A contaminant is any living thing that grows where the mushroom should. Most are other fungi, usually called moulds, but bacteria and flies count too. Penn State Extension says some of these are true pathogens that attack the mushroom mycelium, while others simply outcompete it1. Their descriptions are written for commercial farms. The names and looks still carry over.
Green mould (Trichoderma)
Trichoderma is a genus of fungi, and it's what growers mean by green mould2.
Penn State lists dark green mould patches on the casing that spread to lesions on the stem1. The patches start grey, turn white at the edges and go green as they make spores1. Some Trichoderma species give off a sweet or "coconut" smell2.
Serious? Yes. Penn State says green mould can cause significant yield losses, especially in Agaricus (button), Pleurotus (oyster) and shiitake crops3. Wikipedia adds that infections have caused major crop losses2.
Cobweb (Cladobotryum or Dactylium)
Cobweb is a cotton-like weft of mycelium. It's usually white and sometimes grey, pink or yellow. It can cover the casing, the mushrooms or the pins, the tiny first mushrooms1. It can also leave flesh-coloured spots1.
Bush, bread or barber mould (Syzygites)
This one is bushy and hair-like, with light flesh-coloured spots. It's usually white, turning grey or yellow, and it can cover casing, mushrooms and pins1. Penn State lists it in the same guide as cobweb.
Dry bubble (Lecanicillium)
Dry bubble, once called Verticillium, shows up as bubble and spot symptoms1. Penn State says the most noticeable symptom is often a globe-like mass of mushroom tissue3. On commercial white button farms it's described as the most common fungal disease3.
Bacterial blotch
Bacterial blotch isn't a mould, so it won't be fuzzy. The pathogen forms lesions on the mushroom itself. They're pale yellow at first, then golden yellow or chocolate brown4. The colour is superficial, no more than 2 to 3 mm deep, and the tissue underneath can look water-soaked4.
Other names are brown blotch and bacterial spot4. Penn State names the pathogen as Pseudomonas fluorescens biotype G and lists Pseudomonas tolaasii as an outdated name4.
Flies
Sciarid flies are a concern in the mushroom industry. Adult sciarids don't feed on mushrooms, but they can carry mites, nematodes and mould spores3. Flies aren't a contaminant in themselves. They're a way contaminants travel, which is covered on why contamination happens.
Quick comparison
| Problem | What the sources describe |
|---|---|
| Green mould | Dark green patches on casing, starting grey1 |
| Cobweb | White, cotton-like weft, flesh-coloured spots1 |
| Bush mould | Bushy, hair-like, white turning grey or yellow1 |
| Dry bubble | Globe-like mass of tissue, or spots3 |
| Bacterial blotch | Yellow to brown lesions on caps or stems4 |
Limits
These descriptions come from commercial crops, mostly white button mushrooms. A home block of another species can look different, and several problems overlap. Sorting white fuzz from healthy growth is a separate question, covered in mould or mycelium.
Cited material
- Basic IPM Practices for Organic Mushroom Farms: Fungal Diseases of Mushrooms, Penn State Extension
- Trichoderma, Wikipedia
- Pests and Diseases, Penn State Extension
- Bacterial Blotch Disease, Penn State Extension
More notes
Mould or mycelium? How to tell them apartWhite fuzz on a mushroom substrate can be healthy growth or a contaminant. Here's what extension sources say to look for, and where colour alone misleads.
Why contamination happensWhere mushroom contaminants come from, how spores and bacteria travel, and why wet surfaces and shared equipment raise the risk, with extension sources.
Preventing contamination: clean habits that matterExtension and industry sources on cutting mushroom contamination risk: clean equipment, dry caps, careful watering, fly control and removing dead tissue.