Q: In October we installed fescue sod over our entire yard. It remained absolutely beautiful until May. It seems that overnight it got big brown patches. After closer review under a magnifying glass it looks like the blades of grass are rotting right at the root.
A: The brown border around the tan spots on the leaf blades is a definitive sign for brown patch disease.
In my experience, overwatering and/or overfertilizing are the common causes of this disease.
You can prevent and control the disease with thiophanate – methyl (Infuse Lawn & Landscape Granules) (click for sources) or propiconazole (Bioadvanced Fungus Control for Lawns) (click for sources).