The best way to solve many lawn problems is to prevent them from occurring in the first place, by watering, mowing, fertilizing, aerating, and dethatching regularly. Some of the more common problems are described below. If you're not sure what's causing the symptoms you see, consult a local nursery or your Cooperative Extension Office, or hire a professional to inspect your lawn and make an assessment.
Gasoline spills or dog urine. Round patches of dead grass can be caused by gasoline spills (did you fill the mower while it was sitting on the grass?) or dog urine. To remedy the problem, leach the area thoroughly with water; then gently rake away the dead grass. If the bare patch doesn't fill in on its own, reseed it or replace it with a patch of sod.
Uneven fertilizer application. Dead or yellow patches can also result from uneven fertilizer application. Parts of the lawn that received the right amount of fertilizer turn dark green; areas that didn't get enough are pale green or yellow, while those that received too much burn and turn brown. Remove the dead patches and water well. If the grass doesn't come back on its own, reseed or resod.
Chinch bugs. Chinch bugs are 1/4-inch-long, gray-black insects that suck juices from grass blades. They cause brownish-yellow patches in lawns, primarily in St. Augustine and zoysia grasses (and sometimes in blue grass and creeping bent grass), especially in hot or drought-stressed conditions. To diagnose, sink an empty can (open at both ends) into the ground at the edge of a patch. Fill the can with water. If chinch bugs are present, they'll float to the surface. To minimize spread, keep the area well watered. Chemical controls include chlorpyrifos and diazinon.
Sod webworms. Sod webworms aren't worms at all, but small, hairless gray caterpillars ― the larval form of tiny, buff-colored moths that, if present, can be seen flying close to the lawn's surface in the evening. Sod webworms feed on grass blades. Symptoms are small dead patches of lawn that appear in spring and enlarge during summer. To make a diagnosis, drench an area of lawn near the dead spots with a solution of 1 tablespoon liquid dishwashing detergent diluted in 1 gallon water. The larvae will come to the surface. If you find more than 15 larvae in a square yard, treat the lawn. For chemical control, use chlorpyrifos or diazinon. If you don't want to use chemicals, you may be able to reduce the pest population by improving lawn care. Don't overwater or overfertilize; maintain a regular dethatching and aeration schedule.
White grubs. "White grubs" is a catchall name for the soil-dwelling larvae of various kinds of beetles, including June bugs (named for the month when they are usually noticed), rose chafers, and Japanese beetles. All these larvae feed on lawn roots. They're typically white with brown heads; when exposed, they curl up in a C-shape. Signs of their presence include distinct, irregularly shaped brown patches in the lawn; damage is usually most severe in late summer. Because the roots have been eaten, the dead patches pull up easily. Remove a patch and dig into the soil; if you find more than one grub per square foot, treat the soil. Correct identification of the grubs will help you choose the best means of treatment; for assistance in identifying them, take a few to a local nursery or your Cooperative Extension Office.
Fairy rings. Fairy rings are small circular patches of dark green grass surrounding areas of dead or light-colored grass; mushrooms may or may not be present at the perimeter of the green area. The rings result from a fungal disease common in lawns growing in soil high in organic matter. To control the problem, aerate the soil; then apply a nitrogen fertilizer formulated for lawn care and keep the lawn wet for 3 to 5 days.
Rust. Rust is a fungal disease. Among lawns, it affects primarily blue grass and rye grass. Grass blades turn yellowish to reddish brown throughout; small reddish pustules form in groups on older blades and stems, and the blades eventually die. The best solution for rust is to apply a nitrogen fertilizer formulated for lawn care, water regularly, and mow more frequently.
Weeds. Chances are that any weeds infesting the rest of your garden will also attempt to establish themselves in the lawn. A healthy lawn isn't at high risk: its grass stems grow thickly together, making it difficult for weed seeds to reach soil, germinate, and take root. But if the lawn is in poor condition and patchy soil is exposed, weed infestation is likely. Making the wrong grass choice (growing a type that can't tolerate shade in a semishaded area, for instance) also leads to sparse turf and invites weed invasion.
Some warm-season grasses ― Bermuda and zoysia, in particular ― can themselves be weeds if accidentally introduced into a lawn of a different grass type. The controls for Bermuda grass are also effective against zoysia.
Other lawn weeds include common mallow, crabgrass, dandelion, oxalis, plantain, quack grass, and spotted spurge.