Contamination after harvest, mid-flush

The first flush was clean and good. Ten days later there’s green on the block, or grey fuzz over the surface, and the second flush is clearly not happening.

This is extremely common and it is not bad luck. The moment of harvest is the most vulnerable point in a block’s life, and most mid-flush contamination is introduced by the harvest itself.

Why blocks contaminate after fruiting

Four things change at harvest, all in the wrong direction:

The surface is wounded. Pulling a cluster leaves torn mycelium and exposed substrate. Intact mycelium is a defensive barrier; broken mycelium is an open door.

Nutrition is depleted. The block has spent a substantial fraction of its available energy building the first flush. It is a weaker competitor than it was three weeks ago, against contaminants that are exactly as strong as before.

Dead tissue is left behind. Stem butts, torn mycelium, aborted pins. This is the single biggest factor. Dead organic matter on a warm humid surface is where cobweb mold starts, almost every time.

The block was opened. Room air, hands, tools, and whatever spores were in the chamber all reached the substrate directly.

Is the block worth saving?

Depends on what you have.

Cobweb mold, caught early: yes. Treat it — 3% peroxide, remove, watch. Blocks recover from cobweb regularly and go on to give a useful second flush.

Trichoderma: no. Green on a block is the end of that block, and leaving it in the chamber puts every other block at risk. See green mold.

Surface mold on an otherwise healthy block, unidentified: judge by economics. A second flush is typically 50–60% of the first, and a third maybe 25%. A block that needs daily attention and threatens its neighbours to yield half a pound is usually not worth the risk to the rest of the room.

The commercial answer is unambiguous: most operations run one or two flushes and compost the block, because block space and contamination risk cost more than the marginal yield. Home growers can afford more patience, but not unlimited patience.

Harvesting so it doesn’t happen

This is where the problem is actually solved.

Take the whole cluster, not individual mushrooms. Twist and pull at the base. Picking the big ones and leaving the small ones leaves wounds and dying tissue.

Get the stem butts out. All of them. Any stub left in the substrate is dead tissue in a warm wet place. This is the highest-value habit on this page.

Harvest slightly early rather than slightly late. An over-mature mushroom drops spores across the block surface, and a heavy spore layer is itself a nutrition source for contaminants. It also degrades faster once picked.

Use clean hands and a clean knife. Not sterile — clean. Wash, and wipe the blade with alcohol between blocks.

Don’t leave aborts in place. Pull them at the same time.

Reduce standing water afterward. Blocks between flushes need less humidity than blocks in pinning. Running 95% RH over a wounded surface for ten days invites both cobweb and bacterial blotch.

Between flushes

• Let the surface dry slightly for a day after harvest. A dry wound heals; a wet wound doesn’t.
• Resume normal misting once the surface has recovered and pins begin to set.
• Some growers soak blocks between flushes to rehydrate. It works, but soaking a wounded block in non-sterile water is a real risk — if you do it, use cool clean water and no more than a few hours.
• Keep contaminated blocks physically away from clean ones the moment you suspect anything.

The pattern worth noticing

If your first flushes are always clean and your second flushes are always contaminated, nothing is wrong with your sterile technique or your pressure cooker. The problem is entirely in how you harvest — and that’s good news, because it’s the cheapest thing on this site to fix.

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Pull the whole cluster. Get every stem butt out. Let the surface dry for a day. Three habits, and most mid-flush contamination stops happening.