The term 'fairy ring' is used to describe a number of situation where soil-borne fungi cause certain symptoms to develop on the surface of the sward. Fairy rings are cause by the activity of many fungi classified as basidiomycetes.
Not all basidiomycete fungi cause fairy rings but all fairy rings are caused by basidiomycete fungi!
Each if the individual fungi grow through the rootzone, feeding off the organic material as it increases in size. The effects seen on the turf surface are roughly correlated with the amount of fungal mycelium within the rootzone. Since the fungal mycelium is made up of compounds that naturally repel water (in the same way that our fingernails do), the more mycelium that is present, the more water that can be repelled by it. Therefore, the larger the amount of fungal mycelium, the more severe or noticeable the effect on the turf.
We arbitrarily classify fairy rings in to four types: Type 1, Type 2, Type 3 and superficial fairy rings (or thatch fungi). Within each of these four groups there may be many individual fungi that cause fairy ring development, but within each group the symptoms that these fungi cause will be very similar.
Type 1 fairy rings typically have a ring of dead grass which is bordered on both sides by a band of stimulated grass growth. Fungal fruiting bodies (mushrooms or toadstools) may or may not be found associated with these rings.
Type 2 fairy rings are typically a ring of stimulated grass growth with or without the fungal fruiting bodies.
Type 3 fairy rings generally show no change in the growth of the grass but are evident only by the presence of rings of fungal fruiting bodies.
Superficial fairy rings are caused by fungal activity in the thatch (hence, thatch fungi). They can cause rings or patches of yellowed or greened-up turf and may or may not be associated with slight depressions of the turf surface. Fungal fruiting bodies will not be seen with superficial fairy rings.
In many cases, the presence of these fungi can be confirmed by smelling the rootzone which will take on a characteristic musty smell.
Although all fairy rings will start from a single point and increase in size annually, eventually forming recognisable rings, the growth of these rings will not stop unless the fungus hits a solid surface or disturbed ground. As rings grow together (or coalesce), the definite ring pattern will change to become broken rings, arcs or ribbons across the turf surface.