Red thread is a common fungal disease affecting cool-season turfgrasses, such as perennial ryegrass and fescue, causing noticeable discoloration. The disease appears when environmental conditions favor fungal growth over grass health. Understanding the signs and implementing proper management techniques can help homeowners restore their lawn’s appearance. This guide provides steps for identifying, treating, and preventing red thread outbreaks.
Recognizing the Signs of Red Thread
The disease is caused by the fungus Laetisaria fuciformis, thriving during cool, wet weather, typically between 60°F and 75°F. Symptoms begin as irregular patches of grass that look water-soaked before fading to a bleached or tan color. These affected areas range from two to ten inches in diameter and can coalesce into larger patches.
The defining characteristic is the presence of small, pink or red thread-like structures, known as stromata, extending from the tips of infected grass blades. These reddish strands are especially visible when the grass is wet. The fungus primarily infects the leaf blades and sheaths, rarely killing the grass plant entirely. However, the resulting straw-colored patches create an undesirable appearance.
Immediate Cultural Control Strategies
Addressing an outbreak requires correcting the environmental factors that allow the fungus to flourish. The most common factor is a deficiency in nitrogen, which stunts grass growth and recovery. Applying a quick-release nitrogen fertilizer stimulates rapid turf growth, allowing the grass to quickly outgrow the infected leaf tissue. A light application of 0.5 to 1.0 pound of nitrogen per 1,000 square feet is recommended to encourage recovery.
Proper irrigation practices are important for reducing the surface moisture the fungus needs to thrive. Water the lawn deeply but infrequently, only during the early morning hours. This ensures the grass blades dry completely before evening, minimizing leaf wetness and making the environment less favorable for fungal development. Improving air circulation and reducing thatch buildup near the soil surface, perhaps through aeration, also limits moisture retention.
When to Use Fungicides
Fungicide application is generally considered a last resort for managing red thread because cultural control methods are often sufficient for recovery. Since the disease is largely cosmetic and rarely kills the entire grass plant, chemical treatment is usually unwarranted for the average homeowner. Fungicides may be justified only in cases of severe infection, on high-value turf areas, or when cultural controls have failed.
Effective chemical options include active ingredients like propiconazole (a DMI fungicide) and azoxystrobin (a QoI fungicide). These products minimize existing symptoms but must be applied according to label instructions for timing and rate. Because the fungus can develop resistance, turf managers should alternate between products from different chemical groups when repeat applications are necessary. For most home lawns, focusing on long-term fertility and cultural practices is more beneficial than relying on chemical intervention.
Preventing Future Red Thread Infections
Long-term prevention focuses on maintaining a healthy, vigorous turf stand naturally resistant to disease. This proactive approach centers on sustained nutrition, achieved through a consistent, balanced fertilization schedule throughout the growing season. A comprehensive fertility program over two to three years drastically reduces the likelihood of future outbreaks, going beyond correcting immediate nitrogen deficiency. Soil testing is valuable, ensuring the soil maintains proper pH and adequate levels of all necessary nutrients, including phosphorus and potassium.
Selecting disease-resistant turfgrass cultivars is another powerful preventative measure when establishing a new lawn or overseeding. Certain varieties of fine fescue and Kentucky bluegrass have been bred for higher resistance to Laetisaria fuciformis. Continued good maintenance practices, such as routine mowing at the correct height and prompt removal of grass clippings, help reduce the organic material where the fungus can survive and spread.