Dead patches in a lawn are distinct areas of necrotic or dormant turf that stand out sharply against the surrounding healthy green grass. Determining the precise cause is the necessary first step toward any successful recovery plan. Dead patches can be the result of non-living environmental stressors, the destructive feeding habits of insect pests, or the presence of specific turfgrass pathogens.
Environmental and Cultural Causes
Localized environmental stress or improper maintenance often cause dead spots. Issues related to water management are common culprits, where either too little or too much moisture can damage the turf. Underwatering leads to brittle, dry grass that often fails the “footprint test,” where the grass blades do not spring back up after being stepped on. Conversely, overwatering can lead to mushy soil and root rot, essentially suffocating the grass roots by depriving them of oxygen.
Chemical burns are a significant source of sudden, localized damage, often resulting from the over-application of fertilizer. Fertilizer contains mineral salts and high concentrations of nitrogen. When applied in excess, these salts draw moisture out of the grass plants through osmosis. This process dehydrates the cells, causing the grass to turn yellow or brown and become crispy to the touch. These burns frequently appear in distinct patterns, such as stripes or checkerboard shapes, indicating where the spreader overlapped or where concentrated product was accidentally spilled.
Pet waste also creates distinct patches due to the high concentration of nitrogen and salts in urine. This concentrated nitrogen acts as an overdose of fertilizer, leading to a brown, dead spot where the urine was deposited. Often, the dead patch is ringed by a collar of dark green, lush grass, which occurs because the urine at the outer edge was diluted enough to act as a growth stimulant. Physical damage, such as heavy foot traffic or the placement of shallowly buried construction debris, can also cause localized dead patches by creating areas of severe soil compaction. Compaction restricts the movement of air and water to the root zone.
Damage from Insect Pests
Insects that feed on the grass can cause dead patches that are frequently misdiagnosed as drought stress. Two of the most destructive insect culprits are grubs and chinch bugs. Grubs are the C-shaped larvae of beetles that live beneath the soil, where they chew through and sever the roots of the grass. This root-feeding activity leads to irregular brown patches that feel spongy or soft when walked upon.
A reliable diagnostic method for grubs is the “tugging test,” where the damaged patch can be easily lifted or rolled back like a piece of loose carpet because the roots are completely detached from the soil.
Chinch bugs are tiny, black insects, about four millimeters long, that feed above ground by inserting a piercing mouthpart into the grass blades. They suck the sap out of the plant and inject a toxin that blocks the grass’s ability to absorb water, causing it to wilt and die.
Chinch bug damage typically starts in the hottest, sunniest areas of the lawn, particularly near sidewalks or pavement where heat is reflected. The grass will not recover even with deep watering, distinguishing it from drought stress. To confirm a chinch bug infestation, a visual inspection of the base of the grass blades is needed, or the “tin can float test” can be performed to force the bugs to the surface of the soil.
Pathogens and Fungal Diseases
Pathogens, typically fungi, thrive under specific environmental conditions, leading to distinct patterns of dead or discolored turf. These diseases require high humidity, specific temperature ranges, and prolonged periods of leaf wetness. Brown Patch, caused by the fungus Rhizoctonia solani, is a common warm-weather disease that favors temperatures between 70 and 90 degrees Fahrenheit. It creates circular patches of brown, wilted grass that can range from a few inches to several feet in diameter.
In the early morning, when dew is present, active Brown Patch can sometimes be identified by a dark, grayish border known as a “smoke ring” around the edge of the affected area. Dollar Spot, caused by Sclerotinia homoeocarpa, is favored by moderate temperatures between 60 and 85 degrees Fahrenheit and conditions of low soil nitrogen. This disease is named for the small, straw-colored spots it creates, which are typically the size of a silver dollar and often merge to form larger, irregular areas of damage.
Diagnosis of Dollar Spot is often confirmed by examining individual grass blades within the patch, which display a distinct lesion that looks like an hourglass with a tan center and a reddish-brown border. Finally, subterranean fungi can cause Fairy Rings, which manifest as circular bands of dark green, fast-growing grass surrounding a ring of dead or stressed grass.