Necrotic Ringspot of Kentucky Bluegrass

Ned Tisserat  
  Extension Specialist, Plant Pathology

Symptoms

Necrotic ringspot, caused by the soilborne fungus Ophiosphaerella korrae, is primarily a problem of Kentucky bluegrass, but may also be found on red fescue and annual bluegrass. In our region, necrotic ringspot symptoms usually appear in late spring or early fall, but they may continue into the summer months.

Necrotic ringspot results in numerous circular to arc-shaped patches of dead or dying turfgrass roughly one foot or more in diameter (Figure 1). The affected turf in the ring is slightly matted, whereas the turfgrass in the middle of the "frogeye" remains healthy and green. The disease often shows up two years after sodding and may reappear in the same location year after year. Affected plants exhibit extensive root discoloration and rot. Necrotic ringspot symptoms may be confused with a more common bluegrass patch disease called summer patch. However, summer patch is a hot weather disease with symptoms appearing in late June or July. Frogeye patches associated with summer patch also tend to be smaller than those caused by necrotic ringspot.

Figure 1.  Necrotic ringspot of Kentucky bluegrass.

Conditions for Disease Development

Necrotic ringspot is caused by a soil-borne fungus called Ophiosphaerella (Leptosphaeria) korrae. The fungus primarily colonizes roots during the spring and fall resulting in thinned turf and frogeye symptoms. Small pear-shaped fruiting structures (called ascocarps) of the fungus may form on decaying crowns or roots, but they are rarely observed in the field (Figures 2 and 3). The fungus persists on infected roots and causes recurring symptoms. The disease severity may begin to decline naturally with time.

Disease Management

Management practices that reduce turfgrass stress will help suppress, but not completely control necrotic ringspot. Turfgrass should be maintained in a vigorous, but not overstimulated, growing condition. A balanced fertilization program is important. Avoid excessive nitrogen fertilization during the summer months. Seventy-five percent of the nitrogen fertilizer should be applied in the fall. Slow-release nitrogen fertilizers should be used for spring fertilization. Mowing heights should be maintained above 2 1/2 inches on residential lawns. Thatch reduction is important for suppressing disease development, and infested lawns should be dethatched yearly and/or core-aerated. A few older Kentucky bluegrass cultivars have been reported to be resistant to necrotic ringspot, although resistance of most newer cultivars has not been determined. Fungicides can suppress necrotic ringspot, but these applications are expensive and may not be warranted in most landscape situations. Fungicide applications should be made in the spring or fall when soil temperatures reach 65 degrees F.

Heritage

Limited testing

Rubigan

good

Eagle

good

Banner MAXX

good

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Web updated 9/01/06
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