Wet slimy patches and a sour smell: bacterial blotch

Brown, wet-looking patches on the caps, slightly sunken, sometimes with a slimy film. Left a day, the patches darken and spread and the smell becomes obvious — sour, or faintly of fish.

This is a bacterium, it is on almost every grow already, and it only becomes a problem when you give it standing water to work in.

What you’re looking at

Pseudomonas tolaasii produces a toxin called tolaasin that breaks down the cell walls of mushroom tissue. The result is a sunken, discoloured lesion rather than a growth sitting on top — the tissue is being digested, not overgrown.

Progression is characteristic:

1. Pale yellow patch, slightly darker than the surrounding cap
2. Deepening to golden brown, then chocolate brown
3. Surface goes slick and greasy-looking
4. Lesion sinks below the level of the cap around it
5. Smell becomes noticeable

It stays mostly on caps and upper surfaces — the parts where water sits.

Telling it apart

Bruising from handling is blue, grey or brown depending on species, appears exactly where the mushroom was touched, and doesn’t spread or smell. It’s a physical reaction, not an infection.

Metabolite droplets are discrete amber beads on healthy tissue. Blotch is a flat spreading stain. See yellow liquid on mycelium if you’re unsure which you have.

Normal ageing browns the whole cap edge evenly and dries it out. Blotch is patchy and wet.

Can it be saved?

The flush: partly. Affected fruits should be harvested and the blotched tissue cut away. Unaffected mushrooms on the same block are fine to eat.

The block: yes. Blotch is a fruiting-surface problem. It doesn’t penetrate the substrate and it doesn’t end the grow. Fix the conditions and the next flush usually comes through clean.

Do not eat visibly blotched tissue. It won’t poison you, but it tastes bad and it’s a sign of a bacterial population you have no reason to consume.

What caused it

There is essentially one cause with several routes to it: water sitting on the mushroom surface for hours at a time.

Pseudomonas needs a film of free water to move and multiply in. Dry caps are not infected regardless of how much bacteria is present.

The usual routes:

Misting directly onto the fruits. The single most common cause. Mist the walls of the chamber, not the mushrooms.

Humidity too high with too little air movement. Above about 95% RH with stagnant air, condensation forms on caps and stays there. Blotch is one of the few problems caused by too much humidity.

Condensation dripping from the lid. Water running off a tub lid onto the fruits below keeps a permanent wet patch. Tilt the lid, or wipe it.

Cold surfaces. A tub against an exterior wall in winter condenses on the cold side. Move it.

Contaminated water. Rare, but standing water in a humidifier reservoir grows Pseudomonas readily. Empty and refill it rather than topping it up.

Fixing it

1. Harvest affected fruits now. Don’t leave rotting tissue on the surface — it seeds the next flush and invites cobweb mold.
2. Stop misting the substrate and fruits. Walls only, from now on.
3. Increase fresh air exchange. Moving air dries cap surfaces between mistings, which is the entire mechanism of control.
4. Drop target humidity from 95% to around 85–88% during fruiting. Pins need the high end; developed fruits don’t.
5. Deal with condensation — wipe the lid, tilt it, or raise the chamber temperature a degree or two so the dew point stops being met on the inside surface.

Preventing the next one

The instinct that causes blotch is a good one — keeping humidity up — applied without the corresponding increase in air movement. High humidity plus moving air is fine. High humidity plus still air puts water on every surface and keeps it there.

Commercial growers control this with a salt solution wash, but at home the answer is simply drier caps: mist walls, run more air, and stop opening the chamber to admire it.

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If the mushroom is wet for hours, blotch is possible. If the mushroom is dry within the hour, it isn’t. That’s the whole disease.