Necrotic ring spot is one of three patch diseases caused by root pathogens that are problems of cool-season turfs. The other two are summer patch of bluegrasses and fine fescues and take-all patch of creeping bentgrass. Though not all caused by the same fungus, these diseases have similar patch-type symptoms, the causal fungi are related and similar in appearance, and these fungi attack grass roots and crowns in a similar manner.
Symptoms of necrotic ring spot appear as circular, ring-shaped, or serpentine patches of dead or dying turf. Affected areas may be a few inches to a foot or more in diameter. These patches may at times coalesce, or they may stand out as individual dead rings. Leaves and stems of affected turf appear yellow or red, then turn a light tan as the disease progresses. Roots and crowns of diseased plants are rotted and recovery of affected areas is slow. Necrotic ring spot seldom occurs in newly planted turf but can occur on turf that has been recently sodded. It may begin during the fourth or fifth year following seeding and can become progressively more severe.
Ophiosphaerella korrae grows on the surface of grass roots for most of the growing season without causing visible symptoms. When conditions become favorable for the disease, the fungus attacks and destroys the roots. Environmental conditions that favor necrotic ring spot may vary from one location to another. In some locations, the disease is more severe during the cool periods of spring and fall, whereas in other areas, the disease occurs only in midsummer. Necrotic ring spot is generally more severe on drought-stressed turf, but can damage turf growing in moist soils as well.