Mycelocity
Biology

Preventing and Managing Contamination

The common sources of contamination, how to estimate a realistic contamination rate, and what "effective yield" means once losses are applied.

Contamination is the largest source of lost yield for most growers, and the one that most distorts a business plan when it is left out. A farm modelled at zero contamination will always look more profitable than it can possibly be. Planning for a realistic loss rate is not pessimism — it is just accurate.

Where contamination comes from

Most contamination traces back to a handful of repeat offenders:

  • Substrate that was under-sterilised or pasteurised, or cooled in dirty air.
  • Spawn that was already compromised before it went in.
  • Air — unfiltered room air during inoculation is the classic culprit.
  • Handling — hands, tools and surfaces during the open steps.
  • Moisture — waterlogged substrate, as covered in the hydration guide.

The two usual symptoms are green mould (Trichoderma) and wet, sour-smelling bacterial blotch. Both win when mycelium is slow to colonise, which loops back to spawn rate, hydration and substrate quality.

Estimating a realistic rate

Your contamination rate is simply the share of blocks you start that you do not harvest from. The only reliable way to know it is to track it: count blocks in, count blocks lost, over several batches. Early-stage and hobby grows often sit in the 5–15% range; dialled-in operations push below 5%. If you have no data yet, model a deliberately cautious number and tighten it as real batches accumulate.

Effective yield

This is the number that actually matters for planning. The Contamination Loss calculator applies your rate to blocks started:

effective = started × (1 − rate / 100)

Those surviving blocks, multiplied by the per-block yield from the Biological Efficiency calculator, give your effective yield — the harvest you can actually sell. A farm that looks healthy on paper at 0% loss can swing to marginal at 15%, which is exactly why the two calculators share inputs.

Reducing the rate

There is no single fix, but the highest-leverage moves are consistent:

  1. Sterilise/pasteurise properly and cool in clean, still air.
  2. Inoculate in still, filtered air — a still-air box or flow hood.
  3. Raise the spawn rate so colonisation outruns competitors (see Costing a Block for the cost trade-off).
  4. Hold field-capacity moisture, not waterlogged.
  5. Cull visibly contaminated blocks early before they sporulate near clean stock.

Every point you shave off the contamination rate flows straight through to effective yield and, from there, to margin.