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Turf Diseases Can Give Lawn A Bedraggled Look

By Laura Pottorff, horticulturist and plant pathologist with Colorado State University Cooperative Extension

If your lawn isn't a candidate for the cover of House and Garden magazine, it could be because one of several turf diseases is taking its toll.

A couple of turf diseases are capable of making ring-like patterns in the lawn. It's important to diagnose the cause of these problems so you get effective control.

Fairy Ring is one of the most common turfgrass diseases to create large rings or circles in the lawn. It is caused by a number mushroom-producing fungi that live in the soil and thatch layer. Early symptoms of infection appear as circular or partial ring-bands of stimulated (lush green) grass that vary in size from 1 foot to many feet in diameter.

Most rings vary from 3 to 12 feet in diameter. Eventually the deep-green grass in the ring-band dies. Following periods of wet weather, mushrooms, the reproductive form of the fungus, may appear in the ring of affected grass.

How fairy rings begin is unknown, but the fungi that cause them are nourished by decomposing organic litter that is abundant in turf thatch. The lush grass occurs because the fungi release nitrogen as they decompose the organic material in the thatch layer. As the fungus matures, it produces a thick fungal mat in the thatch layer. This mat keeps water from penetrating, causing the grass in the area to die.

To control a fairy ring with dead rings, aerate the entire diseased area every 4 inches, plus an additional 2 feet beyond its visible limits. After aerating, hand water the affected area. So that areas of dark green grass blend with adjacent turf, add light applications of nitrogen fertilizers that stimulate turf growth.

Fungicides drenched into the soil are not recommended as their success in Colorado has been very limited.

Necrotic ring spot or NRS, is another ring-or circle-forming fungus common on Colorado lawns. Rings, arcs or circles caused by the NRS fungus usually are about 12 inches in diameter. Often tufts of apparently healthy grass remain in this circular area of semi-dead grass, producing a "frog-eye" pattern.

Kentucky bluegrass is most often the victim of NRS. Lawns with heavy thatch layers and soils that are heavy and compacted predispose turf to the disease.

When establishing a new lawn, it pays to spend extra time and money preparing the soil. A correctly repaired soil is one with 3 cubic yards of organic matter incorporated per 1,000 square feet. Rototill it to a depth of 6 to 12 inches. The most common reason for NRS to develop is sod that's been put down on hard, compacted clay soil.

As with most turf diseases, NRS is principally a disease of stressed turf. Research at Colorado State University shows that the disease can be controlled with a combination of proper turf culture, overseeding with resistant grass varieties and application of fungicide.

Usually you can control NRS within two years by doing the following:

  • Overseed with resistant grass varieties such as Adelphi, Eclipse, Midnight, Park or Wabash I-13 (all are Kentucky bluegrasses). Note: Resistance does not necessarily mean immunity in these cases. Another overseeding option is to use perennial ryegrass to overseed the entire lawn. Perennial ryegrass is not susceptible to NRS.
  • Core aerate the lawn at least once a year to help decrease thatch layers.
  • Mow grass as necessary to maintain a height of 2 1/2 to three inches.
  • Water lawn to depth of 6 to 8 inches as infrequently as possible without creating water stress.
  • Avoid excess applications of nitrogen fertilizers. Slow-release formulations are often the best in this situation.

Photo: Judy Sedbrook

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Date last revised: 01/05/2010