Red Thread and Leaf Spot are Active!

May. 19, 2011

Red Thread is Active in Much of Ohio!!!

By Joe Rimelspach

Red thread is a foliar disease that usually occurs during mild temperatures and long periods of wet turf. The disease is more prevalent in spring and fall but can occur in any season. The recent weather pattern has been conducive for the disease to become wide spread. Susceptible grasses include perennial ryegrass, fine fescue and some cultivars of Kentucky bluegrass BUT all cool-season grasses have been noted to have the disease including tall fescue and creeping bentgrass. Red thread symptoms create an undesirable appearance, but crowns and roots are not infected, so plants are not killed and turf eventually will recover.

Malnourished turf often has a chronic case of red thread. Deficient nitrogen and/or phosphorous fertility levels can result in serious outbreaks.

Red thread management:

  • Genetic resistance to red thread infection is limited. Turfgrass varieties with different levels of red thread susceptibility are listed on the National Turfgrass Evaluation Program (NTEP) Web site:
  • The most important nonchemical (cultural) control option involves implementing an adequate fertility program. A good fertility program implemented over two to three years will drastically reduce further red thread problems. If soil is low or deficient in phosphorous the disease is often severe.
  • Other cultural practices that promote healthy turf and vigorous growth also help suppress red thread. Outbreaks may be reduced further by avoiding irrigation practices that extend dew periods (such as watering in the late afternoon and early evening).
  • Fungicides may be used to control red thread if outbreaks occur on high maintenance turf or high value properties. For fungicide recommendations check out the Families of Fungicides table at:
5-19picture1Red Thread in fine fescue lawn, May 2011, Columbus, Ohio 5-19picture2Close-up of Red Thread fungus in fine fescue lawn, May 2011, Columbus, Ohio

Leaf Spot – Is Wide Spread on Many Cool-Season Turfgrasses.

Extended periods of cool, wet spring weather this year has trigger extensive leaf spot on lawns. This is a troublesome spring diseases on Kentucky bluegrass (especially common bluegrasses), fescues and other lawn grasses. Some leaf spot can be found on most home lawns in the spring, but it normally does not cause significant damage to the lawn. This year due to the prolonged cool wet weather conditions, leaf spot has occurred and progressing down the stem tissue and into the base of the plants. With warmer dryer weather, extensive thinning and “melting–out” of lawns may most likely will occur.

Leaf spot is caused by several different fungi. The fungus overwinters in the thatch layer or in small lesions on leaf blades. In spring, the fungus infects young succulent leaf tissue and causes the development of small elliptical dark colored spots. The spots eventually turn light tan but remain bordered by a dark brown outer edge. The leaf spot phase of the disease usually does not damage the plant significantly. However, during continuous cool, wet conditions, the fungus invades the leaf sheath and crown. The fungus also may invade the crown, rhizomes, and roots. As daytime temperatures increase, leaves on crown- infected plants begin to turn light green or yellow, similar to nitrogen deficient turf. Eventually these plants die and turn brown or straw colored. This is referred to as the melting-out phase of the disease. Severe melting-out can result in irregular patches of dead turf. Damaged lawns often appear "thin" or uneven and tend to have weed problems. Excess thatch, heavy spring nitrogen fertilizing, excess shade, mowing too close and excessive herbicide applications can promote leaf spot and melting out.

Maintenance procedures to help manage leaf spot & melting out:

  • Mow the turf high (2.5 – 3 inches) to provide leaves that produce food for the plant and maintain a healthier lawn.
  • Avoid excessive nitrogen fertilization in spring which promotes lush growth. Once in the melting out phase maintain the lawn with a complete fertilizer at modest rate to encourage healthy turf and recovery. Often a starter fertilizer is recommended.
  • Manage thatch by frequent and heavy core cultivation (aeration) of the lawn. This will also promote a deeper healthier deeper root system.
  • Plant resistant cultivars of Kentucky bluegrass. For information on these refer to the National Turfgrass Evaluation Program, at
  • Fungicides can be applied but for best results need to be made early in the disease cycle or as a preventative treatment. This is done based on a history of the disease in the lawn. Unfortunately the most effective fungicides are no longer registered for use on residential lawns. For fungicide recommendations check out the Families of Fungicides table at:
5-19picture3Leaf Spot on Kentucky Bluegrass, Columbus, Ohio