Red Thread is a common turfgrass disease caused by the fungus Laetisaria fuciformis. This pathogen infects the grass blades, leading to a noticeable decline in the lawn’s appearance. While the infection can make a lawn look sickly, it is generally considered a cosmetic issue that rarely kills the entire grass plant. The fungus primarily attacks the leaf tissue, meaning the crown and root systems usually remain undamaged. Recovery is often expected with proper lawn care.
Visual Identification and Impact
The presence of Red Thread is first noticed as small, irregular patches of light tan or bleached turf within the lawn, ranging from a few inches up to two or three feet in diameter. The overall pinkish or reddish tint of the patches can sometimes be mistaken for other diseases, such as pink patch or dollar spot.
The most distinguishing sign is the reddish or pinkish threads that give the disease its name. These threads are the fungus’s mycelium or antler-like structures called sclerotia, which are bright coral-red when moist. These structures are often visible on the tips of the infected grass blades and can sometimes bind them together.
Specific grass types, including perennial ryegrass and the fine fescues, are more susceptible to this fungus. The infected leaf blades begin to wither from the tip down. The widespread discoloration and straw-like appearance of the patches can severely detract from the lawn’s uniform green color.
Conditions That Encourage Red Thread Development
The disease is most active during cool, mild weather, typically when temperatures are between 60°F and 75°F, conditions often occurring during the spring and fall seasons in many regions.
Prolonged periods of leaf wetness also contribute to the disease’s development. High humidity, extended morning dew, and excessive rainfall create an ideal environment for the fungal spores to germinate and infect the grass blades. Poor air circulation that prevents the grass from drying out quickly can exacerbate this issue.
Red Thread is strongly associated with turf suffering from poor soil fertility, particularly a deficiency in nitrogen. Nitrogen is a necessary nutrient for vigorous leaf growth, and when it is lacking, the grass plant becomes weakened and stressed.
Effective Management and Prevention
Managing Red Thread involves cultural practices that address grass stress and nutrient deficiency. The most effective non-chemical strategy is implementing an adequate nitrogen fertility program. Applying a supplemental source of nitrogen, such as 0.2 pounds of nitrogen per 1,000 square feet, stimulates new, healthy leaf production, helping the grass grow out of the disease quickly.
Proper water management is a foundational control method to reduce the leaf wetness that promotes fungal growth. Lawns should be watered deeply and infrequently to encourage deep root growth, rather than shallowly every day. Watering should only be done in the morning, allowing the grass blades to dry completely before evening dew sets in.
Aeration and dethatching can decrease soil compaction, improve drainage, and enhance air circulation around the grass crowns. Removing excess thatch is particularly important, as the fungus can survive within this organic matter layer as sclerotia.
While cultural controls are usually sufficient for residential lawns, fungicides are available for severe or persistent outbreaks. Fungicide application is considered a last resort and should be applied preventively or at the earliest sign of disease to be most effective. Homeowners with recurring issues may consult a professional to apply products containing active ingredients like propiconazole or azoxystrobin.