Lawn blight affects lawns that have been sodded more readily than those have been seeded, but a buildup of decomposing material on any lawn can invite disease and parasites. Lawns consisting of mostly bluegrass have a higher instance of blight, although fescue and perennial ryegrass are also susceptible.
Fungi that live in moist soil can infect stressed lawn grasses with blight during bad weather. A lawn that hasn't been maintained properly is also a prime candidate for blight. A number of fungi cause grass blight. The fungi can survive in the soil, making it difficult for the lawn to recover. Thatch buildup, overwatering and improper fertilization can all lead to the conditions favorable to disease-causing fungi to reproduce and spread.
Fasuriam blight attacks the lawn when overnight temperatures stay above 70 degrees Fahrenheit. It is considered a patch disease because the areas of dead grass in the diseased lawn appear in crescent or donut shapes.
The fungus Rhizoctonia cerealis causes yellow patch. Heavy thatch and compacted soil allow the fungus to survive and spread. As the thatch within the diseased area decomposes, the affected parts of the lawn appear to sink. Yellow patch can also affect newly laid sod. The blades of grass infected with yellow patch may have a purple or yellow tint. Yellow patch does not cause black strands on the grass, but tan spots bordered with black may occur. Treat lawns suffering from yellow patch by thinning thatch and aerating the compacted soil.
Necrotic Ring Spot
Necrotic ring spot usually occurs in lawns when temperatures are between 70 and 80 degrees Fahrenheit, although sometimes signs of damage can appear in the spring and fall. It is caused by the fungus Leptosphaeria korrae. Straw- or red-colored rings of grass surround the 6- to 24-inch patches of grass that have been covered with black strands of the fungus. It often takes several years to get rid of necrotic ring spot. Thin the thatch from the lawn annually, and change the fertilizer to one containing less nitrogen. While sufficient moisture is needed to help the lawn recover, more frequent, light watering is advised.
Melting out, sometimes called leaf spot, is a blight caused by the fungus Drechslera poae. Once established, the disease is difficult to control. It easily spreads over lawns that have received too much nitrogen fertilizer and grasses that are watered in the evening. Any buildup of thatch in the lawn should be removed. Extensively damaged lawns should be reseeded with resistant cultivars of grass seed.