Necrotic Ring Spot

Causal Agent:

Ophiosphaerella korrae

Susceptible Turfgrasses:

Creeping red fescue; Kentucky bluegrass

Symptoms:

Necrotic ring spot (NRS) first first appears as small, light-green spots and progresses to thinned, circular patches approximately 3" to 15" in diameter. Patches can expand up to 3' in diameter; eventually turn brown or straw-colored. The "frog-eye" symptom of living grass within a ring may be prominent, but arcs and rings can be diffuse, especially in red fescue. Disease is severe in shaded and sunny areas in bluegrass, but mostly restricted to shade for red fescue. The pathogen attacks roots and rhizomes, and there are no distinctive leaf lesions or foliar mycelium. Roots of infected plants turn brown to black.

Conditions Favoring Disease:

Necrotic ring spot typically develops in moist areas in spring, but may not become apparent until heat and drought stresses place a lethal stress on infected roots in the summer. Seeded, as well as sodded sites, in newly cleared woodlands/sub-divisions are most susceptible, but older stands also are prone to damage, especially red fescue. Compacted soil, shade, high soil pH and high nitrogen fertility promote the disease.

Management Tips:

• Minimize shade!
• Raise mowing height.
• Reduce soil compaction through core aeration.
• Maintain adequate nitrogen and balanced phosphorous and potassium fertility.
• Avoid drought stress.
• Reduce thatch.
• Overseed with resistant cultivars or immune species (e.g., perennial ryegrass, and hard or tall fescue) where appropriate.
• Apply a proper systemic fungicide in late fall on a preventive basis where the disease is chronic.

Frequently occurs in:

All regions where cool-season grasses are grown.