Red thread is caused by the fungus Laetisaria fuciformis. This disease occurs on turf that is growing slowly due to low fertility, lack of sunlight, drought, or weed pressure.
Symptoms of red thread appear as roughly circular, pinkish-red to tan patches, 2-4 in. in diameter. The red thread pathogen produces pink to red, thread-like structures (sclerotia) which can be seen on infected leaves with the naked eye or a hand lens. In dry conditions, these structures become brittle, are easily detached, and may be spread by mowers or fall into the thatch, where they remain viable for up to two years. Under humid conditions, pink, cottony flocks of mycelium may be produced.
Red thread occurs on Kentucky and annual bluegrasses, perennial ryegrass, and fine-leaf fescue. Disease development is favored by cool, wet weather, heavy dews, or fog. It tends to occur in the spring, but may persist into summer if conditions remain conducive to fungal growth. Symptoms are limited to the plant foliage and individual blades die from the tips down. Dead leaves may be mixed with healthy leaves, which can give the turf an overall scorched or ragged appearance.
L. fuciformis survives unfavorable conditions in infected debris, or in the thatch. Reproductive structures are spread by water, wind, equipment, and human activity. They require a film of moisture on leaves for germination and infection. The pathogen is capable of growth over a wide range of temperatures.
Cultural Management
- Maintain adequate and complete fertility (potassium, phosphorus, calcium as well as nitrogen) as determined by soil tests.
- An application of water-soluble nitrogen reduces disease severity and is generally the only treatment needed in residential lawns.
- Maintain soil pH between 6.0 and 6.5.
- Water deeply and infrequently while preventing moisture stress.
- Avoid late afternoon, evening, and light, frequent irrigations which extend periods of leaf wetness.
- Control weeds.
- Prune trees and shrubs to improve air circulation and increase light penetration.
- Keep mower blades to sharp to minimize leaf wounding which can enhance infection.
- Collect grass clippings when the disease is active or turf is growing slowly to reduce the amount of inoculum that is incorporated into the thatch.
Management with Fungicides
Fungicides are available; however, fungicide applications are seldom necessary for management of red thread in residential settings.
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