Watery stipe or water soaked stem is a phenomenon that often occurs from the second harvesting day in the first flush. The problem is often greater in a heavier flush than in a lighter one. Contrary to the idea of many growers watery stipe occurs on casing soil that is too dry and not on wet casing soil! During harvesting the compost temperature rises and more water is transported to the mushrooms by the mycelium. The casing soil dehydrates during harvesting as the first flush takes most of its water from the casing soil. The drier casing soil makes evaporation more difficult and the only possibility is to shed water is via the mushrooms. This results in water accumulating in the stem.
There are several ways growers can prevent this problem. Selective harvesting helps prevent the mushrooms being too tightly packed on the third day and will keep the rise in compost temperature in check. Spraying the mushrooms in the first flush can also help. This keeps the casing soil damper and encourages evaporation through the casing soil. Spraying should only be done if a slight increase in compost activity has been noticed.
Henk van Gerwen, AdVisie ‘The mushroom cultivation advisors’