Lawn diseases are most common in grasses that are stressed because of weather conditions or poor management practices. The first step in management is to understand the nature of the disorder. There are a number of reasons a lawn can develop dead circles. Among them are bacterial and fungal diseases and poor adaptation of the grass.
Dollar spot is a fungal lawn disease common to fescue, ryegrass, Bermuda grass and annual bluegrass. The disease is characterized by 1- to 5-inch circles of dead grass that gradually grow larger. The disease is more frequent in late spring to early fall in lawns that get excessive moisture. Temperatures of 60 to 80 degrees F. are most conducive to this fungus. Use a compost top dressing to control the disease, and reduce watering. Fungicides are best applied in early spring or fall. The Colorado State University Extension website recommends reseeding a top-dressed lawn with a more resistant type of grass.
Fusarium patch is also called pink snow mold. The most susceptible grass varieties includes annual bluegrass, fescues, ryegrasses, zoysiagrass and bluegrasses. The fungal disease causes the development of 1- to 2-inch circles of dead grass that can grow as large as 12 inches. Fusarium patch is common in lawns fed with high-nitrogen fertilizers. Grass is most susceptible in temperatures of 40 to 60 degrees. Temperatures exceeding 70 degrees inhibit the growth of the fungus. Control includes reducing shade and aerating the grass. Avoid the use of nitrogen-rich fertilizers. If fungicides are used, the best time to apply is immediately after the appearance of symptoms.
Leaf spot is most frequently seen in fescues, ryegrasses and bluegrasses. It causes the development of dead-brown circles with dark-brown or purple borders. The crowns and roots of the grass blades often have dark brown rot. The disease spreads more aggressively in temperatures of 70 to 90 degrees F., high humidity and in lawns that get excessive applications of high-nitrogen fertilizers. Control includes reducing shade and improving soil aeration and drainage. Though fungicides are available, they are a poor control strategy for this disease.