March 2016 Plant Disease Advisory

As cool season lawns come out of dormancy in spring, it is not uncommon to see small, straw-colored patches of grass with a reddish or pink tinge. Grasses commonly affected include Kentucky bluegrass, tall and red fescue, and perennial ryegrass. The reddish color of the grass blades is due to the brightly colored fungal pathogen Laetisaria fuciformis.

Red Thread Mycelium and Sclerotia
Reddish colored mycelium and sclerotia of the red thread fungus on cool season turf.

Threadlike structures (mycelium) of the fungus form on the surface of infected leaves, and structures called sclerotia often extend in stringy masses out of the tip of the grass blade. This fungus prefers wet weather and cool temperatures of spring and fall. The disease kills only the leaves, so grass recovers when temperatures rise in early summer and conditions are less favorable for the fungus.

Red Thread on Turf
Red thread patches on cool season turf.

Because inadequate nitrogen favors disease development, supplemental nitrogen applications in spring can help alleviate symptoms. However, it is important to avoid applying too much fertilizer as this can favor development of other turf diseases. It is best to have soil tested and follow recommendations provided by a soil testing lab to determine fertilizer rates. Fertilizer applications should be timed as recommended for cool season grasses. Fungicides are usually not necessary for red thread control because grass greens up again in response to warmer temperatures and adequate nitrogen fertility.