Diseases can form a resistance to fungicides after repeated use. Try to minimize this through using various types of fungicides. There are three types of fungicides available.
CONTACT FUNGICIDES: once applied, remain on the plant's surface and kill spores that come in contact with it.
SYSTEMIC FUNGICIDES: applied to the leaves and then moves throughout the plants circulation system including its roots. Has a longer residual life span than contact types.
PENETRANT FUNGICIDES: similar to Systemic types, but act as a preventative treatment to stop the growth of pathogens.
Best cure is an ounce or two of prevention The best prevention though is maintaining healthy turf, be careful of over-watering, or watering at the wrong time of day (see watering info). Here then are a list of common lawn diseases and what can be done to prevent future infections.
Ask a professional lawn care provider for specific treatments recommended for your specific geographic area. (See Selecting a Lawn Pro in your area)