A contamination is an impurity in the air, the soil, and/or the water which can cause (serious) harm to the germinating spores, the growing mycelium and eventually the mushrooms themselves. A contamination can occur naturally or by the hands of humans. Actually anything unwanted in your substrate and mycelium can be considered as a contamination.
When a contamination takes the lead this can have enormous consequences for the growing mycelium and the forming of mushrooms. Therefore when discovered, the contaminated jars, agar cultures etc. should immediately be removed from the other non-contaminated ones. Contaminations can spread very quickly and are sometimes hard to get rid of. A contamination is something no cultivator wants and always desperately tries to avoid. However, even the best cultivator can encounter a contaminated batch or a contaminated mushroom culture.
Most contaminations are pretty easy to identify. The mycelium of mushrooms has a completely white color. If you see any other color in the mycelium, you can consider this, in most cases, as a contamination. A lot of contaminations can also be discovered by their strange penetrating smell. There are 2 exceptions. The colors blue and yellow do not always indicate contamination:
1. Blue. When the mycelium has been bruised, it turns bluish. This is no contamination and has no consequences for the quality of the mycelium.
2. Yellow. When mycelium gets older it can start to form small yellow dots, sometimes this even becomes a yellowish slimy substance. This mostly happens with fully colonized jars/bags which have been in the incubation room for too long. This yellow transforming is a natural resistance of the mycelium and acts as an extra shield against bacteria and other contaminations. It is important that this mycelium is immediately placed into the fruiting conditions.