There are various things that can make a lawn turn yellowish. It may be high phosphorus, high pH, or anything that is damaging the roots. I suggest that you take a sample from the plant along with a photo of the plant in its growing location (if possible) to your County Extension Office to see if they can help diagnose the problem and recommend an appropriate course of action. Call them first as they can direct you as to how to take an appropriate sample. Most likely you will cut out a 4" by 4" plug from an affected area and place it in a zip close bag.
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