How to Deal With Red Thread on Your Lawn

Red thread grass
Kris Lord / Flickr / CC By 2.0

Does your lawn have what appears to be dying patches with pink or reddish fibers in them? This may be the telltale sign of red thread. Red thread is a turfgrass disease most commonly caused by low levels of nitrogen in the soil. This disease typically appears between late April and mid-June but can show up during most of the growing season. Its most active times are in the months of May and June when nighttime temperatures are hanging around 70 F and lawns have grown aggressively, using up most of the nitrogen in the soil.

Susceptible Grasses

Cool season grasses tend to be the most susceptible to red thread. Common varieties include red fescue, ryegrass, Kentucky bluegrass, and bentgrass. Some turf varieties are naturally thread-resistant. These include " Bilart" and "Claudia" fine fescues, "Chateau" Kentucky bluegrass, and "Pennant" perennial ryegrass.

Does Red Thread Kill Grass?

While affected areas of grass appear to be dead, red thread doesn't kill grass. The fungus that infects the turf lives in the thatch and soil and can be spread by dead infected plant material and by mowing and other mechanical maintenance. The pink growths that appear in the grass do not infect the crown or the roots of the plant, so the grass is not killed by the fungus.

Conditions Favoring Red Thread

Red thread can occur in many different climates but is more common in areas with high rainfall or high humidity in late spring to summer. The primary conditions favoring red thread are:

  • Low levels of nitrogen
  • Temperatures between 68 F and 75 F 
  • High humidity

Identifying Red Thread

Red thread is a growth of the fungus Laetisaria fuciformis and appears in two forms on the blades (leaves) of infected grass. One form has threadlike branches, sometimes compared to antlers, called sclerotia. The other form appears as fuzzy pink clusters of mycelium. Affected areas typically are round and range from 4 to 8 inches in diameter. The grass is usually tan or light brown beneath the red thread growths.

Red thread is commonly confused with a disease called pink patch. It also has a similar appearance to pink snow mold and dollar spot, and all of these diseases can appear at the same time of year. However, red thread is positively identified by its antler-like structure or mycelium clusters.

Why Your Lawn May Be Affected

Almost any lawn is susceptible to the red thread turf fungus. However, it is not uncommon for some lawns to have more issues than neighboring properties, due to different soil conditions, maintenance, and water patterns. The cause of red thread and other turf diseases can be understood by looking at the factors of the disease triangle. This is used to illustrate how fungi grow on plants.

The three factors in this disease triangle are the susceptible host, the pathogen or disease itself, and an environment favorable to disease growth. The hosts are grass plants that are lacking in nitrogen and are experiencing slower growth than in prior months. The pathogen is present in the thatch layer of a lawn and is impossible to completely remove. When the plant is weak, and the temperature and relative humidity are favorable for fungal growth, red thread will appear and continue to grow.

Treatment for Red Thread

Property owners often wonder what can be done to get rid of red thread on lawns when it shows up each year. The remedy is to disrupt the disease triangle so that the fungus stops growing. Since it’s not possible to remove the pathogen or control the weather, the most effective way to manage red thread is to address the susceptible host plant and strengthen it with fertilizer.

The basic treatment for red thread is to fertilize the lawn with the proper amount of nitrogen as part of an ongoing feeding program. Nitrogen applied to the lawn in the fall will nourish the re-emerging turf in the spring, but much of that nitrogen will be gone by late spring. At that point, it's often effective to apply 0.2 pounds of nitrogen per 1,000 square feet of lawn, and this can be adjusted for your soil and climatic conditions. Be aware that it may take two years or more of careful feeding to prevent red thread from coming back in the spring.

Treating red thread with chemicals is generally not necessary and not recommended for residential lawns, but chemicals are a viable option for extreme cases. This involves the application of a fungicide, usually a type of Qol inhibitor (strobilurins). If you're interested in treating red thread with chemicals, consult a lawn-care professional for recommendations.