Probably few attempts have been made to grow the natural spawn with certainty in this country, though it does not appear to be an impracticable thing to do, since formerly this was one source of the virgin spawn in Europe. It is usually obtained by search through stables and barn yards or other places where piles of horse manure have accumulated and have remained for several months. In some cases the growers keep men employed through the summer season searching the yards and stables over a considerable area for the purpose of finding and gathering this natural spawn. It is probably termed virgin spawn because of its origin under these natural conditions, and never having been propagated artificially.
The natural spawn, as indicated above, is employed for a variety of purposes. It is used for inoculating the bricks in the manufacture of brick spawn. It is used for propagating once or twice in the mushroom beds, for the purpose of multiplying it, either in the manufacture of brick spawn, or for flake spawn, which is planted directly in the beds to be used for the crop. In some places in America it is collected on a large scale and relied on as the chief source of spawn for planting beds. In such cases the natural or virgin spawn is used directly and is of the first and most vigorous generation. It is believed by growers who employ it in this way that the results in the quality and quantity of the crop exceed those produced from the market spawn. But even these growers would not always depend on the natural spawn, for the reason, that collecting it under these conditions, the quantity is certain to vary from year to year. This is due probably to varying conditions of the season and also to the varying conditions which bring about the chance inoculation, or the accumulation of the material in the yard for a sufficient amount of time to provide the mycelium.
It would be interesting, and it might also prove to be profitable to growers, if some attempt were made to grow natural spawn under conditions which would perhaps more certainly produce a supply. This might be attempted in several different ways. Stall-fed horses might be fed a ripe mushroom every day or two. Or from the cap of ripe mushrooms the spores might be caught, then mixed with oats and fed to the horse. Again, the manure piles might be inoculated by spores caught from a number of mushrooms. Manure might also be collected during the summer months from the highways and aside from the probable natural inoculation which this material would probably have from the spores blown from the meadow and pasture mushrooms, additional inoculation might be made. The manure obtained in this way could be piled under sheds, packed down thoroughly, and not allowed to heat above 100° F. These piles could then be left for several months, care being used that the material should have the proper moisture content, not too dry nor too wet. This is given only as a suggestion and it is hoped that some practical grower will test it upon a small scale. In all cases the temperature should be kept low during the fermentation of these piles, else the spawn will be killed.
One of the methods of obtaining natural spawn recommended by Cuthill ("Treatise on the Cultivation of the Mushroom") is to collect horse droppings all along the highways during the summer, mixing it with some road sand and piling it in a dry shed. Here it is packed down firmly to prevent the heat rising too high. A "trial" stick is kept in the pile. When this is pulled out, if it is so hot as to "burn the hand," the heat is too great and would kill the spawn. In several months an abundance of the spawn is generated here.
Mill-track spawn.—"Mill-track" spawn originated from the spawn found in covered roadways at mills or along tram-car tracks where horses were used. The accumulation of manure trodden down in these places and sometimes mixed with sawdust or earth, provided a congenial place for the growth of the mycelium. The spawn was likely introduced here through spores taken in with the food of the horse, or brought there from highways, if they were not already in the soil from mushrooms grown there. It would be then multiplied by the growth of the spawn, and from spores of mushrooms which might appear and ripen. The well tramped material in which the mycelium grew here, when broken up, formed convenient blocks of spawn for storage and transportation, and probably led to the manufacture of brick spawn.
Manufactured spawn.—The manufactured spawn, on the other hand, is that which is propagated artificially by the special preparation of the substratum or material in which the mycelium is to grow. This material is inoculated either with a piece of natural spawn, or with pieces of previously manufactured spawn. It is put upon the market in two different forms; the brick spawn, and the flake spawn. The latter is sometimes known as the French spawn, while the former, being largely manufactured in England, is sometimes spoken of as the English spawn.
Figure 233.—Brick spawn. Three "bricks," one marked to show into how many pieces one brick may be broken.
Brick spawn.—The brick spawn is so called because the material in which the mycelium is present is in the form of bricks. These bricks are about 5 by 8 inches by 1-1/2 inches in thickness, and weigh about 1-1/4 pounds each when dried. The proportions of different kinds of material used in the manufacture of brick spawn probably vary with different manufacturers, since there is a difference in the size and texture of bricks from different sources. One method of making the brick spawn is as follows: Equal parts of horse dung, and cow dung, and loam soil are thoroughly mixed together to a consistency of mortar. This is pressed into the form of bricks and stood on edge to dry. When partly dry, a piece of spawn about an inch in diameter is pressed into one side of each brick. The bricks are then stood up again until thoroughly dried. They are then piled upon a layer of fresh horse manure about 8 inches deep, the pile of bricks being about 3 feet high. This pile is then covered over loosely with fresh horse manure, a sufficient amount to produce, when heating, a temperature of about 100° F. They are left in this condition until the mycelium or "fiber" has thoroughly permeated the bricks. The spawn is now completed, and the bricks are allowed to dry. In this condition they are put upon the market. The bricks made with a very high percentage of soil often have the appearance of dried soil, with a slight admixture of vegetable matter.