The first stage of mushroom cultivation involves culturing a strain of the fungus you want to grow onto a medium that will allow for reliable expansion of the mycellium. Making cardboard spawn is a very simple way of doing this. However, it only works for some species and it will contain other micro-organisms/contaminates that would cause contamination if used to inoculate a sterilised medium. In order to obtain a “pure culture” of the speices it is necessary to use of a sterilised medium and, if necessary isolate the mycellium from any contaminates. This work is carried out using a petri dish that contains a nutified media.
Agar (or Agar Agar) is a seaweed extract that is used as a gelatin substitute in the food industry. It is also used in laboratories as a solidifying agent for this nutrified media to culture bacteria, fungi and other micro organisms. The recipes of nutrient enrichment can very in complexity but for fungal growth sources of sugar, starch and b vitamins must be added to the agar. This mixture is then sterilised to remove all source of competitive micro organisms. When hot the mixture flows like a liquid but will solidify when cooled. After sterilising and before it cools it must but poured into perti dishes. Once completely cooled the perti dish can be used, under aspetic conditions, to culture a species. The mycellium will grow over the surface of this solidified agar.
To read more about how to prepare and use agar media see: