This disease is caused by the fungus Microdochium nivale (formally Fusarium nivale). It is one of the most damaging diseases of turf grasses and can be difficult to control. It is found most frequently during autumn, winter and early spring, but attacks can occur at any time of the year.

The disease is sometimes very noticeable after thaws of snow, when it is given the common name of snow mould.

Fusarium patch is particularly troublesome on annual meadow grass (Poa annua), but can also affect bents (Agrostis species), fescues (Festuca species) and perennial rye-grass (Lolium perenne).

What to look out for;

• The disease is first noticed as small patches of yellowish, dying grass that later turn brown
• Patches increase in size and may reach 30cm (12in) or more in diameter, often merging together so that large areas can be affected
• During wet conditions a white or pinkish, cottony fungal growth may be noticed, particularly at the margins of the patch. This is not to be confused with another fungal disease called red thread, or with slime moulds in turf

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Non-chemical control

• Ensure that the lawn dries rapidly after dews or rainfall by improving the aeration and drainage. This can be done by a combination of scarifying, spiking and hollow- or solid tining
• Improve general airflow over the lawn by pruning back overhanging trees or shrubs
• Remove heavy dews in the morning with a switch (a long, pliable rod) or bamboo cane
• Avoid high doses of nitrogen fertiliser in late summer or autumn – use a proprietary autumn lawn feed instead

Chemical control

No fungicides are currently available to amateur gardeners for the control of diseases of lawns.

Some lawn maintenance companies offer treatment for certain 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 fungus can survive temperatures as low as -20°C (-4°F), although the optimum temperature for growth is 12-19°C (54-66°F). Infection can, therefore, occur in summer but is usually masked by rapid grass growth. The disease becomes more apparent as the weather cools and grass growth slows. The fungus requires surface moisture to infect leaves, and grass that is particularly lush due to high nitrogen levels is more prone to attack. Alkaline soil conditions also favour the disease.

Snow mould develops under prolonged snow cover. The combination under snow of a fairly constant temperature of about 0°C (32°F) and increased levels of carbon dioxide are particularly favourable for Microdochium nivale. Another fungal disease called grey snow mould, caused by Typhula incarnata, can also develop under these conditions. The patch symptoms are similar but any fungal growth seen is greyish-white rather than pink. Typhula also produces tiny, brown, hard resting bodies, often looking like small seeds, scattered over the affected grass.

Fusarium patch is spread across the lawn by the transfer of spores of the fungus or infected grass debris on equipment or shoes.