0.3 µm, 20mm Synthetic filter discs for mushroom spawn jars, bags and monotubs

Self adhesive PTFE synthetic filter discs with a filtration level of 0.3 μm to prevent contamination from airborne spores. Do note however that if the substrate is filled too high in a jar or it is left too long without moving to fruiting conditions the mycelium can colonise the filter disc and grow through it or produce fruiting bodies on top of the filter.

eBay