The most damaging fairy rings are slowly spreading colonies of the fungus Marasmius oreades, which lives amongst the roots of turf, altering the appearance of the grass and producing toadstools at certain times of the year, mainly in late summer and autumn.

Other colonies of fungi cause fairy rings as well, but most have no effect on the turf, or even seem to enhance the growth.

All types of lawns and areas of rough grass can be affected by fairy rings.

Symptoms are variable, depending on the fungi responsible. Most have virtually no effect on the turf and are only noticed if toadstools appear.

You may see the following symptoms:

• An irregular ring or part-ring of dead grass gradually expanding across the turf, caused by Marasmius oreades
• Immediately adjacent to the dead grass, the grass may be greener than usual
• Crops of brown toadstools at certain times of the year. Those of M. oreades are light brown, with a domed cap and a thin but tough stalk

Non-chemical control

The toadstools produce huge volumes of airborne spores, but there is little point in removing them from the lawn to try and reduce the risk of infection, since the spores can travel long distances and are likely to arrive from elsewhere anyway, even if this is done.

In principle, excavating the active ‘front’ of the colony (ie the ring) and replacing with fresh soil would remove the ring. In practice this would mean removing turf and soil to at least 30cm (1ft) depth, replacing with fresh soil and returfing or reseeding, which is usually impractical, particularly for large rings.

Where dead patches of grass occur, spiking and watering to break up the water-repellent properties of the fungal colony is helpful, as is feeding. Wetting agents are also sometimes used to aid water penetration into the affected area.

Chemical control

No fungicides are available to gardeners for fairy ring control. Some lawn maintenance companies offer treatment for diseases with professional fungicides, but there is strict legislation controlling the application of professional products to domestic gardens. You will therefore need to satisfy yourself that the company is operating within the law.

The fungi, which are spread by airborne spores liberated from the toadstools, form colonies below ground in the root zone of the turf. Most cause little damage and are only noticed when they produce fresh toadstools.

Marasmius oreades is the most important fungus which forms fairy rings. Its mycelium spreads gradually outwards, dying out in the centre, so that the colony is a roughly circular advancing ‘front’ of fungus growing through the soil.

The extent, if any, to which the fungal mycelium actually kills grass roots directly is unclear. However, the mycelium is water repellent and patches of water-stressed dead grass may appear in drier weather. As the dead grass decays it liberates nutrients, so that greener-than-usual areas occur next to the dead ones. Since the colony is an expanding front it is approximately ring-shaped, and therefore when conditions are suitable for toadstool production, these also appear in rings.

Colonies expand by about 30cm (1ft) per year, and in large areas of turf may exist for hundreds of years.