Q. What is happening to my lawn? It has developed yellow spots that seem to expand in a circular manner. I have dug up the grass and find no grubs under the yellow section of grass.
Your great images helped me diagnose the problem. Your lawn is most likely infected with brown patch disease. Read this fact sheet to make sure that the symptoms of this disease match up to the symptoms in your lawn: http://edis.ifas.ufl.edu/lh044.
You can treat with a fungicide listed in the fact sheet but most fungicides will only help prevent, not cure, the spread.
Why does your lawn have this disease? When lawns are overwatered, overfertilized and cut too short, the grass is stressed and more susceptible to disease and insect pests.
It may be best to avoid fertilizing plants during the winter, late October through mid-March since the grass and other plants are usually not growing during this time. When plants are not active, fertilizer is wasted and can become a water pollutant.
Here is a University of Florida webpage with fact sheets on how to care for your lawn and tell when the grass needs water: http://edis.ifas.ufl.edu/topic_lawn_care.
Because weeds are likely to start growing in the damaged area, you may need to take care of them before they get out of control. Also, you can consider removing the damaged area and replanting.