Red thread lawn disease (laetisaria fuciformis)

By | 17/01/2018
Red thread fungus infecting grass

Red thread (laetisaria fuciformis) is a parasitic fungal disease which infects grass. It can appear in conjunction with pink patch lawn disease, causing similar symptoms.

A circular patch of red thread (laetisaria fuciformis) fungus on a lawn
A patch of red thread fungus on a lawn

The disease causes areas of grass to discolour. These are usually circular, but they can coalesce into larger areas. These patches can appear over a few days, and may stay for several weeks. The patches will seem to be in random areas on a lawn.

When examined, the grass looks necrotic, with leaves knitting together. The red cotton-like growths protruding from dead grass leaves. In severe cases, this can give the lawn a red tinge.

Red thread is harder to identify than pink patch. The fine, translucent, thread-like growths be up to 1 cm long and a subtle red colour. The early stages of a red thread infection can be difficult to see if you don’t know what your are looking for.

Which grass species does red thread infect?

Slender and strong creeping red fescues (festuca ssp.) are most at risk from infection. These fescue grasses are often a common species in turf. Recently laid fescue turf is at a very high risk of pink patch and red thread infection.

Perennial ryegrass (lolium perenne sp.), bentgrasses (agrostis spp.) and poa (poa sp.) can also become infected. Red thread is a big problem on bowling greens, cricket squares and golf courses.

When is the grass most at risk?

Red thread is a parasitic fungal infection which thrives in mild and damp conditions. It flourishes in an air temperature of between 15 and 24°C. It can infect grass at much lower temperatures if the grass is weak and the air is damp. Mild winters can increase cases of red thread infections.

In England it is most often a problem in a damp spring and summer.

Overfeeding with nitrogen (especially in autumn) can cause weak, unsustainable growth in grass. This can leave it vulnerable to red thread infection. Sustained wet soil and low light levels which weaken grass can also contribute.

Red thread fungal disease life cycle

Red thread is a fungus and reproduces through releasing microscopic spores. These spores float on the wind until they land on your lawn.

Red thread fungus (laetisaria fuciformis)
Red thread fungus infecting grass

Once they’ve found a suitable plant to infect they grow mycelium. These are root-like structures which seek out nutrients for the fungus. They enter the grass plant through natural openings and cut leaves. These mycelium fibres grow between leaves and plants, spreading the infection. This is why infected areas appear circular, spreading from the initial infection point.

The fungi overwinter as a dried gelatinous mycelium. It will hibernate in the thatch of the lawn on infected dead leaves or in clipping debris. Ready to re-emerge when the conditions are correct.

It can also spread through physical transportation around and between gardens. Infected debris on a mower will spread the infection around a lawn and to other lawns. It can stick to boots and tools, ready to infect when it finds vulnerable grass plants.

If you have a weak, vulnerable grass species it is almost impossible to avoid if the infection is in your area.

Once you notice the disease it will already be in its advanced stages. The red growths only appear once the fungus has taken hold inside the grass plants.

Fungal filaments on grass leaves
Distinct fungal filaments on grass leaves.

Does the fungus harm the grass?

Most red thread infections only result in cosmetic damage to the grass. Most grass species will recover and grow new leaves in a few weeks once the infection has gone. This is unlike fusarium patch which can kill grass.

Severe, prolonged attacks will damage grass to an extent that the grass may die due to lack of leaf growth. But that is only in very extreme circumstances. Environmental factors often reduce the infection long before the grass dies.

Control of red thread disease

Good selection of fungal resistant grass species will reduce the chance of infection. This means avoiding turf grass which contain fescues and sowing modern grass seed varieties. Cheap grass seed often contains grass species which are not disease resistant.

Also keeping the lawn as healthy as possible will reduce the chance of attack, but will never prevent it 100%. Never overfeed, or apply high nitrogen feed in the autumn or winter.

Keeping lawn thatch under control and maintaining good horticultural hygiene will also help. Keep your mower clean and wash your boots after walking over infected areas.

The best way to control an established fungal infection is to apply a fungicide. Modern lawn fungicides can be very expensive, but you get excellent results. Contact your local lawn care technician for advice on applying lawn fungicides.

Red thread is one of the most common lawn diseases I see. If you find your lawn has an infection, don’t panic. Contact you local lawn care technician who can treat it for you. It is very unlikely to damage your lawn in any serious way.

Further Reading

A lawn infected with red thread fungus
A overall look at a lawn infected with red thread fungus

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