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NRS is chemically treatable when combined with proper cultural practices. There are about 4 or 5 different fungicides that work. I have used Banner, it seemed to work fine. The fungicide route is expensive and they are also one of the more nasty chemicals to work with. Try the cultural route, then go for the chemical.
Pathogen is active in cool weather of spring and fall, damaging root system. Symptoms appear in heat of summer because roots cannot translocate enough water to leaves. Summer management is to cool the grass: daily application of 0.1" of water during the hottest part of day, probably 3-5 PM out there. In times of extended drought, increase to 0.2" per day.
Since you are dealing with root damage, recovery depends greatly on adequate levels of fertilization for the specific site. Slow release nitrogen formulations are more effective in disease mgmt than fast release types.
Curative fungicides - benzimidazoles: benomyl or thiopanate methyl.
Preventive fungicides - DMI's: fenarimol or propiconazole; or iprodione. Not effective unless adequate nitrogen is applied.
Above paraphrazed from Management of Turfgrass Diseases, Second Edition, by Dr. J. M. Vargas, Jr. Great book for management, cultural and chemical, of many diseases in warm and cool season grasses.
Dr. Vargas and others have also determined exact soil temps to start preventative treatments in spring, but I forget where that info is.
Do not waste any time or $$ on control chemicals if you do not have a positive lab I.D. Without any intervention, in our area patch diseases have a 5 year cycle: yr 1: noticeable rings; yr 2: Very bad dead spots; yr 3: HORRIBLE lawn; yr 4: like yr 2; and yr 5: like yr 1. The high population of the fungus might attract a predator organism that wipes it out in 3 years.
Cultural modifications will provide the quickest victory over your disease, if it is Necrotic ring spot for sure. Since this is a root disease, remove root stresses from your cultural practices: eliminate preemergent application (but in my experience you will not see results until 20 - 24 months after last app) and reduce postemerg weed controls: spot treat instead of cover spray. If you could state the age of this lawn, and if it was seed or sod installation, I might be able to give more pointed cultural practices.