Turfgrass Diseases: Red Thread (Causal Fungus: Laetisaria fuciformis)

This disease is caused by the fungus Laetisaria fuciformis, and during rainy periods in late spring and early summer, it is one of the most common lawn diseases in Pennsylvania. Red thread is usually associated with perennial ryegrass, fine fescue, and Kentucky bluegrass lawns that do not receive adequate amounts of nitrogen fertilizer.

Figure 1. Red thread symptoms on a perennial ryegrass lawn (left); and in a stand of non-mowed creeping red fescue (right). Photos: Peter Landschoot, Penn State

Symptoms and Signs

Red thread disease symptoms appear as circular or irregularly shaped patches of blighted turf, typically with a red or pink cast. Red thread disease is usually restricted to the leaves, leaf sheaths, and stems of turfgrasses, but in severe cases may kill the entire plant. In the early stages of the disease, symptoms appear as small, blighted areas on leaves that can enlarge rapidly to cover most of the leaf surface. In the advanced stages, leaves become covered with bright red thread-like strands of fungal mycelium, 1/16 to 1/4 inch in length, at the tips of the affected leaves. Diseased plants eventually dry out and fade to a bleached straw color.

Figure 2. Thread-like strands of red and pink fungal mycelium on red-thread diseased perennial ryegrass. Photo: Peter Landschoot, Penn State

Disease Cycle

The causal fungus of red thread disease, L. fuciformis, overwinters as dormant mycelium on dead leaves and stems of previously infected plants. The mycelium resumes growth when sufficient moisture and warmer temperatures return in spring. The fungus enters grass leaves through natural openings and cut tips and spreads rapidly through the remainder of the foliar portion of the plant. Mycelium on infected plants can be broken loose and transported by mowers and foot traffic to other plants.

Disease Development

Red thread disease tends to develop when air temperatures are 65° to 75°F, with prolonged periods of rainy or humid weather in late spring and early summer. At times, the disease occurs in warmer, drier weather. Red thread frequently occurs with another disease called pink patch, which has similar symptoms. Both diseases develop under the same environmental conditions and control measures are similar for both diseases.

Cultural Control

Where red thread causes turf damage, fertilization with adequate nitrogen for good turf growth usually will reduce the problem. However, turf managers should be aware that excessive nitrogen additions can lead to other disease problems later in the season, such as brown patch and Pythium blight. In most cases, turf affected by red thread will recover within a few weeks, but lawns planted to highly susceptible cultivars of perennial ryegrass, Kentucky bluegrass, and/or fine fescues can sustain severe damage.

Chemical Control

In most situations, red thread is not sufficiently severe to warrant fungicide treatment. Occasionally, stands of perennial ryegrass on high-value sports fields, institutional grounds, and golf courses may become severely blighted during wet weather. In such cases, fungicide treatment will minimize symptoms.

Some penetrant fungicides labeled for control of red thread disease.

Some multisite contact fungicides labeled for control of red thread disease.

Some combination product fungicides labeled for control of red thread disease.

*FRAC is an abbreviation for Fungicide Resistance Action Committee. The FRAC code/resistance group system consists of numbers indicating classes or groups of fungicides based on mode of action, and letters that refer to broad classifications of fungicides (P = host plant defense inducers; M = multi-site fungicides; and U = unknown mode of action and unknown resistance risk). Due to the risk of fungicide resistance, turf managers should avoid excessive use of fungicides within the same FRAC code/resistance group and alternate products among different FRAC codes/resistance groups.

**Plant mobility classification refers to a fungicide's ability to penetrate plant surfaces or remain on plant leaf or stem surfaces without penetration. Fungicides that penetrate plant surfaces and are translocated mostly upwards through plant xylem tissues are called acropetal penetrants (acropetal = toward the apex). Fungicides that enter plant cuticles or move limited distances in internal plant spaces, but do not translocate through vascular tissues (xylem and/or phloem) are called local penetrants. Contact fungicides do not penetrate plant surfaces and only inhibit fungal pathogens residing on leaf and stem surfaces.

***Follow label precautionary statements, restrictions, and directions regarding tolerant turfgrass species, rates, and timing of applications.

References

Buhler, W. Fungicide spraying by the numbers.

Clarke, B.B., P. Koch, and G. Munshaw. Chemical control of turfgrass diseases 2020. University of Kentucky, Rutgers University, and University of Wisconsin.