Red Thread

red thread fungus


Red Thread

What is red thread?

Red thread is a fungal infection caused by the fungus Laetisaria fuciformis. Most lawns, good and bad, will have it at some stage, and it is easy to spot although it stands out much more on a better-quality lawn. Like any plant disease, however, red thread is a sign of stress on the plant, and hence a really good indicator to the health of your lawn grasses.

How can I identify red thread?

Red thread attacks the grass in two stages and is visible in both. In the first stage you simply see a reddish tinge to leaf blades, and in the second you’ll spot cotton wool-like mycelium (see pictures).

DHG red thread
Reddish tinge visible in lawn
red thread fungus
Dead leaf blades with red thread
red thread disease on a lawn
Red thread close-up

What causes red thread?

Red thread is a disease encouraged by humid conditions, especially on a poor lawn (for example, lacking nutrition). Any fluctuation between warm and cool temperatures and wet and dry periods can cause the disease to flare up. And of course, our weather today sees plenty of these sudden changes, giving the disease a lot more opportunity to take hold.  

When does red thread appear?

In the past you would have seen it in humid periods any time from March to October. However, with today’s changeable weather patterns, this ‘summer disease’ can now occur at almost any time in the year in the UK. And when the grass is naturally under more stress (winter for example, with cold, wet, frosts etc), it will be harder to cure the red thread or lessen the aesthetic disturbance. 

Does red thread mean I have a poor lawn?

No, not necessarily.  Look carefully at some of the healthiest grass at the world’s finest sports venues and you may well spot red thread!

Will red thread kill my grass?

It won’t kill the grass as it does not affect the crown of the plant. Given favourable conditions or intervention from lawn-owners the grass will survive and return to health. It does, however, look unsightly as it affects a large number of the individual leaf blades.

Can I prevent it from coming in the first place?

Red thread is primarily an environmental symptom, not the result of how well you look after your lawn. So, whilst keeping a lawn healthy will certainly give it the best chance of recovery, with even the healthiest grass there is no guarantee you will avoid red thread in the first place.  

Can I make an infection worse?

If you allow it to develop to the second stage of the disease, the ‘mycelium’ or pink cotton wool-like spores are easy to spread with your feet, your mower or even your animals, so yes, you can make it worse. Much better is to spot the red-tinged leaves and intervene early to slow down the spread.

What can I do about red thread?

There are a number of things that can help, aid or cure. 

Firstly, the best cure is nature itself.  If the weather is stable enough, the disease will clear it up by itself. However, this is not a reliable solution for any disease caused by fluctuations in weather.

The next best option is simply a small dose of ferrous sulphate or sulphate of iron.  It’s what we refer to as a ‘moss killer’.  What this does is simply to dry the leaf blade out, making it hard for the fungus to survive. This won’t ‘cure’ the disease, but it can encourage nature do the rest.

People often think that feeding a lawn is a cure, but this really only works for full-time lawn or sports-turf professionals. An an application of nitrogen, especially when the symptoms are still around, can actually exacerbate the problem enormously, so you would need to keep a vigilant daily watch on your lawn.

If you want to attack it directly, you need a fungicide treatment, but these are only available for licensed professionals to use on your behalf.  It is a viable investment, however, as it will cure the disease for anything from 4 to 12 weeks, which can be long enough to not see the disease again (depending on time of year).


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