Cobweb mould, ink caps and black specks on your grow kit: what is what

The common sights on a grow block, white fuzz, yellow droplets and green patches, were covered in an earlier piece. This one is about the rarer guests. They appear less often, but when they do, almost nobody recognises them, which makes the fright bigger than necessary. Four phenomena, from annoying to harmless.

Grey cobwebs over your mushrooms: cobweb mould

A thin, greyish white, gossamer web that grows over the mushrooms and substrate within a day or two, as if someone pulled cotton wool over them. That is cobweb mould, a parasite that lives on the mushrooms themselves and turns them soft and brown. You recognise it by speed and texture: ordinary mycelium is dense and white, cobweb mould is thin, greyish and fluffy like spider silk, and it sits on top rather than inside.

What to do: gently remove the affected mushrooms and the web with a damp piece of kitchen paper, without blowing on it or spraying it hard, because the spores travel through the air. Then dab the spot with a little hydrogen peroxide or salt water. And above all, remove the cause. Cobweb mould comes with humidity that is too high and air movement that is too low, so mist less and give more fresh air. The next flush is usually clean.

A strange mushroom from the block: ink cap

Sometimes a grow block, or more often a bucket of straw, produces a mushroom that looks clearly different from what you ordered: a thin stem with an elongated, pointed cap that turns black within a day and melts into an inky mess. That is an ink cap, a fast decomposer whose spores are in virtually all straw and most compost. On a pasteurised bucket of straw it is a common hitchhiker, on a sterilised grow block it is rare.

What to do: remove it as soon as you see it, before it turns black and drips ink over your oyster mushrooms. It is not poisonous, but not what you wanted either. Ink caps indicate the substrate is too wet and too rich; let the bucket dry out a little. Your own species nearly always wins in the end, because the ink cap is fast but weak.

Black specks: usually not what you think

Black specks on the substrate or plastic are the most misunderstood observation. Three possibilities, in order of likelihood. First: droppings from fungus gnats, the tiny black flies from potting soil that are drawn to grow blocks too. Harmless, though a sign you have gnats (see below). Second: spores from your own mushrooms. An overripe oyster mushroom leaves a fine white to lilac layer, but with dark-spored species like shiitake or wine cap that is a dark brown to black deposit under the caps. Also harmless. Third, and the only one needing attention: a grey fluffy layer with black knobs on it, like pinheads stuck in cotton wool. That is a bread mould, and you treat it like green mould: cut it away, move the block somewhere cooler and airier, and send us a photo if the patches are large.

Tiny black flies: fungus gnats

Minute black flies hovering around the block and landing on the substrate are fungus gnats, the same ones that come from your houseplants. The flies themselves are harmless, their larvae less so: they feed on mycelium and can further exhaust a weakened block. They usually arrive from a plant in the same room, not from the block.

What to do: yellow sticky traps next to the block, keep the houseplants in the same room a little drier for a week, and do not make the block wetter than necessary. For a stubborn infestation, a thin layer of dry sand over exposed substrate helps, the larvae cannot get through it. A strong block takes little notice of them.

When is it really serious?

With cobweb mould that returns across the whole block within days of removal, with black bread mould larger than a hand, and with anything that stinks. For those three: photo and order number via the contact page. For the rest of this list: tidy up, improve the spot, carry on. Most of these guests are a message about conditions rather than a threat to your harvest.