Red thread

What you are looking at

Patches of the lawn go pale, bleached to a straw or tan colour, with ragged edges rather than clean circles. They run from the size of a coin to a dinner plate and they join up.

Kneel down and look at the individual blades. Fine threads of a pink or coral red stand out from the leaf tips, a few millimetres long and branched at the end like miniature antlers. After rain, the same fungus produces pale pink cottony flocks that bind blades together.

Part the grass at the base of a patch. The shoots down there are green and alive, which is the key to the whole page.

Ryegrass and fescue lawns show it most, and a mild damp spell brings it on. Britain gets the worst of it from early summer through to a wet autumn.

It is telling you the lawn is hungry

Red thread is a disease of grass that is short of nitrogen. A well-fed sward grows away from it faster than the fungus can work up the leaf, and an underfed one does not.

The consequence is unusual on this site. The treatment for red thread is a bag of lawn fertiliser. Feed the lawn in spring or summer and the patches grow out within a few weeks, and it stays gone as long as the lawn is fed.

Everything else on this page is about how long the grass stays wet, since the fungus needs prolonged leaf wetness to spread. Thatch, compaction, shade, still air and evening watering all lengthen that period, and dealing with them lowers how often the problem returns.

The other thing worth saying is that it kills nothing. The fungus attacks the leaves and does not touch the crown or the roots, so a lawn covered in it in August is a full lawn again by October. Nothing needs reseeding and nothing has been lost.

The one that does kill grass

Fusarium patch, also called snow mould, is the lawn disease worth telling apart from this, because the advice is opposite in one important respect.

It makes small brown or yellow patches that darken and merge, with white or pale pink cottony growth at the edges in damp weather. It works mainly from autumn through to spring, often after snow or a long wet spell, and it does kill the grass, leaving bare ground that has to be reseeded.

Where red thread comes from too little nitrogen, fusarium patch is encouraged by too much of it applied late in the season. Feeding a lawn heavily in autumn produces soft growth going into winter and invites it, which is why autumn lawn feeds are formulated low in nitrogen and high in potassium.

Feed for growth in spring and summer. Feed for hardiness in autumn.

What to do about it

Apply a balanced lawn fertiliser and wait a few weeks.

Scarify out the thatch and aerate anything compacted.

Cut back what is shading the lawn and let air move across it.

Water in the morning if you water at all.

Mow regularly and box off the clippings while the patches are visible.

Brush the dew off on still mornings.

Overseed a repeatedly affected lawn in September with improved ryegrass cultivars.

Do not go looking for a fungicide. None is available to gardeners in Britain for this, and none is needed.

What it gets mistaken for

Drought browns the whole lawn together, worst over buried rubble, pipe runs and the thinnest soil, and there are no threads on the leaf tips.

Leatherjackets and chafer grubs kill the grass from below, so the turf lifts away like a mat with no roots holding it, and birds and badgers tear the patches up to get at them.

Dog urine leaves small dark green rings around a dead brown centre.

Fairy rings produce dark green arcs of lush growth, often with toadstools, and sometimes a bare ring between.

Fusarium patch kills, works in the cold half of the year, and follows too much autumn nitrogen rather than too little.