Necrotic ring spot

Circular straw-colored patches and rings 8-60 cm, often with green centers (frog-eye). Roots and crowns are dark and rotted; recovery is slow.

The symptoms of necrotic ring spot (NRS) include circular patches of yellow or orange leaves. These patches usually start as small light green spots, 2-4 inches in diameter, but can enlarge to a foot or more. The affected turf may turn reddish-brown to bronze and then fade to a straw color before dying. The patches may also leave a sunken crater and blackened roots and rhizomes.

Signs of necrotic ring spot include the presence of runner hyphae and perithecia (rare to see in nature) which can be observed under a microscope.

The biology of O. korrae of cool season turfgrasses is not well understood, however it is believed to be a soil-borne fungus that infects the roots and crowns of cool season turfgrasses such as Kentucky bluegrass, annual bluegrass, and fine fescue. The fungus can survive in the soil for several years and can infect the turfgrass through wounds or natural openings in the plant. Once the fungus infects the plant, it causes necrosis (death) of the roots, crowns, and leaf tissue, resulting in circular patches of dead or dying grass. It is most active in cool, moist conditions, and can cause significant damage to turfgrass in these conditions.

Necrotic ring spot (NRS) is a turfgrass disease that occurs during cool, wet weather in the spring and fall. It infects and colonizes roots and rhizomes and is most severe in compacted soils. The disease is most prevalent on younger lawns (2-4 years old) and occurs over a wide range of soil pH from 5-8. The severity of the disease declines after several years.