Homeowners often notice gray mold growing on their lawn grass. The presence of gray mold on grass is usually a type of slime mold, which often occurs after periods of rainy weather in times of high humidity. Slime molds are not harmful to lawn grass and often disappear on their own.
Slime mold is caused by a fungus known as Physarum cinereum, which affects all weeds and lawn grasses such as Bermuda and St. Augustine. While this mold does not attack lawn grass, it lives on the surface, feeding on fungi, bacteria and dead organic matter in the grass. Slime molds usually appear during the warm, humid summer months after extended periods of rain. Some grasses get covered with patches of slime mold each year when weather conditions are favorable.
Slime mold is characterized by a creamy white or gray fungus on the surface of lawn grass. After a few days, this fungus turns into a gray or white powdery substance. Black fruiting structures known as sporangia are tiny sacks of black fungi that develop within the gray moldy substance. Lawn grass covered with slime mold may turn yellow from lack of sunlight if mold is present for several days. Slime mold is usually four to six inches in diameter, but has been known to grow to several feet across. The presence of slime molds for several days may place the lawn grass at an increased risk of attack by damaging organisms or diseases.
Slime mold occurs after rainy periods and is often worsened when homeowners apply dry mulch over the top of mulch colonized with mold fungi. Apply mulch to a depth of no more than 2 inches, and a coarse, fresh, woody mulch causes fewer fungal problems. Bark nuggets from pine trees and cypress trees are the most resistant to slime mold fungi.
Slime mold is not harmful to your lawn grass or garden plants and will usually disappear on its own after a few days. Many homeowners, however, cannot tolerate looking at gray, slimy patches of mold on their grass. Raking or mowing slime mold will remove the mold on your grass. Spraying grass with a hard stream of water is not recommended to remove slime mold, because the water may aggravate the condition if the humidity is high. Slime mold that develops in mulch may produce toadstools, which can be poisonous. Be sure to mow over toadstools to remove them, especially if you have small children at home. There are currently no chemical control products recommended in the treatment of slime mold fungi.