Over time older lawns will typically have a variety of grasses or grass like plants that will have established themselves. This is normal and expected. You could be describing annual bluegrass, nimblewill, maybe a fescue or a bent grass. In general they all have a spreading habit with limited root system. Nibblewill and fescues seem weak in habit. Proper Indentification will be key in determining if it is the grass plants themselves or you do have a fungus disease infecting a particular kind of grass in your lawn. Some grasses thrive in cooler weather and do very poorly in hot weather. Your watering practices would not likely promote disease, especially if you are watering early in the morning and allow the lawn to be dry by nightfall. Locate your local County Extension Office and take a good sample with you. They should be able to help you.
Sincerely, Richard
