound
baskets and in the West largely in 1-pound paper cartons.
When packed fresh they are usually sorted according to
varietal characteristics, sizes, and freedom from blem-
ishes. In most large cities they are sold on a commission
basis through produce dealers. In smaller cities they are
sometimes shipped directly from the grower to hotels.
Shipment from surrounding states into Chicago is usually
made by express, whereas nearly all of the mushrooms
shipped from Pennsylvania into New York City are taken
directly from the grower to the commission dealer in auto-
mobile trucks.
Often both the grower and the consignee benefit from
the use of United States standards for mushrooms when
handling the best grade. Mushrooms may be graded and
the containers marked United States No. 1 provided they
contain good quality mushrooms larger than 1 inch in
diameter. They may be marked United States Small,
United States, Medium, United States Large, and United
States Extra Large if they conform to the quality require-
ments under United States No. 1 and the following size
specifications: Small, under 1 inch in diameter; medium,
1 to 1 5-8 inches; large, 1 5-8 to 3 inches; extra large, over
3 inches. Packages in any of these grades should contain
fresh mushrooms of similar varietal characteristics which
are not badly misshapen, are free from disease, insect injury,
open cap, spots, and damage caused by dirt, or by mechanical
or other means, and having stems properly trimmed and not
more than 1 1-4 inches long. *
Mushrooms are canned in Pennsylvania, Delaware,
Ohio, Minnesota, and Colorado. Usually they are received
at the cannery and processed on the day they are picked.
Button mushrooms are preferred for canning. These
are sorted out on a moving belt, carried immediately to
vats where they are preheated for canning. These
about 40 to 50 per cent in bulk, placed in cans, weighed, and
A few growers have tried marketing dried mushrooms,
but this product has not seemed to find much favor with the
consumer and must meet the competition offered by dried
mushrooms from the Orient and southern Europe.
Copies of the latest United States specifications for mushroom grades and
sizes, including the definition of terms and the percentage of tolerance -f off
types, can be obtained on reguest from the Bureau of Agricultural Economics,
U. S. Department of Agriculture, Washington, D. C.



Mushroom growers must meet many of the same eco-
nomic difficulties that confront the producers of other
perishable crops. The cost of production is difficult to
predict and the sale price is almost entirely out of the growers
control. Although the cost of raising a crop is largely fixed,
the cost of producing a pound of mushrooms often varies
considerably from one crop to another, depending on
the yield per square foot of bed space. The price differs
from one locality to another and from one season to
another. In the East prices usually somewhat higher
during the summer than in the winter, because fewer
mushrooms are grown in the summer. Warm spells in
the early fall and late spring may greatly increase the
supply of mushrooms for several days at a time by raising
the temperature in mushroom houses. The temperature rise
is reflected in an increased rate of growth of the mushrooms
and in the production of a larger proportion of buttons. In
congested centers of mushroom growing this usually occurs
in hundreds of mushroom houses at the same time and the
grower often finds himself in the untenable position of pro-
during the most mushrooms when the price is below the cost
of production.
If a yield of 1 pound per square foot is assumed, the cost
of producing mushrooms in eastern Pennsylvania in 1930
was about 26 cents a pound. This may be divided as fol-
lows: Interest on investment, depreciation, and upkeep of
buildings, 5 cents; raw materials, 14 cents; and labor, 7
cents. (Table 1.) The estimated cost of manure may differ
as much as 3 or 4 cents from one locality to another; the
interest and depreciation charge may be deduced when
abandoned buildings or caves are used; and labor cost will
vary in different localities and in different years. Otherwise
costs are fairly comparable in different localities. In some
localities spent manure or "mushroom soil" is sold to truck
gardeners, but this rarely returns an income of more than
enough to pay for hauling it away.


Table 1-Estimated Cost of Raising Mushrooms in Chester County,
Pennsylvania, 1930

Cost Per Pound of
Mushrooms When
Yield Is
1 Pound 1 1-2 Pound
Per Square Per Square
Fiot Foot

Interest on investment (6 per cent of $5,000
equals $300.00) .......................................... $0.0214 $0.0143
Depreciation (5 per cent) ................................1- .0178 .0119
Manure, at $5.25 per ton (for 70 square feet
of bed space) .................................... .0750 .0500
Freight on manure, at $1.25 per ton............ .0178 .0119
Hauling manure to house, at 60 cents per
ton .............................................................. .0085 .0057
Composting manure, three turns ........................ .0081 .0054
Filling house ...................................... ..... .0063 .0042
Emptying house .......................................... .0060 .0040
Fumigating and disinfecting ...................... .0036 .0024
H eat and light ............................................. .0113 .0075
Spawn and planting .................................... .0200 .0133
Casing .............................................................. .. .0040 .0027
Ventilation, watering, and miscellaneous ...... .0050 .0033
Picking and cleaning beds ......................... .0266 .0222
Packing ................................................... ..... .0116 .0116
Baskets, wire, and paper, at 5 cents per
basket .............................................. .... .0166 .0166
T otal ................................................................ 2596 .1870
Total 1 .2596 1 .1870

The following cost information, based on the estimates
(made in 1930) of several commercial growers, was used
in calculating the detailed cost per square foot of bed space
shown in Table 1.
Cost of building a double house (approximately 14,000
square feet) in a row of 10 is estimated at $4,000 on a
basis of wood construction, 100 feet long, 6-foot beds six
high, including heating plant. Equipment for this unit, in-
cluding trucks, is estimated at $1,000.
Cost of composting manure is based on the turning and
watering of a 90-ton heap three times by hand labor with
six men in 27 hours, at 35 cents per hour. (A heap of this
size fills a 7,000-square-foot house.)

Filling a house of 7,000 square feet, by hand labor, takes
seven men 18 hours (at 35 cents per hour), and costs
$44. 10.

r.c -


Emptying a house of 7,000 square feet takes six men 20
hours (at 35 cents per hour), and costs $42.00.
The cost of fumigating a house of 7,000 square feet is
estimated at $25 for sulphur, pyrethrum, labor, etc.
The cost of heat and light is estimated at $80 for 7,000
square feet, using soft coal.
Spawning and planting cost is based on the use of one
bottle of spawn, costing 70 cents, for 35 square feet.
The cost of casing 7,000 square feet with soil from land
belonging to grower includes that of screening (requiring 20
man-hours at 35 cents per hour) and hauling and casing
(requiring five men 12 hours), and totals $28.
The cost of picking and cleaning beds is based on one
man picking four to six baskets per hour, at 40 cents per
hour (including cleaning the beds).
Ventilating, watering, and miscellaneous costs are estimated
at $35 for each 7,000 square feet.
Packing cost is estimated on the basis of 10 baskets per
man-hour, at 35 cents.
Except in isolated cities where one or two growers con-
trol the supply, mushrooms are sold on consignment and the
price received for them is based on the prevailing values, as
indicated by sales from receivers and wholesalers or retail-
ers. In New York and Chicago the receiver charges a com-
mission of 10 per cent for handling mushrooms. The white
variety commands a better price than cream or brown and
large mushrooms bring a higher price than buttons. On this
market a higher average price is maintained in the summer
because of the greatly reduced supply.
Canners specialize in high-quality button mushrooms and
often fix a purchasing price in the fall that remains fairly
constant throughout the winter.

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