Fungal pathogens are a common cause of discolored or damaged turf. Correctly identifying the specific issue is the first step, as various fungal diseases, such as those caused by Rhizoctonia or Sclerotinia, require different environmental adjustments. Misdiagnosis is common because many lawn problems share similar visual symptoms, making a careful, systematic inspection necessary. This guide focuses on the visual signs and environmental contexts needed for accurate identification, not on the methods of treatment.
Differentiating Fungal Disease from Other Lawn Problems
A preliminary inspection can help determine if the problem is biotic (fungal) or abiotic (environmental or mechanical). Fungal issues often present as distinct, spreading patches with clear signs of infection on the individual grass blades. In contrast, many non-fungal issues affect the lawn uniformly or follow non-biological patterns.
Drought stress initially causes the grass to turn a dull blue-green or grayish color before wilting and becoming brittle. If footprints remain visible on the grass for ten seconds or more, the turf lacks sufficient moisture to spring back (the “footprint test”). Unlike a fungal disease, a drought-stressed lawn typically recovers quickly after a deep watering.
Insect damage, particularly from root-feeding pests like grubs, leads to irregular, sporadic patterns rather than the circular shapes common in fungal disease. If the damaged grass pulls up easily like a loose carpet (the “tug test”), the roots have been severed by feeding larvae. Surface-feeding insects, such as sod webworms, often leave a ragged, chewed appearance on the leaf blades.
Mechanical injuries and chemical burns have distinct margins. Mower blight, caused by dull blades, results in consistently brown or shredded tips across the entire lawn. Chemical or fertilizer spills usually create sharply defined, unnatural patterns, such as stripes or squares, corresponding to the application method or spreader path.
Identifying Common Fungus Types by Visual Symptom
The most reliable way to identify a lawn fungus is by examining its unique visual signature on both the patch and the individual grass blade. These distinct markers separate one fungal pathogen from another.
Circular Patches
Brown Patch (Rhizoctonia solani)
This disease appears as circular or irregular patches of light brown or straw-colored turf. On closely mown turf, the patches may be surrounded by a dark purple or grayish-brown border known as a “smoke ring.” This ring of active mycelium is most visible in the early morning dew. The individual grass blades within the patch often have tan lesions with dark brown margins.
Dollar Spot (Clarireedia jacksonii)
Dollar spot is characterized by small, distinct, sunken circles. These straw-colored spots often merge together to form larger, irregularly shaped areas of damaged turf. A close inspection of the individual blades reveals light tan lesions that are constricted by a reddish-brown band. Fine, white, cobweb-like mycelium may be visible on the patches when heavy dew is present.
Thread-Like Growths and Powdery Coatings
Red Thread (Laetisaria fuciformis)
Red thread is easily identified by the presence of pink or red thread-like structures, called sclerotia, extending from the tips of the grass blades. From a distance, the disease appears as circular or irregular patches of tan or pink-tinged turf. The infection is primarily foliar, which gives the patch a ragged, scorched appearance.
Rust (Puccinia spp.)
Rust leaves a powdery residue that easily rubs off onto shoes or clothing. The disease starts as small, yellow flecks on the leaf blades that enlarge into raised pustules containing masses of colored spores. These spore masses rupture the leaf surface, giving the entire turf area an orange or rusty tint. Rust is common on slow-growing, stressed grass, and the spores are easily spread by wind and traffic.
Slime Mold (Physarum cinereum)
Slime mold appears as slimy, colored masses that sit on top of the grass blades. This organism is not parasitic; it uses the blades for support while producing spores. As the mold dries, it turns into a crusty, gray or white powdery substance. The coating can temporarily shade the grass, causing slight yellowing beneath, but the turf is not killed.
Environmental Triggers and Conditions for Fungal Growth
Accurate identification is confirmed by correlating the visual symptoms with the prevailing environmental conditions that favor the fungal pathogen. Fungi require three elements to thrive: the host grass, the pathogen itself, and a conducive environment.
Prolonged leaf wetness is a universal trigger. Extended periods of leaf wetness, often exceeding ten hours, are particularly favorable for most turf diseases.
Temperature requirements vary significantly between specific fungi. Brown patch is a high-temperature disease, thriving when daytime air temperatures exceed 80°F and nighttime temperatures remain above 65°F, especially with high humidity. Dollar spot prefers moderate conditions, typically between 60°F and 85°F, often with warm days, cool nights, and heavy morning dew.
Nutrient availability also plays a role in disease activation. Dollar spot is known to be more severe in turf that is deficient in nitrogen. In contrast, excessive nitrogen fertilization, particularly during summer heat, can increase the susceptibility of grass to brown patch. These nutrient imbalances create stress that weakens the plant’s natural defenses, allowing the dormant pathogens to become active.