Necrotic ring spot, caused by the fungus Ophiosphaerella korrae, is one of several patch diseases that affect turf in the United States. For many years a patch disease causing circular to serpentine necrotic patterns in turf was believed to be caused by Fusarium fungi, and hence the name "Fusarium Blight Syndrome" evolved. Current research shows that Fusarium fungi are not the cause of this disease. "Fusarium Blight" consists of at least two separate patch diseases--summer patch caused by the fungus Magnaporthe poae, and necrotic ring spot caused by O. korrae.
As with most turf diseases, necrotic ring spot is principally a disease of stressed turf. Research at Colorado State University has shown that the disease can be controlled by a combination of proper cultural practices to maintain the turf grass at optimum vigor, and application of a fungicide. Dense turf with heavy thatch tends to be more prone to infection. Lawns with ring spot have had 3-inch thick thatch layers. One-quarter inch is normal. Cultural Control--Use resistant varieties when establishing or re-establishing a lawn. The following Kentucky bluegrass varieties show some resistance to NRS: Adelphi, Eclipse, Midnight, Park and Wabash I-13. --Core aerate the lawn once a year (spring or fall) to help reduce thatch buildup and improve soil condition. --Mow grass as necessary to maintain a height of 2 1/2 to 3 inches. Make sure mower blades are sharp. Never remove more than one-third of the grass blade at a time. --Water to a depth of 6 to 8 inches as infrequently as possible without creating water stress. Water in the morning or midday so the leaf blades dry as quickly as possible. --Water grass lightly (in addition to step 4) during hot periods. Run the sprinkler for one to two minutes during hot weather to help cool the turf and avoid stress. --Avoid excessive applications of nitrogen fertilizer that induce tender, succulent growth and more susceptible tissue. Apply nitrogen according to soil test results or at the rate of 1/2 to 1 pound per 1,000 square feet, four times a year: mid-May, June, September and two to three weeks before frost. Never apply more than 4 pounds of nitrogen per 1,000 square feet in an entire year. If grass clippings are allowed to lie (not bagged), fertilize three times a year: June, September and two to three weeks before frost. For more information on necrotic ring spot see the fact sheet: Necrotic Ring Spot.
Rings in a lawn may also be the result of fairy ring, a common disease of Kentucky bluegrass and many other turf grass species. It is caused by many mushroom fungi that live in the soil and thatch layer. Damage often is unsightly and sometimes a serious problem even on well-maintained lawns. Fairy rings develop over a wide range of fertility levels and soil and climatic conditions. The disease tends to be worse on lawns maintained at low soil moisture and fertility levels. To prevent the disease, don't bury organic debris, such as stumps and waste lumber, before establishing a lawn and maintain optimum growing conditions for turf grass with proper watering, fertilization and thatch control. To control an established fairy ring, aerate the entire diseased area every 4 inches, plus an additional 2 feet beyond its visible limits. Disinfect core cultivators after use to prevent accidental spread of the fungus into healthy grass. Following aeration, soak the infected area with water. Add a wetting agent to help water penetrate. Hand water these areas to prevent over watering of adjacent healthy turf grass. For more information on Fairy Ring, see the fact sheet: Fairy Ring.