Red Thread
Laetisaria fuciformis
Red thread is a fungal turf disease that causes pink-red patches in cool, wet, humid weather, mainly on undernourished lawns.
Red thread is caused by the fungus Laetisaria fuciformis, which attacks the leaf blades of turf grasses rather than the roots or crown. The most recognisable feature is the fungal growth itself: fine, needle-like or antler-shaped strands of pink to red gelatinous fungal tissue (called stroma) that extend from the tips of infected grass blades. These threads are usually 1-2cm long and are easiest to spot in early morning when the lawn is damp, or by parting the grass and looking closely at blade tips. In humid conditions, a pale pink, cobwebby mycelium can also be seen covering affected patches.
The disease shows up as roughly circular patches of blighted, bleached or pink-tinged grass, ranging from a few centimetres to half a metre or more across. Patches often have a water-soaked, dull appearance at first before fading to light tan or straw colour as the leaf tissue dies. Unlike many root and crown diseases, red thread rarely kills the grass plant outright; it damages the leaf blades, so lawns usually recover once conditions change and the grass resumes active growth.
Red thread favours cool, humid, overcast weather with extended leaf wetness, so in Australia it's most common in autumn, winter and spring in temperate and cool coastal regions, particularly on cool-season grasses such as ryegrass, fescue and bentgrass. It's less common on warm-season turf but can still occur on kikuyu or couch during cooler, damp spells. The disease is strongly linked to low nitrogen fertility, so lawns that are underfed, or coming out of a period of slow growth, are far more susceptible.
Management is mostly about correcting the conditions that favour the fungus rather than treating it directly. Maintaining adequate, balanced nitrogen fertility is the single most effective long-term control, since well-fed, actively growing turf can usually outgrow the disease. Improving air movement and light penetration, avoiding late afternoon or evening watering, mowing at the correct height and removing excess thatch all help reduce the leaf wetness the fungus needs. Collecting clippings from infected lawns during an active outbreak can help limit spread, since the fungal strands and thread-like structures can be carried in clippings, on mower blades and on shoes.
How to identify
- Patchy discolouration: irregular, roughly circular patches of pink-tinged, bleached or straw-coloured grass, from a few centimetres to 50cm or more across.
- Pink or red thread-like growth: fine, coral- or antler-shaped strands of fungal tissue extending from the tips of grass blades, most visible in early morning dew or humid weather.
- Cobwebby pink mycelium: a faint pink, web-like coating may be visible over affected patches when conditions are very humid.
- Leaf blight without dead crowns: affected leaves die back but the plant's crown and roots usually stay alive, so the lawn tends to recover with new growth rather than leaving bare, dead patches.
- Field check: kneel down and part the grass in an affected patch, looking closely at leaf tips for the small pink or red threads; they're a reliable diagnostic feature not seen with most other lawn diseases.
- Lookalikes: pink patch (caused by a different fungus) produces similar pink, patchy symptoms but lacks the distinctive thread-like fungal strands and tends to have a more ragged, doughnut-shaped patch outline. Dollar spot causes small, sunken, straw-coloured patches with an hourglass-shaped lesion on individual blades, but no pink thread structures. Nutrient or drought stress can cause general lawn discolouration but won't show the fungal threads on close inspection.
