Red thread is a common and problematic disease that affects turfgrass. Red thread, scientifically known as Laetisaria fuciformis, can cause significant damage to turfgrass, particularly in cool and humid environments. Throughout this module, we will explore the disease cycle, symptoms, signs, management practices, and control strategies associated with red thread. By the end of this module, you will have a comprehensive understanding of this disease and be equipped with valuable knowledge to effectively manage and mitigate its impact on turfgrass. Let’s dive in and explore the fascinating world of Red Thread!

Credit: VCE Master Gardener

Causal agents

Laetisaria fuciformis fungus

Primary Hosts

Agrostis, Festuca, Lolium, and Poa spp.

Disease Cycle

Red thread’s disease cycle begins with the presence of fungal mycelium in the thatch and upper soil layers. Under favorable conditions, such as cool temperatures ranging from 15 to 25°C (59 to 77°F) and high humidity, the fungus becomes active and starts to infect the turfgrass.

The primary mode of infection is through the aerial parts of the grass, particularly the leaf blades. The fungus penetrates the leaf tissue and colonizes it, leading to the development of characteristic symptoms. As the infection progresses, the affected grass blades develop red or pinkish patches or threads, which give the disease its distinctive name.

During wet or humid weather, the fungus produces visible structures called sclerotia. These are small, compact masses of fungal hyphae that serve as survival structures. Sclerotia can range in color from pink to red and may be visible on the affected grass blades or within the infected patches.

The disease cycle of red thread is typically favored by prolonged periods of leaf wetness, which provide optimal conditions for fungal growth and infection. Poor turfgrass nutrition, particularly a deficiency of nitrogen, can also contribute to the development and severity of red thread.

Epidemiology

Red thread disease is commonly observed in cool and humid regions, particularly in areas with prolonged periods of wet or damp conditions. It occurs most frequently during spring and autumn when temperatures range from 15 to 25°C (59 to 77°F) and humidity levels are high. The disease is favored by a combination of factors, including extended leaf wetness, poor soil drainage, and nutrient imbalances.

The fungus Laetisaria fuciformis responsible for red thread can spread through the air or be transported by water, wind, or human activities. It can survive as mycelium or sclerotia in infected plant debris, thatch, or soil. Once favorable conditions are met, such as cool temperatures and moisture, the fungus becomes active and initiates new infections.

Red thread can affect a wide range of turfgrass species, but it is most commonly observed on fine fescue, perennial ryegrass, and Kentucky bluegrass. It is often seen in lawns, golf course roughs, and other low-maintenance turf areas.

Symptoms

Red thread is characterized by distinct symptoms and signs that help in its identification. The affected grass exhibits water-soaked patches that vary in shape and size, ranging from circular to irregularly shaped patches with diameters of 5-60 cm (2-24 incehs). Initially, the dead leaves of the grass turn straw brown to tan in color, giving an indication of the disease. The necrosis typically progresses from the leaf tip toward the base.

Credit: John Kaminiski/Penn State University

Signs

Upon closer inspection, one can observe unique signs of red thread. The pathogen produces colorful, threadlike structures known as sclerotia, which protrude from the affected leaves. These sclerotia are branched and exhibit a pink to pale red or orange coloration. In moist conditions, the sclerotia may extend up to 10 mm (0.4 in) beyond the leaf tip, adding to the distinct appearance of the disease. However, in dry weather, the sclerotia become bright red and brittle, making them easily detachable and prone to spreading through mechanical means or falling into the thatch layer.

During periods of high humidity, the pathogen may also produce mycelial mats and pink cottony flocks of mycelium. These additional signs contribute to the reddish or pink cast of the affected patches. It’s important to note that in the absence of sclerotia or cottony flocks, red thread can be challenging to distinguish from other diseases like dollar spot or pink patch.

Credit: John Kaminiski/Penn State University

Management

Management of red thread rarely requires fungicides. Cultural practices, including the addition of nitrogen in the spring, are generally able to reduce disease symptoms to acceptable levels.

Cultural

Cultural control practices are essential in managing red thread disease in turfgrass. Proper fertility management is crucial, and it involves maintaining adequate fertility levels based on soil or tissue tests. It is important to avoid excessive nitrogen rates and to use quick-release nitrogen sources. Maintaining an appropriate soil pH within the recommended range for the specific turfgrass species is also important, generally falling between pH 6.0-6.5.

Irrigation practices play a significant role in controlling red thread. Water should be applied as needed to prevent drought stress, and it is recommended to water deeply and infrequently. Early morning irrigation is preferred to avoid prolonged leaf wetness. Light, frequent irrigation or late afternoon/early evening applications should be avoided as they can lead to extended periods of leaf wetness, creating favorable conditions for disease development.

Promoting sunlight penetration and improving air movement over the turfgrass is crucial in reducing red thread disease. This can be achieved by selectively pruning trees or shrubs or arranging the landscape design to allow for better sunlight exposure and airflow. Proper mowing practices are also important, as using sharp blades minimizes leaf tip wounds that can serve as entry points for the pathogen.

Collecting turfgrass clippings during periods of active red thread disease can help reduce the spread of sclerotia. By removing the clippings, the potential source of inoculum is minimized. Additionally, selecting turfgrass cultivars that exhibit resistance or tolerance to red thread disease can be beneficial. Choosing cultivars that are less susceptible to the disease can reduce the overall impact and severity of red thread.

Chemical

Fungicides are generally not needed to reduce red thread as nitrogen applications usually aid in recovery. If disease pressure is high, however, fungicides may be warranted. Highly recommended fungicides for effective disease control include:

• Flutolanil (Prostar, Pedigree)
• PCNB (PCNB, Turfcide, etc.)
• Triadimefon (Bayleton)
• Tebuconazole (Torque, Mirage, etc.)
• Myclobutanil (Eagle)
• Propiconazole (Banner Maxx)
• Iprodione (Chipco 26GT)