There are 14 species of fungi that thrive in the roots and on the blades of common lawn grass. Barring obvious mushrooms and toadstools, these fungal infections appear most often as a dying brown patch of grass or a ring of wilting grass roughly 1 foot in diameter. Lawn fungi take advantage of weak or stressed lawns and can be very difficult to get rid of once they've become firmly entrenched in your yard.
The cheapest and most effective methods of treatment are preventative measures. Water your lawn only in the morning when the grass can quickly take up moisture to avoid puddles in which fungi can propagate. Aerate the lawn often and rake up any thatch pressed against the ground to make sure fungi can't take advantage of the dark areas on the soil's surface, just above the grass. Sharpen the blades of your lawnmower once a year. If the mower cuts cleanly, it does not stress the grass as much, which enables the grass to better fight off infection. When fertilizing your lawn, be careful to follow the fertilizer's directions closely. Too much nitrogen in the soil encourages fungal growth.
Systemic fungicides such as Ridomil, Aliette and Phostrol are liquid suspensions that are absorbed by lawn grass on contact. As the fungicide passes through the vascular system of the host grass, it kills any fungus it touches. It should be noted that systemic fungicides, while fast-acting, can cause a fungal infection to develop a resistance to it if overused.
Contact fungicides such as Bravo, Dithane, and Orbit, come in a powder that's sprinkled across the lawn. As the fungicides only work when they directly contact the fungi, the lawn is then watered. While fungi do not develop resistances to contact fungicides, it can take much longer for them to work. They do not kill the fungus; they inhibit the fungus's spores from germinating. This means the fungus cannot reproduce, and once the present fungus finishes its life cycle, the infection is gone. While some fungus can live for decades, those that infect lawns typically do not last more than six months.