Necrotic ring spot (NRS) (Ophiosphaerella korrae) is a serious fungal root rot that affects Kentucky bluegrass, annual bluegrass, fescue and bentgrass. NRS can devastate cool-season grasses throughout the northern tier states of the United States and throughout Canada. The unsightly visual symptoms and persistence of the disease rank it high among the list of problems that professional turfgrass managers want to correct or avoid.
Until recently, several patch diseases, including NRS, were grouped together under Fusarium blight, or various patch disease syndromes. Other diseases, such as Fusarium patch, take-all patch and summer patch, are often confused with NRS. Pathologists now know that NRS has a specific behavior and can be isolated as a single disease.
The disease manifests during the cool-weather months of March through May, and again September through November. It usually appears as a doughnut-shape damaged area that can range from several inches to over 1 to 2 feet in diameter. The descriptive term “frog-eye” is often used because the center of the damaged circle consists of green grass resulting in the eye-like appearance. Similar diseases mentioned earlier are also described as a frog eye and can result in misdiagnoses. The most reliable step in identification requires a microscopic examination of the roots infected with dark hyphae of the fungus. Hyphae are thread-like filaments of the growing fungi. There are private and university plant pathology labs that offer diagnostic services.
Due to the serious nature of NRS and the economic impact to lawns, private companies and turfgrass organizations have invested money to support university research on the biology, management and control of NRS. Publications on NRS are available from a number of universities.
Dead or dying circular patches are not always NRS related. They may be localized dry spots or another disease. NRS is often confused with several patch-like diseases, the closest being summer patch. Seasonal observations can help in making the correct diagnosis. Summer patch disease symptoms first occur in late summer during hot weather. Fairy ring disease symptoms may also appear similar to NRS and occur any time of the year. Fairy rings often have a dark green ring of grass on the outside of the expanding ring. Like fairy rings, NRS spots tend to occur in the same exact patches the following year, just larger in diameter. This can help distinguish NRS from summer patch, which has randomly occurring patches each year. Another similar disease is take-all patch, but that disease only affects bentgrass.
Symptoms of NRS appear in late May/early June, particularly if the lawn has experienced environmental stress, such as drought. Be aware that symptoms can continue through the summer, or disappear and reappear in the fall. Checking for patches in localized areas of the lawn may also give some clues to the correct identification. NRS is often more severe in dry or compacted areas such as slopes, mounds or near areas where heat is radiating from walls or walkways or in areas afflicted by tree roots.
Planting grass varieties that resist infection is the primary line of defense against NRS. Research by Dr. Doug Brede, director of research at Jacklin Seed by Simplot, reports that mixing turfgrass species, such as perennial ryegrass to reduce NRS in stands with Kentucky bluegrass, may not be as effective a control strategy as simply utilizing resistant bluegrass cultivars and this strategy provided better, long-term solution to cultural control of NRS. This advice does not help much for existing lawns that have the disease unless total renovation is planned.
Below are some Kentucky bluegrass varieties that have shown good performance against NRS, as well as susceptible varieties from www.ntep.org. If you can find out what grass varieties were planted, you may be able to narrow your identification of the disease. Check with your local extension agent for varieties that have performed well in your area.
Resistant varieties: Award, Impact, Liberator, Midnight, Total Eclipse, Apollo, Odyssey, Princeton 105, America, Sidekick, SR2000, Unique, Baron, Monte Carlo, Abbey and Midnight II.
Susceptible varieties: Shamrock, Raven, Compact, Envicta, Serene, Bodacious, Chateau, Blue Knight.
NRS control requires an approach that considers both cultural and chemical methods for management. Nathan Ruffer with Earth Services and Supply in western Michigan says, “We suggest organic fertilizers in conjunction with fungicides to address both the microbial health of the soil, as well as effective fungicides that can be applied to minimize the damage to lawns.” The use of organic amendments and compost is a common approach to managing NRS and has been documented to reduce disease severity. However, suppression might be inconsistent and is not guaranteed. Nevertheless, fertilization is important in recovery from NRS symptoms. Without NPK, the spots will persist almost indefinitely.
