Basic Pinning Strategy Mushrooms fruit indoors in response to much the same conditions that trigger fruiting in the wild. Several environmental factors, working in combination, provide an ideal environment in which mushrooms flourish. Most, if not all cultivated mushrooms fruit at lower temperatures than the optimum for the growth of mycelium. Usually, a drop in temperature is accompanied by rain or an increase in humidity. Water is essential for the absorption of nutrients by the mycelium. And vaporous water creates the humid microclimate that is so critical for the developing primordia. Primordia have a low tolerance to CO2 and need ample fresh air. And while the mycelium has no requirement for light, many species need light to initiate pinheads and to mature into healthy mushrooms. Mushrooms form only when there is a coincidence of all these factors. Cultivators create an artificial environment that prolongs these optimum conditions so that mushrooms are given the best possible environment in which to grow.
Primordia formation strategies are well defined for species now under cultivation. These procedures are similar in their approach and differ only in certain environmental requirements. Given that the substrate has sufficient nutrients, the interaction of water, humidity, temperature, fresh air, CO2 and light all play determining roles in the fructification process. (In some cases, specific microorganisms must be present before fruiting can occur). The modification of any one of these factors beyond the fruiting requirements can inhibit or stop the process. Hence, the cultivator must have precise control over conditions within the growing room if this critical phase is to be carried out successfully.
PRIMORDIA FORMATION PROCEDURES Agaricus brunnescens culture illustrates the interplay of environmental factors in pinhead initiation. It serves as a useful model for setting primordia in many species, especially those using a casing layer. In each of the following stages, the main considerations are highlighted and then discussed in detail. Although Agaricus does not require light, and since most cultivated mushrooms do, this requirement has been listed as the last parameter.
Stage I: Preparation Following its application, the casing is conditioned to allow even mycelial growth into it. Once mycelial growth is well established, the casing layer microclimate and the growing room are carefully managed to meet the following requirements.
1. The casing layer is at optimum moisture capacity.
2. The casing layer surface is rough and porous.
3. The relative humidity of the growing room's air is 95%.
4. The substrate is incubated in total darkness.
During the casing colonization period, the casing layer is being conditioned for pinhead initiation. Gradually, the moisture content is brought up to the optimum and a microclimate with high relative humidity is carefully maintained. Water in the casing moves by capillary action to the surface where it is drawn into the air by evaporation. This constant movement slowly depletes the casing of the moisture needed to protect pinhead development. Therefore, in conjunction with an optimum casing moisture level, the relative humidity of the room must be held at 95%. Lower humidities must be accompanied by light but regular waterings. The higher the humidity (rH), the less water will be lost to evaporation.
Given optimum moisture conditions in and directly above the casing layer, the next step is to prepare the casing surface. Whether by initial application or by ruffling at a later time, the casing surface should be rough and open - with minute mountains and valleys. A rough open casing has more surface area where pinheads can form, provides a humid environment conducive to that formation and allows the diffusion of metabolic gases.
Stage II: Environmental Transition?The Prelude to Setting Primordia Pinhead initiation techniques should begin when the mycelium reaches the valleys of the casing surface. Once the mycelium is clearly established in the valleys, the cultivator can begin the first steps leading to the setting of pinheads. Within this one to two day period, the
1. Substrate and air temperatures are lowered to the fruiting range.
2. The humidity is maintained at the 95% level.
3. The carbon dioxide content of the room is reduced by the introduction of fresh air.
4. The room is lighted on a 12 hour on/off cycle.
Mycelium breaking through the casing surface early should be lightly sprinkled with moist casing. Uneven growth through the casing layer is usually an indication of a casing with irregular depths. By "patching" shallow areas, an even mycelial spread is assured. Note that the more even the distribution of the mycelium in the valleys of the casing's surface, the more even the pin-set and the greater the first and second flushes.
The exact time for initiation varies with the strain and according to the experience of the individual grower. Some strains continue to grow vegetatively for a period after the initial temperature shock whereas others stop immediately. For this reason, some cultivators initiate when 20% of the valleys show mycelial growth while others wait until 90% are run through with mycelium. Normally within 12-48 hours from the time the mycelium is first visible in the valleys, the initiation sequence is started.
