Ack, try not to use fungicides. Not only may it kill off fungi that will benefit your grass, it doesn't treat the source of the fungi. The symptom of this is dead grass.
Typical causes of fungi is too much water or fertilizer, or perhaps improper mowing or adding fertilizer to the grass at the wrong time.
To best deal with it, what does the fungi look like? Knowing the specific type of fungi you're having problems with will help narrow down the cure for it.
