October is prime time for lawn fungus. Keep a watchful eye out for circular patches of yellow to brown grass forming. If you spot this turf behavior, treat it that day with fungicide to prevent large dead patches of grass. The affected grass at the center of these circles are too damaged to live and will have to fill in next year or have sod added. These large patches of thin or dead grass is what lead to a lot of the headaches people have with weeds in their lawns. Open soil is where new weed seeds tend to thrive.
If you see mushrooms growing in your lawn, this simply means you have correct conditions for fungus, but mushrooms by themselves do not lead to lawn damage. Circular rings of mushrooms are called fairy circles and are also not a cause for alarm. Homeowners tend to be nervous about mushrooms since most are poisonous to both pets and humans.