Question
Recently (September) I had a new irrigation system installed using lake water, plus new top soil, seeding, and strawing of yard throughout. All the grass came up beautifully within 2 weeks, and soon thereafter it was growing healthy and green.  However in October I began noticing areas (not the entire yard) that were turning yellow and thinning.  Now some of the yard is showing typical green grass and other parts yellow, thin areas.  Can you tell me both what might be the problem, and treatment as well.  We've invested a lot and do not want to lose it.  It is now December and we live in North Carolina (Triad area).  

Answer
I cannot tell you what the problem is but I can suggest how you can find out.
1. Consider the edges of the bad areas. Are they straight and meet at angles? If yes, the problem might be 'abiotic'. A deficiency in that patch of ground or something toxic in there. A soil test of the bad areas compared with the same from the good area might provide clues. If the edges are curved or circular then you might have an insect or disease problem. You can eliminate bugs by doing a 2x2x2 'soap flush' on random spots on the lawn. Mix 2 ounces of dishwashing liquid in 2 gallons of water and pour it slowly over 2 sq. ft. of lawn. Choose an area that includes both brown and green grass. In a few minutes, all the bugs in the area will try to come to the surface. Some will be harmful, others not. If no harmful insect is present, then you might have a fungus problem.
2. Pull gently on the crown of some brown grass. If it feels slimy or it comes away easily, that is a good indication of a fungus. Most fungus diseases show up as circular patches of dead grass.
If none of the above is a contributory factor, and the problem continues to spread, then it will probably be because of something in the water or soil.