Red Thread

Need to know: 

  • Symptoms include irregularly shaped patches of grass becoming blighted. 
  • Signs include bright thread-like strands at the end of infected leaves. 
  • Spread by water splash, wind, movers, shoes and other mechanical means. 
  • Manage by avoiding mowing when turf is wet, raising mowing height, and avoiding frequent sprinkling in late afternoon. 
  • Fungicides may be necessary if turf has history of red thread.  

Predisposing Conditions for red thread

Water-saturated environment associated with rainy weather and slow shoot growth caused by nitrogen deficiency or water stress (although the disease also may develop in well-fertilized turf) are predisposing conditions for red thread.

Symptoms of red thread  

Circular or irregularly shaped patches of grass first become blighted and begin turning reddish or pinkish. Eventually the grass dries out and turns a bleached straw-like color. Affected areas initially are 1-2 inches, but may coalesce and form irregular shapes. Infected leaves may appear water-soaked. Dead leaves often are interspersed with green leaves.

Signs of red thread

Bright red fungal structures, called "red threads" (approx. ¼ inch or less long), can be observed extending from leaf surfaces, particularly the cut ends of the grass blades. Tufts of pink or reddish web-like fungal growth can also be seen on leaves, sometimes binding leaves together.

Disease Cycle of red thread

Red thread is caused by Laetisaria fuciformis, a fungus. Infection occurs on the leaf and stem parts, not the roots. The pathogen rarely kills plants, but does weaken turfgrass, contributing to decline from other stresses. The disease is spread by water splash, wind, mowers, shoes, and other mechanical means. Red threads, when dry, function as survival structures (sclerotia), allowing the fungus to survive from season to season.

Type of Sample Needed for Diagnosis and Confirmation

The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents.  If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us

Management of red thread

The following are recommended ways to manage red thread:

  • Maintain adequate fertility
  • Raise the mowing height, if possible
  • Avoid mowing early in the morning when the turf is wet with dew
  • Avoid frequent sprinkling in the late afternoon
  • Collect grass clippings when red thread is active to reduce the number of red threads that fall back into the turf

A fungicide program may be necessary on turf with a history of red thread. Labeled products include Banner, Cavalier, Chipco 26019, Cleary’s 3336, Compass, Dithane, Fore, Heritage, Junction, Mancozeb, Pentathlon, Protect T/O, Rubigan, and others

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.

Last reviewed:
April 2022

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on September 1, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.