Sometimes no matter how healthy your turf is, environmental conditions such as moisture and humidity can be ideal for disease growth to occur. The most common symptom is seeing little areas start to wilt and die off. An easy way to tell if it's a disease is to wake up when the dew is still on the grass and take a close look to see if there are any cobweb type looking funguses growing on the plant. There are many types of turf diseases and many ways of diagnosing them. If you start to notice your lawn looking sick call immediately as some types of disease can spread quickly. These can be stopped from spreading using a fungicide spray or granule.
You can also add a preventative fungicide application to your program. It is applied with each fertilizer application. This will help prevent diseases from occurring.