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There is a disease in the Denver Colorado metro area lawns that is very common called Necrotic Ring Spot. It is a disease in the soil caused by spores that can lie dormant for up to 30 years and then become active at any time. It is characterized by what are called scars that are circular dead spots with one tuft of live grass in the center. It can be spread by a mower particularly if you use a service that mows numerous properties on a weekly basis. The only way to prevent this is to bleach the blades and tires after every lawn mowed. So obviously that's NOT going to happen because time is money and those guys need to mow as many lawns as quickly as possible in a day. So what can be done about it? Well, if you were to try to remove the diseased soil, you would have to remove at least 18 inches of diseased soil and if any of the diseased soil touches the clean soil, you have wasted your time. So that is not a good solution. If you tear up the affected grass and re sod it, it will last about a year and then it will come right back because most sod is primarily Kentucky Bluegrass. If you treat it with fungicide, it will at most keep the scars from spreading, but they are not very affective and very expensive and don't get rid of the scars. The fungicide that worked the best was called Rubigan and it was very expensive and they don't make it anymore. So the BEST solution is to seed the affected areas with a different variety of grass that's not affected by this disease. That would be Perennial Rye grass. Bluegrass is affected by it, Perennial Rye is not. So you basically just cover it up with a new type of grass. Perennial Rye is a sunny type of grass and germinates in 10-14 days, blends in well with the Bluegrass and is more disease resistant than the Bluegrass. I love the looks of a Blue grass lawn, but it is best to introduce some Perennial Rye so that you have about a 50-50 mix of Bluegrass and Perennial Rye.