Mucor and Aspergillus Molds
ContaminantsAct now
Very fast, cotton-like white to olive-grey growth that outpaces the mushroom mycelium, later with brown spores; Aspergillus colonies start white and turn yellow-green.
Likely causes
- Damp, poorly ventilated rooms
- Soil, old straw, compost or manure close to the grow
- Badly sterilised or too-wet substrate
What to do
Remove affected bags, ventilate the room to lower relative humidity, keep soil and old substrate away, and dust lime around the room. The handbook also describes injecting 5–10% ammonium bicarbonate into affected areas, but bin the bag if it is spreading.
Easily confused with
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Sources
- Mushroom Growers' Handbook 2: Shiitake Cultivation (MushWorld, 2005; Spanish edition by CERZOS), chapter 5 'Mucor and Aspergillus' (MushWorld). Mucor grows faster than shiitake mycelium, forming thick cottony colonies with vertical white sporangiophores that turn olive-grey to brown, and can take over the substrate and stop mycelial growth; Aspergillus colonies start white and become yellow-green and thick; both live in soil, air, manure, old straw and compost and are likely in damp, poorly ventilated rooms; suggested measures are ventilation to lower humidity, lime on the sides of the room and a 5–10% ammonium bicarbonate solution on affected areas.
Last updated: 2026-10-19