Mushroom Growing

Of the many species of edible mushrooms which grow infields and woods, only one, Agaricus bisporushas been developed as a cultivated crop in the United States. Because of the variety of conditions under which fungi grow naturally, it might be assumed that commercial production would-be a simple process but nothing could be further from the truth. No other crop is as exacting in its requirements. Profitable production requires constant attention to maintenance of favorable conditions and eternal vigilance against the inroads of insects and diseases.

Because the mushroom is a fungus and lacks chlorophyll, it cannot carry on photosynthesis and must depend upon non-living organic matter for its nutrition. Also, because it belongs to a lower order of plants having no true roots, stems or leaves, it produces no flowers or seeds and reproduction depends upon spores. Hence mushroom growing is not a matter of planting a seed to produce a plant.

The spores are borne on the gills on the under-side of the mushroom cap and one mushroom produces millions of spores. Given a favorable environment, the spores develop a threadlike mass called mycelium or spawn, which in turn, under the proper conditions, develops the fruiting bodies which are edible mushrooms.

The early steps in this process, called “spawn making,” are carried on in a laboratory under carefully controlled and sterile conditions. The spores are collected by a trained technician and placed on a potato-dextrose-yeast agar for germination. They are later transferred to bottles containing an organic medium such as tobacco stems, kafir corn, wheat or rye for continued growth. The bottles are kept at 75° F. until the threads of mycelium have covered the grain or other medium after which they are placed under refrigeration until time for use.

Because conditions for mycelia growth are also favorable to growth of molds and bacteria, the bottles are checked frequently and any which show contamination are discarded.

Only the largest producers of mushrooms prepare their own spawn; most growers purchase their supplies from companies which specialize in that function.

Before getting his spawn supply, the grower prepares his houses. A “standard” mushroom house, usually built of cinder block, is 64 ft. x 20ft. and contains 2 tiers of beds with an alley between. Each tier is 6 beds high, giving a total bed surface of approximately 4000 sq. ft. Mushroom houses are now built as “doubles,” that is, each house is wide enough for 4 tiers of beds. Because light is unnecessary for growth, the house has no windows. Temperature and air circulation are important, however, and a series of ventilators is built into the roof.

The organic matter in which the mycelium grows is called compost. In the early years of commercial mushroom production, compost was almost entirely horse manure. Following World War II, because of the scarcity of horse manure, growers adopted “synthetic compost” which is made of hay and chopped corn cobs supplemented with brewers’ grains and gypsum and small amounts of ammonium nitrate or potash.

Since 1960 a decreasing availability of corncobs and an increased number of race tracks with available manure have brought about a combination of “synthetic compost” and horse manure for mushroom compost.

Before being placed in the house, the compost must go through a curing process during which it is turned several times and water added. In this way it is broken down by the bacteria present and is both chemically and physically conditioned for the growth of mushrooms. The composting process requires from 1-3 weeks depending upon the material used and the rate of bacterial action. Over-composting or under-composting can have adverse results on the mushroom crop. The experienced grower knows by the appearance and feel of the compost when it is ready to go into the house; no definite rule can be laid down for this process.

After the beds have been filled to a depth of 6 or 8 in., the compost must go through a pasteurization period known as the “sweat out” or “cook out.” The house is closed tightly except that doors are opened at intervals to replenish the supply of oxygen. Fans are installed to aid in bringing all parts of the house to a uniform temperature. The heat generated by bacterial action in combination with the moisture, causes the temperature in the compost to rise and the resulting pasteurization brings about a final conditioning as well as killing insect and disease organisms which have come into the house in the filling process.

The “cook out” is a critical step in the preparation for a mushroom crop. Much depends upon the condition of the compost when it is put into the house. The bacterial action depends upon the moisture and carbohydrate content of the compost; if conditions are not right, the temperature may rise too rapidly or too slowly. If the temperature does not go high enough disease organisms are not killed. On the other hand, an extremely high temperature can cause chemical changes in the compost which can result in a poor crop. Since air temperature is no indication of the compost temperature, thermometers or Thermo-couples are placed in the beds and checked frequently.

A uniform compost temperature of 140° F. throughout the house is considered sufficient to kill animal pests and molds. As the compost heat approaches this point, heat is usually added in the form of steam to equalize the air temperature and the bed temperature and the house is held at this point for from 4 to 6 hours. The temperature is then allowed to drop slowly, preferably not more than 5° per 24 hours.

When the compost is placed in the beds, it has a strong odor of ammonia. By the time the temperature has dropped to 125° F. after the peak heat, no ammonia should be noticeable. The compost temperature is then allowed to drop to 75° F. before spawning. The entire “cook out” requires from 7 to 10 days.

The spawn is then broadcast on the beds and the mycelium grows into the compost in whitish-gray threads. During the “spawn running” period, the temperature is maintained at not more than 75° F. and the compost should have a water content approaching 70%. Water is applied as a fine spray rather than as a heavy stream and not in quantities that will cause the compost to become soggy. Transparent plastic is sometimes placed over the beds to maintain a satisfactory moisture level in the compost during the spawn run. From 2 to 3 weeks are required for the spawn to grow through the compost.

While the mycelium will grow in the compost it will not develop into the fruiting stage and produce mushrooms until a thin layer of soil (called “casing soil”) has been placed on top of the compost.

Preparation of the casing soil is again a most careful process. Since the mushrooms derive no nutrient from the soil, the physical characteristics of good casing soil are more important than its chemical composition. It must hold water without becoming waterlogged and must remain friable on the beds without “caking.” Topsoil meets these requirements better than subsoil. This means that in areas of concentrated mushroom growing, long-range programs of rebuilding topsoil must be planned.

Although the chemical content of topsoil is not important, its degree of acidity affects the crop. It should test between 7.5 and 7.8 on the pH scale. Spawn laboratories maintain soil-testing services for their customers. If the pH tests below 7, lime is added. Mushrooms tend to produce acid in the soil and the addition of lime neutralizes the acids and aids in preventing the growth of green molds.

The soil is screened in the field to remove stones and debris and is then transported to the wharf at the mushroom house where it must undergo sterilization.

One cause of crop failure in the mushroom industry is the presence of nematodes which are present in all soils and all raw compost. At least 2 species are especially injurious to mushroom mycelium. The soil as it comes from the field may also be infected with organisms which cause mushroom diseases known as “bubbles,” verticillium spot, and “mat” disease (all familiar terms to the mushroom grower but merely names to the inexperienced).

Heating the soil to 180° F. by passing live steam through it will destroy nematodes as well as other animal and disease organisms. Some chemicals such as chloropicrin have also been found effective and chemical sterilization of soil has replaced steaming to some extent. After sterilization, the soil is placed in a clean bin or on a concrete wharf and covered to prevent recontamination.

When the mycelium has grown through the compost, about t in. of soil is placed on top of the beds and leveled off. Again there must be constant attention to watering, temperature and ventilation.

Mushrooms begin to appear as “pinheads” about 3 weeks after casing. The temperature must be held at no more than 70° F. and preferably between 50 and 65° F. The lower temperatures improve mushroom production and discourage the growth of insect and disease organisms.

Throughout the growing period the grower must be alert to invasions by disease or insects. Optimum conditions for mushrooms arc also favorable to growth of molds and weed fungi, hence the precaution to prevent contamination by spores in spawn, compost and soil. Malformed or diseased mushrooms are removed.