Spawn, spawning and spawn growth begin with a little fascinating history. Until the 20th century, mushroom growers used mill-track spawn to seed their substrate. Mill-track spawn was obtained from under the horses driven around a pole to provide power to the flour mills. Spawn from this source had uncertain varietal characteristics and was not free from pest and other competitors. Later "Flake Spawn" was developed. Flake spawn was the mushroom tissue or mycelium transferred to special substrate manure piles. When the spawn had thoroughly grown through this "Manure Spawn" was broken up and spread onto substrate in the house. England was the first to develop this "Manure Spawn" and when the Lambert brothers immigrated from Belgium to Minnesota, they brought the process to America. One brother, L. F. Lambert moved to Chester County Pennsylvania and began producing flat bricks of moist horse and cow manure that were stacked together with bits of mushroom mycelium.
Manure spawn was used worldwide until the 1930's, when a scientific breakthrough occurred in France and later in the United States. Researchers learned how to germinate spores on sterilized media. This procedure, which we all take for granted, allowed pure cultures to be maintained and then eventually used to make "Pure Culture" manure spawn. This spawn was usually made in a bottle that was stoppered with wool plug. The grower had to break the bottle to remove spawn for use. A practice that was probably fun for the kids, but messy and probably unsafe.
The next step in spawn history was "Tobacco Spawn", shredded dry stems of tobacco from Lancaster County, Pennsylvania. Tobacco spawn was more consistent than manure spawn, however the process of using reusable, screw top bottles was labor intensive and tedious. Tobacco spawn was used for about 10 years.
In 1930, Dr. James Sinden was assigned to the mushroom research project at Penn State. He needed a more consistent method of inoculating substrate for his research. He developed a method using sterilized grain inoculated with pure cultures of the mushroom mycelium. At first spawn, makers resisted its adoption, but by WWII grain spawn prevailed in the US. After 1950, it was introduced into Europe along with the process of thorough mixing of spawn into substrate, which was developed by Ms. Hauser and Dr. Sinden. Until 1926, cream-colored mushrooms were the only strains grown commercially. It was Chester County grower, named Downing that found a cluster of white mushrooms and realized its significance. He called L. F. Lambert from whom he had bought the spawn, who then made spawn from tissue cultures that grew only white mushrooms, hence the birth of the white strains.