How to recognise it
This produces a ring of toadstools which is bell shaped at first but later flattens out to be 5cm wide. The gills are thick, widely spaced and white turning buff as the toadstool ages. The mycelium (fungal strands) inside the ring of the infection exhaust the nutrients in the soil and dies, forming an impervious layer depriving the grass roots of moisture.
Just outside the ring the grass grows strongly, probably due to the nitrogen released by the fungal ring when it breaks down organic matter. This gives rise to the ‘fairy ring’ which the disease is commonly named after. The rings can grow in radius by up to 30cm per year and can continue to expand for several hundred years.
Why it’s a problem
The ringing effect is unsightly, but other than this it causes no problems.
Where you are likely to find it
Lawns and other areas of grass.
How to prevent it
Nothing can really be done to prevent this.
How to get rid of it
Excavating and removing the soil from the affected area can reduce the infection, but this would require substantial earth movements to thoroughly remove all trace of the fungus (including mycelium) within the soil.
If the grass in the centre of the ring is severely affected then aerating the soil (eg by spiking) and feeding the grass with extra fertiliser can relieve the symptoms.
Is it good for anything?!
Some of the toadstools produced can be edible, but this shouldn’t be tried without expert advice as others are poisonous.
Other useful information
Folklore suggests that fairy rings are the gateway to elfin kingdoms or, perhaps, places where elves meet and dance and, therefore, where they can be captured.