There are many factors responsible for lawn discoloration, but the few that cause blades to turn black and form a black spot or patch in the grass are usually due to fungal infestations. Different types of fungi ranging from green to black in color appear on grass weakened due to improper cultural practices including underwatering or poor fertilization.
Fungi responsible for leaf spot damage a host of plants including shrubs, trees, fruits, vegetables and grasses. Although they resembles mold, the fungi are actually small to large colonies comprised of tiny mushrooms barely visible to the naked eye that give the infested grass a purplish-black to red-black hue. The fungi responsible for leaf spot do not damage the health of the plant unless they cover large areas of leaf surface that blocks sunlight and inhibits the process of photosynthesis.
Usually infecting perennial ryegrass, blue grass and fine fescue, leaf smut occurs during the fall and spring when temperatures remain consistent between 50 and 60 degrees Fahrenheit. The fungi causes grass to appear yellow and stunted before streaks of black spores become visible through the surface. Although the symptoms of smut usually disappear during the summer, the infestation in the grass remains. Treat smut infestations in grass with systemic fungicides.
The disease pythium blight usually occurs on newly seeded lawns and appears as patches of black, flattened grass. Symptoms of the disease include cobweb-like growths of fungus visible during warm, wet mornings and nights. Patches usually start small but spread and grow wider than 12 inches when left untreated. Control options include spraying infested areas with a registered fungicide.
Although slime mold fungus only causes aesthetic damage and rarely affects grass growth, its sudden appearance on otherwise healthy turf often leaves homeowners baffled and confused. The disease commonly occurs on warm-season grasses in warm, moist areas. Symptoms of the disease include 4- to 8-inch-wide circular patches of fruiting bodies on grass. Because individual fruiting bodies are tiny, thousands are embedded on grass blades. Wash infected parts of the grass with a hard stream of water or mow the area to remove the fruiting bodies of slime mold. Collect grass clippings in a collection bag and discard to prevent its spread.
To prevent fungal diseases responsible for black spots, irrigate lawn grass in the morning so any drops on the blades are dry by evening. Follow a good regimen of fertilizing and watering your grass to boost its strength so it fights off disease-spreading bodies.