Lawn diseases are caused by both living and nonliving factors. Nonliving factors include nutritional imbalances and drought, which weaken plants ability to fight off living disease agents.
Among the living disease-causing organisms are bacteria, fungi, viruses, parasitic plants and nematodes. Of these, only fungi are a major cause of lawn diseases. Viruses cause many plant diseases, but only one major lawn disease: St. Augustinegrass Decline (SAD).
The most important principle in preventing lawn disease is to keep the lawn healthy. This means fertilization on a regular schedule, water as needed, proper mowing practices and maintaining a healthy soil. Avoid planting the wrong type of grass for your location(s).