Grass or lawn fungus appears when temperatures are hot and the ground is too wet. Heavy rain during the summer months over-saturates the soil, providing fungus with the ideal conditions to grow and spread. But fungus will also crop up if dead leaves or dead grass is permitted to accumulate on the lawn, cutting off the air circulation grass needs to remain healthy.
Using a lawn spreader, apply corn meal to your lawn at the rate of 9 kg (20 pounds) of corn meal to 90 metres squared (1,000 square feet) of lawn. The corn meal attracts Trichoderma, a predator fungus that feeds on other fungi but will not damage your lawn. In 10 days or so, the Trichoderma will have eradicated the lawn fungus, and in another 10 days new grass will begin to grow, filling in the bare patches in your lawn. As a preventive measure, apply corn meal about once a month at the rate of 4.5 kg per 90 metres squared (10 pounds per 1,000 square feet) of lawn.
Spray a liquid fungicide over the effected area. Look for a product that has Chloronthalonil as its active ingredient. It will kill the fungus upon contact. Chloronthalonil penetrates the soil, killing off fungus spores for up to two weeks. Using a sprayer attached to your garden hose, apply the fungicide according the directions on the container.
Mix one part hydrogen peroxide with nine parts ordinary tap water. Using a spray bottle, apply the solution to the affected area. Hydrogen peroxide will not burn healthy grass or damage other plants, and it is safe to use around children and house pets.
Thatch your lawn in the spring and again at the beginning of fall. If you have not thatched before and fungus has already appeared, thatch and rake your lawn thoroughly, bag the dead leaves and clippings in plastic so the fungus spores will not spread, and put the bags out with the trash.
The time of day when you water your lawn may be causing grass fungus. Do not water your lawn in the early evening; it will remain wet all night, creating a perfect breeding ground for fungus spores. Instead, water your lawn early in the morning, just before dawn if possible, so the grass and soil will dry gradually as daytime temperatures rise.