Necrotic Ring Spot and Summer Patch
Necrotic Ring Spot and Summer Patch
Two of the most destructive patch diseases of turfgrass are necrotic ring spot, caused by Leptosphaeria korrae, and summer patch, cause by Magnaporthe poae. Necrotic ring spot generally occurs on Kentucky bluegrass during spring and fall. Summer patch occurs during the hot portion of the summer.
Necrotic ring spot most commonly occurs when wet weather is followed by hot, dry periods. Kentucky bluegrass is the primary host, but the disease can occur on red fescue and Poa annua. Summer patch is most active in turfs irrigated by frequent rain or watering. It is common in most turfs, but certain environmental, site, or cultural conditions will enhance development of symptoms. These conditions include heavy thatch, low mowing in midsummer, unbalanced fertility, compaction, a site with a steep slope or which is exposed to heat, and poorly adapted grasses.
Symptoms
Symptoms of this disease are virtually indistinguishable in affected turf areas, making laboratory examination necessary. Laboratory identification takes three to six months and is not a simple procedure. Affected turfs show 6- to 12-inch circular, semi-circular, or serpentine patches giving the area a pockmarked or doughnut appearance. The dead grass is light tan and matted. Many of the patches will have a tuft of apparently healthy grass in the center. This is often referred to as the “frog-eye” symptom. Plants at the edges of the patches may be unthrifty due to a rotting of the roots by either of the pathogens. If symptoms occur in midsummer, the disease is probably summer patch. If they show up in the spring or fall, the disease is more likely necrotic ring spot.
Prevention and Control
On established turfs, the most important control is to eliminate plant stresses that favor disease development. The key to prevention is to avoid management practices that promote rapid top growth at the expense of root development. These practices include reducing thatch, growing improved and disease-resistant cultivars, fertilizing and watering properly, syringing heat exposed turf during midday in July and August, and eliminating compaction. It is always best to use proper mowing height and frequency. These are cultural practices that will prevent serious injury from either disease. On turfs with a history of necrotic ring spot or summer patch, fungicides will effectively control the two diseases when applied from April to early June with one or two additional applications at three- to four-week intervals.
Reference
J. E. Watkins. (1997). Diseases of Cool Season Turfgrass. In F. Baxendale, Ph.D., & R. Gaussoin, Ph.D., Integrated Turfgrass Management for the Northern Great Plains (pp. 125-126). Location: Nebraska