A friend recently asked me about a lawn problem called necrotic ring spot, or NRS. When I first moved to this area, this fungus that was causing dead spots in local lawns had not been identified.
It was often misdiagnosed as being caused by lawn billbugs. These insects are weevils whose larvae or grubs can cause significant damage to lawns by feeding on the roots of grass plants. However, billbugs were seldom found in the effected lawns. In 1984, when dead spots started showing up in more and more lawns around the state, WSU worked to find answers.
Numerous samples from local lawns were sent to the WSU turfgrass specialist, and lab results pointed to a fungus disease that was very similar to Ophiobolus patch or Take-all patch disease. Tri-City residents were not the only ones experiencing ugly lawn spots from this mysterious turf disease. It was also showing up in other parts of Washington, Oregon, Idaho and British Columbia.
We did not know what was causing the spots, but we did know that this disease most commonly attacked 2- to 5-year-old Kentucky bluegrass and fine fescue lawns that had been started from sod. However, it was also found attacking some older lawns. The problem appeared to be worse in vigorously growing lawns with thick thatch or ones being watered improperly. It was also common in lawns growing on a layer of topsoil that was not mixed in with the underlying compacted layer of native soil when the lawn was sodded or seeded.
NRS was originally discovered and named by Australian researchers in 1972. Dr. Gary Chastagner, WSU Plant Pathologist, isolated and identified the offending fungus in Washington lawns as NRS. University researchers theorized that NRS had become more prevalent than in the past because improved bluegrass varieties were resistant to a leaf spot disease. The leaf spot disease had been devastating to lawns, masking the presence of NRS.
With the development of leaf spot-resistant turfgrass varieties, NRS became more noticeable. Researchers noted that certain improved Kentucky bluegrass varieties, such as Baron, Merit, and Sydsport, were highly susceptible to NRS. They recommended avoiding susceptible cultivars and using a blend of less susceptible varieties, such as Eclipse, Alpine, Joy, or Mystic, for starting a lawn from seed or sod.
The symptoms of NRS are similar to those of many other lawn fungi that cause dead circular spots here and there in a lawn. The spots, or sometimes arcs, start out small but can grow to 1 to 3 feet in diameter. The dead areas typically manifest themselves during stressful hot summer weather. This happens because the fungus attacks the grass plant’s roots, rhizomes, and crowns, impairing their ability to function. As the patches grow older, the center of the “spots” often become recolonized by healthy grass or weeds, creating the appearance of a “ring.”
Cultural control is an important step in managing an NRS problem. WSU recommends annual core aeration, removing excessive thatch, watering properly, and having a well-balanced fertilizer program that avoids excessive rates of nitrogen. Proper soil preparation to ensure good drainage when starting a lawn can also help prevent the problem.
There are fungicides that are available as preventatives. These are applied in early spring or late fall, but most of these products are not readily available to home gardeners and need to be applied by lawn care companies. For more on chemical control go to https://pnwhandbooks.org/node/2994/print.
If NRS is starting to show up again in area lawns, it could be related to poor lawn care practices or possibly the use of grass varieties that are more susceptible to the disease. It certainly was a puzzle when it first appeared in the Tri-Cities. Thankfully, we know more about NRS today and how to manage it.
Marianne C. Ophardt is a retired horticulturist for Washington State University Benton County Extension.