Irregularly-shaped dead patches in your lawn could be from a fungal disease called melting-out. Melting-out is one phase manifested by the leaf, crown and root rots that are the most common and serious group of diseases attacking lawn and turf grasses in the US.
When the fungi attack the leaf portions of grass, the symptoms are small dark brown, reddish-brown, or purplish spots which appear on the grass blades from early spring to late fall. The spots increase rapidly in size, become round or oblong, and the centers gradual fade to an ash white or straw color. Sometimes, the spots are described as "eye-spots". Leaf infections are sometimes so severe that the blade is girdled and drops, sometimes entire grass plants are killed.
These fungi can be very destructive during wet, humid weather or in areas where the turf is sprinkled frequently, especially in late afternoon and early evening. The more often grass is wet and the longer it remains wet, the greater will be the chance of disease.
With the arrival of relatively dry weather, the fungi may attack the roots of diseased plants. These symptoms usually appear first in warm to hot weather as a reddish-brown decay of the root tissues. Sometimes the color turns chocolate-brown to black. Such plants lack vigor and often wilt during mid-day, even when soil moisture is abundant. The lawn may have a drouth-injured appearance, then gradually turn brown and begin thinning out. As the disease progresses, large irregular areas are yellowed, then become straw-colored and die out. This is called the melting-out phase. In some instances the entire lawn can be lost. Once the fungi that cause melting out disease become established in a lawn they cannot be eradicated and remain an ever-present problem.
To manage melting-out disease, mow bluegrasses, fescues, and ryegrasses at the recommended maximum height. Avoid close clipping at all times. Mow the grass so that no more than 1/3 of the grass height is removed at one time.
If dense thatch more than one-half inch thick has formed, reduce or remove it with a power rake in the spring or early fall. These machines can be rented at most large garden supply centers. Application of sufficient, but not heavy amounts of slow-release available nitrogen can also help. Too much nitrogen can be detrimental and can leach into the ground water. Soil tests can determine proper amounts. Proper watering practices will also help manage melting-out disease. For information on good watering practices, see the fact sheet in this series, "Watering of Lawns and Trees".
When planting new lawn or turf, buy only top-quality, disease-free seed, sod, springs or plugs from a reputable dealer. Select locally adapted, disease-resistant grasses or combinations (blends and mixtures). Your county Extension agent can recommend grass species to grow.
References:
Shurtleff, M., W. Thomas, and R. Randell. 1987. Controlling Turfgrass Pests. Prentice-