Necrotic Ring Spot (NRS) is a challenging and persistent turfgrass disease that can severely damage otherwise healthy lawns. This affliction is caused by the soil-dwelling fungus Ophiosphaerella korrae, which attacks the roots and crowns of cool-season grasses, notably Kentucky bluegrass. Effective management is possible through a dual approach combining short-term chemical suppression with long-term cultural practice adjustments.
Confirming the Necrotic Ring Spot Diagnosis
Identifying Necrotic Ring Spot accurately is the first step toward effective treatment, as its symptoms can be confused with other lawn problems. The most distinct visual characteristic is the formation of circular or arc-shaped patches of dying grass, often referred to “smoke rings” or “frog-eyes.” These patches typically range in size from a few inches up to three feet in diameter, appearing straw-colored or bronze at the edges. A unique feature is the “doughnut” or “frog-eye” pattern, where the grass in the very center of the ring may remain healthy and green. This occurs because the fungus spreads outward from a central point, killing the roots in an expanding ring while new, healthy grass may regrow in the center. Although the fungus actively infects the root system during cooler, wet weather in the spring and fall, the visible symptoms become most severe during periods of summer heat and drought stress because the damaged root system cannot absorb enough water, causing the turf to collapse visibly.
Immediate Chemical Intervention
When Necrotic Ring Spot symptoms are actively damaging the lawn, a rapid intervention using professional-grade systemic fungicides is often necessary to suppress the disease. These products do not cure the existing damage but work to stop the fungus from spreading to healthy roots and allow the turf to recover. Effective systemic fungicides include those with active ingredients such as propiconazole, azoxystrobin, fenarimol, or myclobutanil.
Because the fungal pathogen resides deep in the soil and attacks the roots, proper application technique is essential. The fungicide must be delivered directly to the root zone, not simply sprayed onto the leaf blades. After application, the treated area must be immediately “watered in,” using a light irrigation (about 1/8 to 1/4 inch of water) to wash the chemical down to the site of infection.
Timing is important for maximizing the fungicide’s suppressive power. Applications should be made preventively in the early spring and fall when the soil temperature at a three-inch depth is consistently between 60°F and 70°F, indicating when the fungus is most biologically active. Repeat applications, often spaced 28 days apart, are required for effective suppression throughout the disease’s active seasons.
Long-Term Cultural Management
Sustained control of Necrotic Ring Spot relies heavily on changing the turf’s environment to make it inhospitable to the fungus. This is achieved through specific cultural management practices focusing on soil health, moisture levels, and reducing overall turf stress. A fundamental shift in watering habits is necessary to manage the disease long-term.
Watering Practices
The long-term goal for a healthy, NRS-resistant lawn is deep, infrequent watering. This technique encourages grass roots to grow deeper into the soil profile, making them more resilient to fungal attack and summer stress. Watering should deliver a full inch of water, soaking the soil deeply, and then only be repeated when the turf shows early signs of drought stress. In contrast, the common practice of light, frequent watering keeps the upper soil layer constantly moist, creating the ideal environment for the Ophiosphaerella korrae fungus to thrive.
Soil and Thatch Management
To promote a healthy root environment, it is necessary to reduce soil compaction and excessive thatch accumulation. Core aeration, which involves physically removing small plugs of soil, relieves compaction and allows water, air, and nutrients to penetrate the root zone more effectively. This practice should be performed in the spring or fall when the turf is actively growing.
Excessive thatch, a layer of undecomposed organic matter between the soil and the grass blades, creates a dense, moist habitat where the NRS fungus can persist and spread. Dethatching or vertical mowing can physically remove this dense layer, improving drainage and reducing the fungal reservoir. For severe cases, it is important to clean equipment after working in infected areas to avoid spreading the fungus to healthy parts of the lawn.
Fertilization and Soil pH
Fertilization should focus on providing balanced nutrition using slow-release nitrogen sources, such as sulfur-coated urea, to avoid sudden flushes of growth. High rates of quick-release nitrogen, especially in the late spring or summer, can lead to tender, stressed turf that is highly susceptible to the disease. It is important to limit total annual nitrogen application and concentrate the feeding in the late summer and fall to promote recovery and root development.
The NRS fungus is known to thrive in soils with a high or alkaline pH. Conducting a soil test is an important step to determine the native pH level and guide any necessary amendments. If the soil pH is above 7.0, applying sulfur amendments, such as elemental sulfur, can help to gradually acidify the soil over time, creating a less favorable environment for the pathogen. A typical application rate may be around 1 to 1.5 pounds of elemental sulfur per 1,000 square feet annually.
Mowing
Maintaining a higher mowing height is a simple yet effective cultural practice to reduce stress on the turfgrass. Setting the mower blade between 2.5 and 3.5 inches allows the grass plant to develop a deeper root system and provides more leaf surface area for photosynthesis. This increased vigor helps the turf better withstand the root damage caused by the fungal infection.