Black spots on your lawn can be caused by a disease known as necrotic ring spot. This disease can produce symptoms throughout the entire growing season, but worsens in the hottest part of the summer.
Necrotic ring spot or NRS is a perennial disease that commonly occurs on Kentucky bluegrass, red fescue and annual bluegrass. NRS is caused by a fungus known as Leptosphaeria korrae, which is active during the spring and fall. Once NRS is present in a lawn, it is often hard to control and may remain problematic for as long as four years. NRS is most common in high-maintenance lawn grass established more than three years.
NRS symptoms include light patches on lawn grass, which measure 6 to 24 inches in diameter. When the grass begins to die inside the patches, it often develops a "frog's eye" pattern. Affected areas of grass become covered with black strands of NRS fungus. Grass decomposes in these areas and looks like a sunken crater. Patches developing in areas of heavy shade typically have severe symptoms.
Once NRS is present in your lawn grass, cultural control methods must be implemented to prevent further damage. Water your grass lightly and often, keeping the top portion of lawn grass moist. Avoid placing fertilisers high in nitrogen content on your lawn grass, as this promotes fungal disease development. Mow your grass to a height of less than 1/2 inch.
Sulphur can reduce the severity of NRS in lawn grass. Apply sulphur during regular fertilisation applications as a sulphur-coated urea. Fungicides are available at garden centres for the treatment of NRS. Apply fungicides in the spring before the symptoms of disease are present. Reapply fungicides one month after the initial application. Severe cases of NRS may require a third application of fungicides to keep it under control. Once the symptoms of NRS are present in your lawn grass, fungicides are no longer helpful and should not be applied until the next spring.