Red thread is a common turfgrass issue caused by the fungus Laetisaria fuciformis. The disease is primarily cosmetic, affecting the appearance of the lawn rather than killing the grass outright. It causes noticeable discoloration and patchiness that detracts from the uniform green color. The fungus infects the grass blades but leaves the crowns and roots undamaged, meaning the turf retains the ability to recover.
Visual Identification of Red Thread
The first indication of this disease is the appearance of small, ragged patches of grass that initially look water-soaked or slightly darkened. These patches typically range from four to eight inches in diameter and quickly fade to a beige or light tan color. From a distance, an infected lawn may look like it has scattered spots of straw-colored or pinkish turf.
The definitive sign distinguishing red thread is the presence of the fungus’s reproductive structures. Look closely at the tips of the affected grass blades for fine, antler-like structures that are bright red or pink. These threads, called sclerotia or stromata, are gelatinous when moist and become dry and brittle as they age, often growing up to a quarter-inch in length. These fungal strands cause the infected blades to look matted or webbed together and give the disease its descriptive name.
The Environmental Conditions that Cause Outbreaks
Red thread outbreaks occur when environmental conditions favor the fungus over the grass’s natural defenses. The fungus thrives in cool, mild weather, with the most active growth occurring when air temperatures are consistently between 60°F and 75°F. This makes spring and fall, when temperatures are moderate and rainfall is frequent, the most common times for the disease to appear.
High humidity and prolonged leaf wetness are significant factors that encourage the fungus to infect the grass blades. Conditions like consistent drizzle, fog, or heavy dew that keep the blades damp for ten to twelve hours or more create a perfect environment for fungal spores to germinate and spread. The fungus survives the winter as dormant sclerotia within the thatch layer, ready to resume growth when moisture and temperatures are favorable.
The disease is strongly associated with turfgrass that is deficient in nitrogen. Grass plants without adequate nitrogen struggle to produce the proteins necessary for strong cell walls and vigorous growth, making them highly susceptible to infection. When nitrogen levels are low, the grass cannot “outgrow” the disease by quickly replacing infected leaf tissue with new, healthy blades. This deficiency is why red thread is more common in lawns that have not received consistent fertilization.
Non-Chemical Management and Prevention Strategies
Since red thread primarily affects the leaves and not the roots, the most effective treatment involves encouraging the lawn to grow quickly to shed the infected tissue. A quick-release nitrogen fertilizer application is the recommended first step to halt the disease’s progression. Applying a nitrogen source at a rate of about 0.5 to 0.75 pounds of nitrogen per 1,000 square feet will stimulate immediate growth.
This flush of new growth allows the grass to effectively mow off the damaged tips, removing the fungus’s reproductive structures and allowing the plant to recover. Consistent nitrogen fertilization, following general guidelines for your turf type, is the most important long-term cultural practice to prevent future outbreaks. Regular applications over two to three years can drastically reduce the severity or eliminate the problem.
Adjusting watering practices can significantly reduce the duration of leaf wetness, which is necessary for the fungus to thrive. Water the lawn deeply but infrequently, completing watering in the early morning hours, ideally between midnight and 6 AM. This timing allows the grass blades to dry quickly once the sun rises, minimizing the hours they remain moist. Avoid watering in the late afternoon or evening, as this extends the period of wetness throughout the night, creating ideal conditions for fungal growth.
Improving air circulation helps to dry the foliage and reduce humidity near the soil surface. Dethatching the lawn to remove excessive organic material, especially if the layer is over half an inch thick, will improve air flow and water penetration. Aerating the soil regularly helps to reduce compaction, which improves drainage and ensures that oxygen and nutrients can better reach the grass roots.
Proper mowing techniques further support the lawn’s health and recovery. Maintain a mowing height of three to four inches, as shorter cuts can stress the grass and make it more vulnerable to disease. Avoid mowing when the grass is wet, as a wet mower deck can transport the fungal material to uninfected areas. Fungicides can control red thread, but they are rarely recommended for residential lawns because the disease is cosmetic and the turf typically recovers through improved cultural practices and fertilization alone.