The first step in the pinhead initiation process is to lower the substrate and air temperature from the mycelial growth optimum to the fruiting range. This temperature "shock" is accomplished by ventilation with a large volume of cool fresh air, thereby lowering the room's temperature to a point 5-20? below the optimum for spawn running. (For Agaricus brunnescens, this would mean dropping air temperature from 70 ?F. to 64 ?F.). Whatever the air temperature may be, the bed temperature is normally several degrees warmer. The length of time needed to affect this change is determined by the total volume of substrate and the temperature of the air being introduced. Within 48 hours, the substrate temperature should fall to fruiting temperatures, effectively slowing vegetative growth. This change signals to the mycelium that it is time to fruit.
Fresh air also removes high concentrations of carbon dioxide and other metabolic gases from the room. Since Agaricus brunnescens does not pin properly at CO2 concentrations above 2000 ppm, lowering the carbon dioxide content of the room's air to under 2000 ppm is critical. The inhibitory effect of carbon dioxide on mushroom formation gives Agaricus growers a high degree of control over the pinning process. Not until carbon dioxide is removed will pinheads form. If carbon dioxide levels remain high, the mycelium will totally cover the casing surface, a condition called overlay.
The mycelial mat formed by overlay makes the casing impervious to water and produces few pinheads. Overlay also occurs if the casing surface is too dry, the humidity (rH) is too low or the air temperature remains too high. Overlay can be counteracted by patching, but the cause must be diagnosed and carefully corrected if the culture is to be revived. Few flushes will be as great from a casing with overlay as from a casing properly managed.
Stage III: Primordia Formation (Knotting) Once substrate temperatures have been lowered and CO2 levels have been reduced, primordia will begin to form. Maintain:
1. A constant fresh air supply to remove metabolic gases, and CO2 at levels less than 1000 ppm.
2. A constant temperature in the growing room that is within the fruiting range.
3. A relative humidity of 95%.
4. A 12 hour on/off light cycle.
The combination of temperature drop, high humidity and reduction of metabolic gases by a constant supply of fresh air now provides an environment conducive to pinhead formation. These parameters should be held constant until the pins are set. Any abrupt changes in temperature or humidity will be harmful to primordial growth. Pinhead initials form in the humid valleys of the casing layer and are visible as small knots of mycelium. This is the earliest stage of fruiting. Within five days these knots enlarge into small mounds or buttons that soon differentiate into mushrooms.
Due to slowed mycelial growth in the cooled substrate, carbon dioxide evolution is greatly reduced. Consequently, the fresh air supply can be moderated to the minimum level necessary to maintain 1000 ppm of carbon dioxide. At this time, oversupply of fresh air can lead to high evaporation rates and excessive drying. The humidity should never be allowed to fall below 90%. If dry air becomes a problem, a light misting of the casing surface, two to five times daily, should keep the microclimate moist. In fact, some growers knock down the mycelium with a forceful watering on the first day of initiation. Others mist daily as a standard practice. However, once pinning has begun, any forceful watering will kill a number of developing pins, and damage others. Given sufficient casing moisture and a high humidity, these watering practices become unnecessary.
Stage IV: Pinhead Development After the pinheads have grown to pea size (3-5 mm.), their further development is primarily dependent on air temperature and relative humidity. To insure that they mature into healthy mushrooms, the
1. Air temperature is held constant within the fruiting range.
2. Relative humidity is lowered to 85-92%.<
3. A constant fresh air supply with CO2 below 2000 ppm
4. A 12 hour on/off light cycle
The humidity is lowered to 85-92%, thereby increasing the evaporation rate, an essential requirement for pinhead maturation. If humidity remains too high, pinhead development will be retarded. The easiest way to reduce humidity is to raise the air temperature by 1 -2 ?F. or to increase air movement within the room. Under no circumstances should pockets of stagnant air be allowed to form. Evaporation is negligible in stagnant air pockets which are also excellent breeding grounds for mushroom pathogens.
At this time, a slightly higher level of carbon dioxide is desirable (in the 1500-2000 ppm range) and fresh air can be cut back accordingly. Given proper CO2 levels, and sufficient evaporation, the pins continue to develop. The exact rate of growth depends on the air temperature in the room. Work done by Lambert (193
has shown that a pinhead of Agaricus brunnescens with a diameter of 2 millimeters fully develops into a mature mushroom in twenty-two days at 50 ?F., in ten days at 60 ?F. and in six days at 70 ?F. Although mushrooms develop more quickly at 70 ?F., overall yields diminish. Optimum temperature for cropping in Agaricus brunnescens is 62-64?F.