If the lawn has large, dead patches that are gradually getting bigger, then this may be a sign of fungal disease.
Get down on your hands and knees and examine the damaged lawn area. Inspect for the presence of insects or damage caused by herbicides, equipment leaks or animals including pets. Make notes by answering the following questions:
What kind of grass do you have (many parasitic fungi are specific to certain types of grass)?
What time of year is it?
When did the damage first appear?
Does the damaged grass appear greasy or water-soaked?
Are there any visible fungal structures (red threads, rusty spots, smoky haze appearing in the morning, etc.)?
When did you last fertilize the grass?
How long and how often is the lawn being watered?
How much sunlight does the area receive (full day, half-day or less)?
How does air circulate in the area? Do trees and shrubs surround the damaged lawn?
Is the damaged area higher than, lower than, or at the same level as the surrounding lawn?
What shape and size is the damaged area? Is it growing?
Are there any differences in the lawn's color around the damaged area?
If you see any signs of fungal disease, remove a square-foot section of turf about 2 inches deep, making sure to take the sample along the outer margin of the diseased area. The sample should be a single piece that has both damaged and undamaged sections. Place the sample in a plastic bag or box and take it to your local county extension service or certified lawn-care expert for identification.
After the problem has been identified, ask the expert what can be used to treat the disease and how its recurrence can be prevented. Research treatment plans on the Internet, where there are many good disease-management websites maintained by the extension service and universities.
A well-maintained lawn is the best defense against lawn disease. Choose a variety of turfgrass that's resistant to diseases common in your area; a local extension agent or lawn expert can make recommendations. Fertilize at the right time of the year since many diseases are spurred by mistimed fertilization. Mow only when the lawn is dry. Keep the mower blade sharp. Mow at the proper height for your variety of turfgrass. Dethatch, aerate and water your lawn regularly.