What is Necrotic Ring Spot?

Necrotic ring spot (NRS), Oph- iosphaerella korrae (formerly Leptosphaeria korrae) is a disease that resembles small fairy rings in lawns when it reaches the advanced stages. NRS is often confused with several other patch diseases and regularly misdiagnosed as Fusarium patch, Magnaporthe poae.
NRS was discovered when a fungicide was unsuccessfully used to treat Fusarium blight, which is a common cool-season lawn disease. When the scientists studied the problem, they discovered NRS was causing the patches instead of Fusarium.
Necrotic ring spot is found in many regions throughout the United States, but especially in the Pacific Northwest, the Great Basin, and northeastern and north central parts of the United States.

Signs and Symptoms of Necrotic Ring Spot

Symptoms of NRS appear as individual circular patches of dead or dying lawn during cool, wet weather. At the primary stages of infection, the grass turns a light green color. As the infection progresses, small patches of the grass turn reddish brown, and then gradually to a straw color. The infected area of grass, then becomes a sunken patch and may range in size from 2 inches to 3 inches, to 2 feet to 6 feet in diameter. As the infected area moves from the center outward, tufts of healthy green grass grow in the middle, creating a small circle of dead grass. In the advance stages of infection, the roots and rhizomes of the turfgrass turn brown, then black. When this occurs, the infected leaves are easily lifted from the soil. At this stage the disease takes-on a doughnut-shaped appearance. At this phase as it grows, it is confused with other common turf diseases, summer or yellow patch, Fursarium patch and fairy ring.

How to Prevent Necrotic Ring Spot

1. Prevention is your first line of defense against necrotic ring spot. Listed below are several ways to reduce infection.
• Maintain a 2-inch or greater mowing height.
• Avoid drought stress. Keep the soil moist but not too wet or dry.
• Core-aerate non-infected lawns to relieve soil compaction. This will allow roots to penetrate through the thatch layer. Roots will grow deeper and become less prone to drought.
• Sterilize tools and mechanical equipment with a mild solution of bleach and water that have been used in infected lawns.
• Reduce foot traffic in infected turf to stop the spread of the disease.
• Apply frequent low rates of fertilizer (half the recommends rate).
2. In early spring or fall use slow- release forms of fertilizers.
3. Use only half the recommended rate (1/2 lb of actual nitrogen /1000 square feet of lawn) of fast-release fertilizers like ammonium sulfate and ammonium nitrate.
4. When over seeding infected areas of the lawn in the spring or fall, choose resistant cultivars.