Why contamination happens

Contamination happens when a competing fungus or bacterium reaches the substrate or the mushrooms and finds food and moisture there. The substrate is meant to feed the mushroom, and it feeds other organisms just as well. The sources below describe commercial farms. Some routes below are named for one disease only, and the text says which.

Where contaminants come from

Trichoderma, the group behind green mould, is present in all soils. Wikipedia calls it the most prevalent culturable fungus in soil¹. So the spores aren't rare. They're everywhere, and a grower's job is mostly to keep them out of the substrate.

Bacterial blotch has a different source. Penn State says casing and airborne dust are the primary means of bringing the blotch pathogen into a mushroom house². It adds that the bacterium is probably present in most casing material, even after pasteurisation². Casing is the thin top layer put over some substrates, and pasteurisation means heating it to kill most organisms.

How they travel

A contaminant that's in the room still has to reach the crop. Sources list several carriers, usually called vectors.

• Air. In its cobweb entry, Penn State says cobweb spores are readily distributed in air currents, and very little air movement is needed to dislodge spores that can then be carried considerable distances³. The source makes this statement about cobweb only.
• Water. In the same cobweb entry, Penn State says cobweb spores are spread by watering³. For blotch, the bacteria spread by splash during watering².
• People and tools. For green mould the listed vectors are people, equipment and plastic³. For blotch, tools used by pickers are a route too².
• Flies. Adult sciarid flies can carry mites, nematodes and mould spores⁴. Female phorid flies are drawn in by the odour of growing mycelium⁴.
• Moved equipment. A UK industry guide says equipment moving between facilities should be cleaned, disinfected and rinsed⁵.

Why some grows fail and others don't

Spores alone don't decide it. Conditions do.

Wet surfaces matter for bacteria. Blotch spotting is typically seen wherever mushroom caps stay wet for 4 to 6 hours or longer after water has been applied². In its cobweb entry, Penn State says to avoid wet mushroom caps or high relative humidity (95 to 100 percent)³. It adds that germination occurs in hours at 60 to 65°F³. These figures are given for cobweb only, not for contaminants in general.

Timing and numbers matter too. Penn State says the appearance of an infection depends on its timing and the number of spores³. A UK guide for large farms picks out the 3 to 5 days after filling compost as a critical period to watch⁵.

Competition decides the rest. Wikipedia says infected substrate leads to lower yields because the contaminant competes with the mushroom¹. If the mushroom mycelium has the substrate to itself, there's nothing to compete with.

Exceptions

None of these sources study a kitchen-table grow. The humidity and temperature figures are for commercial crops, mostly button mushrooms, and a different species may behave differently. What can be done about each route is on preventing contamination.