How to track contamination in a mushroom grow

Steps

1. 1

   Note the contaminant type

   Classify what you see: trichoderma, bacteria, cobweb, yeast, or other if you are unsure.

2. 2

   Count the affected containers

   Record how many containers are affected out of the batch total. This is what makes the rate meaningful.

3. 3

   Capture the date you noticed it

   Log when it became visible, not when you suspect it began. The visible date is what you can compare between batches.

4. 4

   Add a photo

   A photo makes later classification much easier, and helps you tell a bacterial blotch from a wet spot after the fact.

Contaminant types worth separating

Trichoderma is the most common competitor mould in home grows: fast-spreading, bright green, and usually a sign of incomplete substrate sterilisation or a spawn-to-substrate ratio that left the mycelium outcompeted. Bacterial contamination shows as sour-smelling, wet-looking patches and often tracks back to over-hydrated substrate.

Cobweb is a wispy grey growth that tends to appear on damaged mycelium rather than healthy growth. Yeast presents as pale, slimy patches in wet substrate. Separating these categories is what lets a pattern emerge — a run of trichoderma points somewhere different from a run of bacteria.

Contamination rate is the useful number

A single contaminated tub is an anecdote. The share of containers you lose across a batch is a measurement. If you run twelve tubs and lose one, your rate is roughly 8%. If that holds steady across three runs, your process is stable and you can start changing one variable at a time.

Container-level accounting is why the affected-container count matters. Two batches with identical numbers of contaminated containers can have completely different rates if one used four tubs and the other used twenty.

What patterns usually mean

Contamination appearing during colonisation, before the substrate is fully covered, usually points to spawn quality, substrate preparation or inoculation hygiene. Contamination appearing after full colonisation, particularly around the first flush, more often points to environmental conditions — standing water, poor airflow, or temperature swings.

Because the log records dates, you can check the gap between inoculation and the first visible sign. A short gap across several batches is a strong hint that the problem is upstream, in the substrate or the spawn.

Terms used in this guide

Spawn

Spawn is mushroom mycelium growing on a carrier material, most often sterilised grain, used to inoculate a larger substrate.

Substrate

Substrate is the material mushroom mycelium digests and grows through, such as straw, coco coir, hardwood sawdust or supplemented pellets.

Flush

A flush is one distinct wave of mushroom fruiting from a single batch, harvested together before the substrate rests and produces the next wave.

Contamination

Contamination is the presence of an unwanted organism competing with or damaging the cultivated mycelium, such as trichoderma, bacteria, cobweb or yeast.

Trichoderma

Trichoderma is a fast-spreading green competitor mould and the most common contamination in home mushroom cultivation.