Red thread is said to be the first turfgrass foliar disease ever to be described. It is a fairly benign fungal disease, and is most often cured without extraordinary measures. The fungus, Laetisaria fuciformis, is distantly related to the mushroom-forming fungi, with the red threads that emerge from blades of grass being the visual manifestation of the fungus. The organism itself lives inside the leaves, where leaf tissue can be discolored and even killed, but it is the red strands that are most prominent, and which produce the spores that allow the disease to reproduce.

This close-up of red thread in perennial ryegrass shows the red or pinkish strands.
Photos by Paul Vincelli, University of Kentucky.

“The spores are blown about by the wind. It’s very common in humid parts of the world,” says Paul Vincelli, extension professor in the plant pathology department at the University of Kentucky. He says that in Kentucky, red thread operates under “the widest range of conditions I’ve ever seen in turfgrass.” It can emerge as early as February and can last until late fall.

The disease can be a problem in perennial ryegrass, tall and fine-leaf fescues, as well as Kentucky bluegrass. It overwinters well in the infected dead leaves and simply can’t be prevented from spreading as the red threads emerge and release spores into the wind in the spring. It can also be spread by contact, such as being transported by shoes or mowers. The pink to red threads may be gelatinous, and sometimes a cottony, pinkish fuzzy growth containing microscopic spores may also be visible. Red thread may appear at times with pink patch, another species of fungus. Vincelli says both diseases are essentially an aesthetic problem, rather than a real danger to grass. He has never seen grass killed by red thread, which is encouraged by some of the same weather conditions as dollar spot, but is not nearly as hazardous to plant health.

A fungal disease, red thread may look serious, but it is not a turf killer.

The universal conditions that favor red thread are low nitrogen conditions in the turfgrass and dry conditions in the soil.

A combination of those elements, which weaken the plants and make them susceptible to infection, can be a catalyst for a red thread outbreak. Drought can give rise to outbreaks of the disease, as can dewy evenings over dry soil. On the other hand, correcting those conditions is the natural path to bringing the grass to peak health and minimizing the disease. Information from around the country indicates that the best cultural controls are infrequent, deep irrigations, correcting nitrogen deficiencies (too much nitrogen can invite other fungal diseases), avoiding irrigation near nightfall, aerating compacted soils, and thinning vegetation around the site to allow better air and light penetration.

The most likely situations that might call for chemical control are in highly visible outbreaks in a commercial setting, such as a golf course, or in a high-end residential yard.

There are a large number of fungicides that are effective against red thread. The University of Kentucky has compiled a list of the chemicals that have been tested nationally for the treatment of various fungal diseases of turfgrass, and red thread is one of them. Highest on the list for treatment of this particular fungus are the following: azoxystrobin (Heritage); flutolanil (Prostar); pyraclostrobin (Insignia); polyoxin D (Endorse); and triticonazole (Trinity and Triton).

The chemicals listed at the next level of efficacy are several brands of chlorothalonil (Daconil Ultrex, Manicure, Concorde SST, Chlorostar, Echo, Pegasus L); iprodione (Chipco 26GT, Raven, Lesco 18 Plus, Iprodione Pro); propiconazole (Banner Maxx, Spectator, Savvi); and triadimefon (Bayleton). Chlorothalonil and iprodione are not registered for use on home lawns.

Vincelli says that granules are not as effective against red thread as are sprayable liquids, which can reach the leaf blades where the disease lives. If treatment is undertaken, one application is often enough, though more than one application may be needed. Instead of blanket applications, simple spot treatment with a backpack sprayer should be adequate.

“It’s a disease that is minimized pretty easily, if the turf is actively growing,” Vincelli says. If a turfgrass manager is considering a chemical treatment, the best sequence of management events would be to get the fertilizer and water requirements up to snuff first, wait until the grass is beginning to outgrow the red thread, and then spray the remaining highly visible patches only if the appearance is undesirable.

Vincelli has seen instances where a lawn or golf course is healthy, but will still get a flush of red thread. That is very unusual, and probably due to particular soil conditions in those microsites. In those cases, at sites where nitrogen and irrigation levels are adequate, the grass will likely outgrow the flush and soon look good again. He advocates a wait-and-see approach to a disease that isn’t going to kill the lawn.

Don Dale resides in Altadena, Calif., and is a frequent contributor to Turf. He has covered the green industry for more than 10 years.