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Snow mould is caused by fungi which are active at temperatures just above freezing, and in moist conditions. It therefore often becomes apparent as the snow melts and exposes the grass, but it can also occur if anything else, such as fallen leaves, covers the grass. It is partly similar to the disease Fusarium, in that the same weather conditions are responsible for its development.
The problem is caused by the combination of prolonged cold weather slowing the grass plant’s metabolism, and the lawn being covered, preventing sufficient exposure to air.
The symptoms are roughly circular of dead and dying grass leaves, which become matted. In very severe cases the patches may join together, and therefore not be recognisable as circular.
Grey snow mould usually only occurs after prolonged snow cover, and is therefore not common in most parts of the UK. Only the blades of the grass are affected, and therefore the grey variety is not regarded as being a serious problem; the lawn will recover quickly, even if the damage appears extensive. One way of identifying the mould as being the grey variety (other than by the colour) is the presence of a substance called Mycelium, which shows on the grass leaves as tiny brown or black specks.
However, pink snow mould is more serious, as it can damage the grass roots. It is identified by having patches of 2-3” wide brown circles around a pink or straw-coloured centre, with web-like strands of a different strain of Mycelium, which originally look white, but go a faint pink colour as they mature.
The Mycelium produced by both varieties will disappear quite quickly if the lawn is given a chance to dry out.
Treatment should start with aeration (spiking) which will drain the lawn and accelerate the drying process. Once the lawn has dried out, an application of a systemic fungicide is recommended.