Circular patterns, rings, arcs, or spots in a lawn are not random occurrences but distinct indicators of an underlying issue. These patterns vary widely in size and color. Diagnosing the cause requires careful observation, as the problem can range from deep-seated biological activity to chemical exposure or simple physical stress. Understanding these visual cues is the first step toward restoring the health and appearance of the turf.
The Distinctive Pattern of Fairy Rings
Fairy rings are recognizable circular patterns caused by various species of fungi, primarily from the class Basidiomycetes. These fungi feed on decaying organic matter deep beneath the soil surface, such as old tree roots or buried lumber. The fungus grows outward radially from a central point of origin, creating the distinct circular or semi-circular shape.
The visual symptoms of a fairy ring are categorized into three types, all stemming from the outward growth pattern of the fungal body (mycelium). A Type I ring is the most destructive, featuring a ring of dead or brown grass bordered by a band of lush, dark-green turf. This dead zone occurs because the dense fungal growth makes the soil hydrophobic, preventing water from penetrating the roots and causing drought stress.
Type II rings are characterized only by stimulated, dark-green grass. This color results from the fungus breaking down organic matter and releasing a concentrated burst of nitrogen into the soil. Type III rings show no noticeable change in the turf’s health or color. Their presence is revealed only through a ring of mushrooms or puffballs that appear, especially after heavy rainfall. These fungal structures are the fruiting bodies of the organism.
Fungal Diseases Creating Circular Damage
While fairy rings result from fungi feeding on deep organic matter, other common turf diseases create circular patterns by infecting the grass blades and crowns closer to the surface. These pathogens thrive under specific environmental conditions, making them distinguishable by size and appearance. Brown Patch, caused by the fungus Rhizoctonia solani, appears as large, irregular circles ranging from a few inches to several feet in diameter.
This disease flourishes in warm, highly humid conditions when temperatures exceed 80 degrees Fahrenheit, damaging the grass blades. In the early morning, a distinct grayish-brown border, often called a “smoke ring,” may be visible around the perimeter. In contrast, Dollar Spot, caused by Clarireedia fungi, is named for the small, straw-colored spots it creates, which are one to five inches wide.
Dollar Spot is common during moderate temperatures (60 to 85 degrees Fahrenheit), especially with prolonged leaf wetness from dew or light rain, often coupled with low soil nitrogen. On the grass blades, individual lesions with tan centers and reddish-brown margins form. These small circular patches can merge to create larger, irregular areas. A third distinct disease, Necrotic Ring Spot (NRS), is a root and crown infection that creates smaller, sunken rings up to twelve inches across. The characteristic feature of NRS is the “frog-eye” pattern, where a ring of dead, tan-colored grass surrounds a patch of healthy green grass in the center.
Biological and Chemical Non-Fungal Causes
Non-fungal biological and chemical factors can also cause circular damage to turf. Pet urine, especially from dogs, is a frequent culprit, creating spots known as “nitrogen burn.” Urine contains high concentrations of nitrogen salts and urea, which act as an over-fertilizer in the center of the spot, leading to desiccation and death of the grass.
This intense concentration results in a distinct visual pattern: a patch of dead, brown grass surrounded by a halo of dark green, stimulated turf. Similarly, localized chemical spills, such as accidentally dropping granular fertilizer or spilling gasoline, can cause rapid circular death. Gasoline contains toxic hydrocarbons that disrupt the soil’s microbial balance and suffocate the grass roots, leaving behind a clearly defined dead patch.
Another common biological cause is damage from grubs, the C-shaped larvae of beetles that feed on grass roots just beneath the soil surface. A large population of grubs severs the root system, causing the grass above to wilt and die in irregular patches that often appear circular. A simple test for grub damage involves trying to lift the affected turf. If it rolls back easily like a piece of carpet, the roots have been eaten away.
Environmental and Physical Factors
Some circular patterns result from simple physical or environmental stresses. Objects left on the lawn for an extended period, such as kiddie pools or trampolines, create circular or semi-circular patches of yellow or dead grass. This damage occurs because the object blocks the grass from receiving necessary sunlight for photosynthesis, air circulation, and rain. This lack of resources causes the turf to enter a state of dormancy or stress.
Localized soil compaction can lead to circular or patchy turf problems, particularly in high-traffic areas like play zones or around a utility access point. When soil particles are tightly compressed, water and air cannot penetrate effectively. This leads to shallow root growth and increased vulnerability to drought and disease. The lawn’s irrigation system can also be the source of circular issues, especially when sprinkler heads are misaligned, clogged, or improperly spaced.
These malfunctions lead to uneven water distribution. This creates localized circular dry patches where the spray pattern does not reach, or conversely, overly wet patches where spray overlaps are too concentrated. For instance, a clogged head might cause water to pool directly at the base, leading to a small, soggy, and eventually dead circle from oversaturation.