Red thread disease

Red thread is one of the most recognizable lawn diseases because of its striking visual symptom: pink to red thread-like mycelium visible on leaf tips, particularly in low-light or overcast conditions. It's also one of the most mismanaged lawn diseases -- homeowners see the pink threads, conclude.

Red thread is one of the most recognizable lawn diseases because of its striking visual symptom: pink to red thread-like mycelium visible on leaf tips, particularly in low-light or overcast conditions. It's also one of the most mismanaged lawn diseases — homeowners see the pink threads, conclude something serious is happening, and apply fungicides when a light nitrogen application is usually all that's needed.

Pathogen and host range

Per Cornell Cooperative Extension, red thread is caused by Laetisaria fuciformis. It affects all common cool-season grasses, with perennial ryegrass and fine fescue being most susceptible and Kentucky bluegrass slightly less so. Tall fescue is less affected.

The disease is cosmopolitan — found throughout North America, Europe, and wherever cool-season grasses are grown in humid conditions.