There are several fungicides registered for use on home lawns. Check with your state to see which fungicides are legal to use. Fungicide application timing is very important. Applications should be made when soil temperatures at the 3-inch depth average 55 to 70 degrees. This is when the disease organism is active, even though symptoms may not be evident. Fungicide applications should be when soils are moist and the fungus is active in May/June and then followed by water-drenching immediately after application to prevent the fungicide from drying on the foliage. The disease organism is in the root area and the fungicide must be moved into the rootzone to be effective. More than one yearly application is generally needed.
Spokane Pro Care serves over 10,000 residential and commercial lawns in Washington and Idaho. According to Manager Evan Ludeman, “We apply Eagle fungicide with a soil penetrant to assist in moving the fungicide through the thatch into the rootzone. We also add a fish emulsion to improve the microbial balance of the soil and thatch.
“When a customer has NRS, we educate them about all the options, and let them know that 100 percent control may not be possible,” says Ludeman. “We tell them we can manage the disease severity by combining cultural, organic amendments and fungicides.” Ludeman estimates that Spokane Pro Care treats about 120 lawns with fungicides for the management of NRS.
Joe Staker, owner of Evergreen of Utah Lawn Care, services about 500 customers in the Salt Lake City area. His approach includes educating the homeowner about the conditions that favor the disease and adjusting the cultural practices where possible. “In most cases, we achieve 80 percent control in two to three years, and in some cases we achieve 100 percent control. I have a number of lawns where the disease was 100 percent controlled over a two-year control program, and the disease has not reappeared,” says Staker. “If the soil conditions are good, and the customer is willing to implement good cultural practices, I can be reasonably sure we can control the disease with fungicide applications.” Staker applies one application in the spring for moderately infected lawns, but sometimes applies an additional fall application if the disease infection is severe.
Application costs average around $50 per 1,000 square feet per application. Some lawn care companies apply fungicide on the entire lawn, while others spot-treat infected areas.
Treatment for NRS is a long-term program. In some areas of the U.S., NRS severity decreases over time, but in other areas it persists for many years. Follow-up is paramount to success, and even then, control is not always complete.
Cultural management techniques
NRS appears to be most severe when sod grown from susceptible Kentucky bluegrass varieties is laid on infertile, sandy or otherwise poorly prepared soils. This may be because the disease takes two to three years after seeding to appear, and sodding delivers one to two-year-old turf to the homeowner, speeding up the process. Compaction and poor drainage increase disease occurrence. Soils that have balanced fertility, proper drainage, uniform irrigation and good soil conditions will have the best chance to support healthy turfgrass. Even if the disease is present, a healthy lawn may be able to recover from spring and fall disease attacks. While some pathologists report the disease may “run its course” and disappear on its own after about five years, Staker reports lawns that are 20 years old with the disease.
Typical cultural practices recommended for any healthy lawn are recommended for minimizing the negative effects of NRS:
- Mow at a recommended height and frequency.
- Irrigate to wet the rootzone. Avoid shallow, frequent irrigation that only wets the top portion of the rootzone.
- Achieve balanced fertility based on soil tests and avoid applying nitrogen fertilizer on cool-season grasses during hot summer months.
- Do not allow excess thatch to develop.
- Aerate heavily trafficked areas where soils have become compacted.
- Install drainage where necessary.
If possible, purchase sod and seed from Kentucky bluegrass cultivars that show good performance in your area. Overseeding damaged areas with perennial ryegrass may temporarily cover up the scars, but the disease may grow beyond the overseeded area. Seeding or sodding NRS patches will result in a spotty appearance and is not recommended. Lawn care companies may recommend total renovation when lawns are seriously infected with NRS. The renovation program may include soil modification in addition to non-selective herbicide applications followed by new seed or sod that is resistant to NRS.
Necrotic ring spot is a disease that must be addressed early if lawns are to survive the potentially devastating damage that can occur.
The author is president of James Connolly Consulting, Ltd., in Spokane, Wash., an agronomic service company providing practical solutions to turfgrass management and construction for sports fields, golf courses and other recreational turfgrass facilities.