Red thread in lawns is caused by the fungus Laetisaria fuciformis. This fungal disease commonly affects turf grasses, especially when the grass is deficient in nitrogen and growing slowly due to low temperatures, drought, or inadequate fertility

. Key causes and contributing factors include:

• Nitrogen deficiency: Lawns with low nitrogen levels are more susceptible because the fungus attacks nutrient-poor, slow-growing grass

• Environmental conditions: Red thread typically develops in cool temperatures between 40°F and 75°F (about 4°C to 24°C), with prolonged wet, humid weather, especially in late spring, summer, and autumn

• Excess moisture and high humidity: Prolonged leaf wetness from rain, dew, fog, or overwatering encourages fungal growth

• Poor lawn care: Excessive thatch, poor fertilization, drought stress, limited sunlight, and heavy foot traffic can increase vulnerability

• Grass species: Cool-season grasses such as red fescue, perennial ryegrass, Kentucky bluegrass, tall fescue, fine fescue, and bentgrass are particularly prone to red thread infection

How the fungus spreads: The fungus survives unfavorable conditions on infected leaves or thatch and spreads through mycelium growth rather than spores. It can be transmitted by water, wind, and contaminated lawn equipment

. In summary, red thread is caused by the fungus Laetisaria fuciformis , which thrives on nitrogen-deficient, slow-growing turf under cool, wet, and humid conditions, spreading through mycelium and affecting mainly cool-season grasses