Black spots on grass not only look unattractive and serve as an eyesore, they sabotage the health and vigor of the lawn and weaken its structure. Mostly caused by fungal infestations, such spots or patches signal serious health problems that require immediate attention to control spread and help the grass regain its natural color. Improper cultural practices further weaken the structure of the grass, making it susceptible to the fungi that cause disease.
Black spots on lawn grass occur due to fungal diseases such as leaf spot, pythium blight, leaf smut and slime mold. Leaf spot affects many flowering and foliage plants, trees, shrubs and vegetables. Leaf smut is more common on bluegrass, fine fescue and perennial ryegrass, while slime mold occurs on warm-season grasses and rarely affects cool-season ones.
Symptoms of leaf spot include reddish-black to purplish-black spots on grass created by colonies of tiny mushrooms. Although individual mushrooms are not clearly visible to the naked eye, the colonies they form when joined together resemble a moldlike mass on grass. More common on newly seeded lawns than established ones, pythium blight forms cobweblike growths of fungi usually apparent during humid, wet mornings or evenings. Left untreated, the patches that start small grow over 1 foot in width. Initial symptoms of leaf smut appear as yellow patches on grass that stunt growth. Left untreated, black streaks of fungal spores appear on the yellow patches. Slime mold fungi cause 5- to 8-inch-wide patches or large spots of black fruiting bodies on the grass.
Scattered leaf spots on grass do not harm the lawn. Cultural management of the disease includes mowing the grass slightly higher than the required height to reduce stress, avoiding excessive spring fertilization and watering deeply but infrequently. If proper cultural practices do not reduce the severity of the disease, apply a registered fungicide and repeat application every five to 10 days until symptoms disappear. To treat pythium blight, spray a registered fungicide over infected grass. Manage leaf smut with systemic fungicide. Douse parts of the lawn infested with slime mold with a fast jet of water from a hose to remove fruiting bodies.
Maintain a healthy, stress-free lawn so it naturally fights diseases and reduces the occurrences of black spots. Irrigate the lawn infrequently but deeply, preferably in the morning so any droplets on blades are dry by evening. Follow a good fertilizer regimen to boost the grass's immune system so it combats disease-spreading bodies.