Penicillium is a widely occurring mould that grows abundantly on easily decomposable carbohydrates. It is however one of the moulds in the mushroom growing sector that is underestimated. This green mould is often seen as a small colony growing on the compost layer that is visible between the base and side planks. The cause is often over-disinfection which kills off the mushroom mycelium and allows the rapid development of Penicillium on spawn grains or pieces of straw. After a while these patches are recolonised by the mushroom mycelium. In this situation there is hardly any damage.
But sometimes a massive colony of Penicillium develops on the compost and this situation shouldn't be underestimated! It is not a parasitic mould, but it is known as one of the moulds indicating compost quality.
With similiar compost the mycelium growth in the casing soil will be weak and the cac-cing material will also have difficulty developing. Badly fermented compost containing many hard dry blades of straw with too many easily decomposable carbohydrates are more susceptible to Penicillium and obviously the nutrients available for the mushrooms in this type of compost are not sufficient.
Con Hermans, AdVisie ‘The mushroom cultivation advisors’