Superficial fairy rings (also known as thatch fungi) are most common during the summer and autumn months on lawns in the UK.
Normally they have little effect on the on the grass sward, however in certain cases a yellowing ring maybe seen (see photo) caused by plant stress. Fruiting bodies such as toadstools and mushrooms are not associated with superficial fairy rings.
If a soil sample or a core is taken from the lawn, white mycelium accompanied with a strong musty smell may be present.
In some cases where a degree of thatch has accumulated there may be some undulations in the lawn (thatch collapse) hence the name thatch fungi.
The undulations or depressions in the lawn are caused by the fungi breaking down the organic matter in the soil.