Question
Hi, I have a large yard , I planted it 4 years ago and before it was my yard it was a farm field. Last year I noticed large dark green circles in the grass, and white mushrooms grow in or close by these circles. It has spread and is all over the  yard and worse than last year. I have never seen this before and would like to know what I can do to get rid of the circles and the mushrooms. Thank you

Answer
Fairy circles are a common phenomenon caused by colonization of a single fungal growth on some woody material.   In the case of a farm field, it could be old hay in the soil.
They are slow growing and may last many years.  One in northerm Michigan covers thirty acres and is estimated to be 15,000 years old.  Waiting it out may not be an option.
I have read that products like Prostar and Lawn Guardian are effective.  However, I would check with your local ag agent to see what will work for you.  Alternately, a professional spraying service will have probably dealt with this problem before and have a solution.
The other choice is to dig up the turf six or more inches down, replace the soil, and sew new grass.  There must have been a source of carbon (wood) below the surface.