Blue-Green Mould (Penicillium)
A common blue-green mould from airborne spores, the same family as the mould on old bread. It cannot be cured once established.
What it is
Penicillium is a common blue-green mould, the same family as the mould that grows on old bread and citrus. It arrives on airborne spores and settles on grain or substrate.
How to spot it
It shows as blue-green or grey-green patches, often with a white edge, and can look powdery. It is usually easy to tell from bright white mushroom mycelium.
Prevention
Full sterilising, clean air, and a vigorous culture are the main defences, as with most airborne moulds. Keep your work area free of old fruit, bread, and compost, which can be sources of spores.
What to do
There is no reliable cure. Seal and remove the affected jar or block without spreading spores, then clean the area and tighten your sterile technique for the next run.