One of those diseases is called Helminthosporium. Depending on the time of year, this disease has different common names. Starting out I the spring, we call it "leafspot", where the individual grass blades develop distinct spots. Later in the summer the disease has a common name of "melting out" as the fungal disease moves into the grass plant crown and will kill entire leaves. It appears that the grass is just "melting away". Another stage is one of crown and root rot. Once you are there, there is no turning back, the grass plant is dead. This can appear patchy in the lawn or those smaller patches can coalesce and leave you with much larger patches to deal with.
Lawn disease management is quite a challenge for the homeowner. Any fungicide treatments need to start before the disease is present in the lawn and then repeated repeatedly! For example a golf course may treat several times a month while monitoring the weather closely.
There are some lawn care management strategies than can help, so all is not lost. Reducing stress by proper mowing, using lower rates of nitrogen in the fertilizer applications, managing the thatch layer, timing of any watering are a few that help. Better ways to mow include mowing the grass higher which promotes deeper roots. Mowing the lawn short introduces stress to the grass plant (and the opportunity for weed seed germination). Lower rates of nitrogen will slow growth, exposing less new tender cells to fungal invasion. Some thatch is a good thing. Keeping the thatch layer to one half inch or less is the goal. While we have not had to really water our lawns so far this season when you do, water deeply to again encourage deep rooting of the grass plants.
Sodded lawns have the potential of more disease outbreaks than a lawn that was started from seed. Unless the soil was properly prepared, a sod to soil interface can occur, again stressing the grass plant. Sod to soil interface can be overcome by core aeration twice a year for several years. This also will aid in thatch management.
There are a number of other lawn diseases that can impact our lawns. Two that the homeowner can identify are Rust and Powdery mildew. Their names imply exactly how they look. The rust we see are the spores in structures on the leaf blade waiting to be blown around. Powdery mildew can appear on a wide number of plants including our lawns. The white fine powdery substance can be rubbed off, but the disease remains embedded in the grass leaf blade.