Brown spots in a lawn are symptoms indicating underlying stress, damage, or disease affecting the turf. Identifying the precise cause requires careful observation of the spot’s pattern, location, and the condition of the surrounding grass. The distinction between a cultural issue, an insect infestation, or a fungal outbreak is determined by these unique visual clues. This systematic approach is necessary because different problems often mimic one another, making an accurate diagnosis the first step toward recovery.
Water and Soil Management Issues
A common group of causes for brown patches relates to how water is handled by the lawn, often referred to as cultural practices. When the lawn is experiencing drought or underwatering, the grass blades first show signs of stress by turning a dull, gray-blue color before they wilt and eventually turn straw-brown. A simple test to confirm drought is to walk across the area; if footprints remain pressed down for an extended period, the grass lacks the water pressure needed to spring back.
Conversely, too much water can be equally damaging because it saturates the soil, filling up the pore spaces that normally hold oxygen. This creates an anaerobic condition that starves the roots, leading to root rot that manifests as yellowing and then browning patches. These spots often occur in low-lying areas or where drainage is poor, feeling squishy or spongy underfoot long after irrigation or rainfall.
Soil compaction, frequently seen in high-traffic zones, is another cultural issue that leads to brown spots by hindering the flow of air and water to the root zone. The tightly pressed soil particles restrict root growth and nutrient absorption, causing the turf to thin and brown over time. Improper mowing, specifically cutting the grass too short in a process called scalping, also creates brown patches, especially on uneven terrain. This practice removes too much of the photosynthetic leaf tissue, stressing the plant and sometimes cutting into the non-green crown, leading to immediate browning.
Insect Pests and Root Damage
When brown patches appear suddenly and do not respond to watering, the cause may be a subterranean or surface-feeding insect pest damaging the grass roots or stems. White grubs, the C-shaped larvae of beetles, feed directly on the grass roots, essentially detaching the turf from the soil. The clearest sign of grub damage is the “tug test,” where a patch of brown grass pulls up easily like a loose piece of carpet because the root system has been severed. These irregular, wilting patches are often spongy to walk on.
Another common culprit is the chinch bug, a tiny, surface-feeding insect that injects a toxin while sucking the sap from the grass stem, blocking the plant’s ability to transport water. Chinch bug damage often starts as irregular, spreading patches of yellowing grass that quickly turn reddish-brown, particularly in hot, dry, and sunny areas of the lawn. Small burrowing animals like gophers and voles also cause damage, leaving physical evidence such as mounds or visible, shallow runways across the lawn surface.
Fungal Diseases and Pathogens
Fungal diseases thrive when the lawn is stressed or exposed to extended periods of moisture, such as high humidity or long stretches of leaf wetness. Brown Patch disease is a warm-weather disease that appears as circular, sunken patches that can range from a few inches to several feet in diameter. In actively spreading cases, the perimeter of the patch may be ringed by a distinct, dark, grayish-purple border, often referred to as a “smoke ring.”
Dollar Spot is another common fungal disease that is easily identified by its small, silver dollar-sized, straw-colored spots that frequently merge into larger, irregular patches. Upon close inspection, the grass blades will exhibit hourglass-shaped, tan lesions with reddish-brown borders. Early in the morning, especially when heavy dew is present, a fine, white, cobweb-like fungal growth, known as mycelium, may be visible across the affected spots.
The disease known as Rust is recognizable by the orange or reddish-brown dust it leaves on the grass blades. This powdery substance is actually a mass of fungal spores that easily rubs off onto shoes, clothing, or a mower deck. Rust generally affects lawns that are growing slowly and are deficient in nitrogen, leading to a thinning of the turf and an overall yellowish-orange cast to the grass blades.
External Chemical and Mechanical Injuries
Sometimes, brown spots are the result of external agents that create distinct, localized patterns. Pet urine spots are a classic example, typically appearing as a dead, brown patch in the center surrounded by a ring of unusually dark green grass. This pattern occurs because the high concentration of nitrogen salts in the urine burns the grass in the center, while the diluted nitrogen at the edge acts as a potent fertilizer.
Fertilizer burn, which is a form of chemical drought stress, occurs when excessive amounts of fertilizer salts draw water out of the grass roots. This damage is often recognizable by its pattern, appearing as distinct stripes, streaks, or checkerboard shapes that correspond to the path of the fertilizer spreader or areas of overlap. Finally, a complete, sharply defined kill spot, sometimes with a dark or oily residue, can result from spills of household chemicals, gasoline, or motor oil.