Red thread is an extremely common turfgrass disease that can develop at any time of the year during cool, wet weather, but frequently appears most severely during winter. It can develop on most turfgrasses but ryegrasses, meadowgrasses and fescues appear to be more commonly affected.
This disease is often referred to as an indicator of low fertility and symptoms will often develop more severely if nitrogen or potassium is limited.
Red thread is caused by the fungus Laetisaria fuciformis. The initial symptoms of infection are water-soaked areas of leaf tissue, but these often go unnoticed. As the infection progresses, the infected leaves rapidly dry, become straw-coloured and appear as irregular patches across the sward. Patches can range in size from 5 to 50 cm in diameter and will often develop characteristic red needles (sclerotia) throughout the damaged area. The sclerotia are aggregations of countless strands of very pale pink fungal mycelium which grow out of the infected leaf tissue and become wound tightly together appearing pink or red when complete.
The development of these 'needles of mycelium' allows the infection to progress by enabling the fungal mycelium to spread over the turf under humid conditions. Once the sward is dry or the relative humidity around the turf is reduced, the needles become desiccated and brittle. They then become dislodged from the infected plants and fall to the base of the sward where they will remain until favourable conditions return.
The disease can be spread by infected clippings and direct movement of the sclerotia but the fungus also produces arthroconidia (fragmented mycelial strands) that can be windblown over long distances.
Red thread is almost invariably a foliar disease and although the causal fungus has the ability to enter and damage the crown tissues, it very seldom does. Because of this, the symptoms of the disease can frequently be reduced by light nutrient applications and removal of the diseased tissue by boxing off the clippings. However, there are increasing reports of this disease developing on turf that has been maintained under adequate nutrition and in such instances, symptoms will not be satisfactorily reduced by nutrient application alone. Where red thread regularly causes damaging symptoms, it would be worth considering over-seeding with grass cultivars that have been bred with reduced susceptibility to the disease.
Many of the fungicides that are currently available for use on managed amenity turf have shown efficacy against this turf disease and, where necessary, can be used as part of an integrated programme to manage red thread. Always ensure that the disease is correctly identified prior to the application of any plant protection product.
Dr. Kate Entwistle, The Turf Disease Centre