Red thread is a common turfgrass problem caused by a fungal pathogen that weakens grass blades, leading to discoloration and noticeable patches of damaged turf. It is primarily a foliar disease, meaning it affects the leaf tissue and rarely kills the grass outright by damaging the roots or crown. The turf usually recovers once environmental conditions are no longer favorable for the fungus and the grass is growing vigorously.
Identifying Red Thread on Grass Blades
The disease first appears as irregular, bleached patches in the lawn, typically ranging from two to ten inches in diameter. These patches initially have a reddish or pinkish tinge before the affected grass blades turn tan, light gray, or straw-colored. The affected blades often die from the tip down, giving the turf a scorched or ragged texture.
The unique feature giving the disease its name is the presence of coral-pink or bright red, thread-like structures on the infected grass blades. These structures, technically known as stromata or sclerotia, are a form of fungal growth that may look like tiny, gelatinous needles extending from the leaf tips. During high humidity, a pink, cottony, or web-like material (fungal mycelium) may also be seen binding the blades together. These distinctive red filaments are the definitive way to distinguish Red Thread from similar lawn diseases.
The Fungal Culprit and Necessary Environmental Triggers
Red thread is caused by the fungus Laetisaria fuciformis, which survives as dormant structures in the lawn’s thatch and soil. This pathogen requires a specific combination of environmental and nutritional factors to become active and infect the grass. The fungus spreads when its spores or fragments of the red thread structures are carried to new areas by wind, water runoff, or on contaminated equipment like mowers or shoes.
Nitrogen Deficiency
A significant trigger for an outbreak is a nutrient deficiency, particularly low nitrogen levels in the soil. Nitrogen is crucial for vigorous grass growth, and turf starved of this nutrient becomes weak and highly susceptible to infection. The disease is often associated with low-maintenance turf that is not fertilized regularly. This slow-growing grass lacks the necessary energy reserves to rapidly replace infected leaf tissue and outgrow the infection.
Prolonged Leaf Wetness
The fungus requires prolonged periods of leaf wetness to germinate and infect the grass blades. This condition occurs during extended heavy dew, fog, or light rain that keeps the grass surface moist for many hours. Improper watering practices, such as late evening irrigation, also extend the duration of leaf wetness, creating an ideal environment for the fungus.
Moderate Temperatures
Temperature is a necessary environmental factor, as the disease favors moderate, cool conditions. The optimal temperature range for Laetisaria fuciformis development is typically between 55°F and 75°F (13°C and 24°C). This temperature preference explains why Red Thread is most common during the cool, wet periods of spring and autumn. The fungus’s growth rate decreases significantly when temperatures consistently exceed 77°F, and disease activity will often cease entirely above 85°F.
Non-Chemical Management Strategies
Optimize Nitrogen Levels
Adjusting cultural practices is the most effective way to manage and prevent this disease. Since a lack of nitrogen weakens the grass and promotes infection, timely fertilization is the primary non-chemical control strategy. Applying a nitrogen-containing fertilizer encourages the turf to grow quickly. This rapid growth allows the grass to rapidly replace infected leaf tissue and effectively outgrow the disease.
Adjust Irrigation Practices
Proper irrigation is essential to reduce the duration of leaf wetness. Water the lawn deeply but infrequently, which promotes deeper root growth and a healthier turf stand. It is important to water in the early morning, ideally between midnight and 6:00 AM, so the grass blades can dry completely as the sun rises. Avoid watering in the late afternoon or evening, as this extends the period the leaves stay wet overnight, favoring fungal growth.
Improve Air Circulation and Drainage
Improving air circulation and managing organic matter further discourages the disease. Aerating the soil helps reduce compaction and improves drainage by allowing air and nutrients to penetrate the root zone. Dethatching, which removes the layer of dead organic matter between the grass blades and the soil surface, reduces moisture retention where the fungus survives. Pruning surrounding trees or shrubs can also increase sunlight exposure and air movement, helping the grass dry more quickly.