The cause of bacterial blotch - the Pseudomonas tolaasii bacteria - is always present on casing soil and usually on the mushrooms too.
Under favourable conditions this bacteria will develop rapidly and the brown blotches will already be visible within the space of a few hours.
Most blotches are light brown shiny and feel sticky to touch. Other symptoms are little sunken spots that are often confused with green mould on the cap.
Extreme quantities of mycelium in the casing soil combined with loss of moisture during the 1ste flush creates a casing soil that is difficult to bring back to the ideal humidity level after the 1ste flush.
The mycelium will have become very hard, making it problematic to transfer nutrients from the compost to the mushrooms in the second flush. The growth conditions for good mushrooms will be compromised increasing the risk of bacterial blotch triggered by the tiniest of climate mistakes at a further stage in production.
Confront the potential problem right from the beginning by using rougher textured casing soil at filling and allowing less mycelium to develop in the casing soil.
Helpful methods include filling with rougher textured casing soil compacting the casing soil less applying less caccing adjusting the sprinkling pattern quicker blow down etc.
Jos Hilkens, AdVisie 'The mushroom cultivation advisors’