Dollar spot is one of the most widespread and economically significant fungal diseases affecting turfgrass, particularly on highly managed areas like golf courses and lawns. The disease is named for the distinct, small, circular, bleached or tan patches it creates, which are typically the size of a silver dollar on closely mowed turf. The manifestation of this disease results from a combination of three factors: a specific pathogen, conducive environmental conditions, and a susceptible host plant. We explore the specific fungus responsible, the climate that triggers its activity, and the common turf management practices that leave grass vulnerable to infection.
The Specific Fungal Culprit
The primary biological cause of Dollar Spot is a fungus in the genus Clarireedia, which was historically known as Sclerotinia homoeocarpa. This genus includes several species, with Clarireedia jacksonii being the most common agent affecting cool-season turfgrasses globally. The pathogen is a foliar disease agent, meaning it attacks the grass blades themselves and not the roots or crowns of the plant.
This fungus survives unfavorable periods by retreating into infected dead plant debris or the thatch layer as dormant mycelium or thick-walled, compact masses called stromata. When conditions become favorable, the mycelium emerges and directly penetrates the grass blade, often through the cut tip or natural openings like stomata. Unlike many other fungal diseases, the dollar spot pathogen rarely produces spores; instead, its spread relies almost entirely on the movement of these vegetative mycelial fragments.
The disease is readily moved from one area of a lawn to another by mechanical means. Grass clippings carrying the active fungus are a major source of transmission. Equipment such as mowers, as well as human traffic, shoes, and even water runoff, can efficiently carry the fungal strands to healthy grass plants.
Climate Conditions That Activate the Disease
The sheer presence of the Clarireedia fungus is not enough to cause an outbreak; it requires specific weather conditions to become active and successfully infect the turf. Dollar spot development is favored by a broad temperature range, typically becoming aggressive when air temperatures settle between 60°F and 85°F. This range often corresponds to late spring, early summer, and the transition period of early fall.
The most important environmental factor, however, is the presence of prolonged leaf wetness. The fungus needs a continuous period of moisture on the grass blades, usually between 8 to 12 hours, to germinate and penetrate the leaf tissue effectively. This extended wetness is most frequently provided by heavy, cool-weather dew, light drizzle, or ground fog.
High humidity levels, particularly above 65%, further exacerbate the problem by slowing down the natural evaporation of moisture from the grass canopy. These conditions are often met when warm days are followed by cool nights, leading to the formation of heavy dew. Once the fungus has successfully infected the leaf, the disease can continue to progress even if the soil itself is dry, which is a common occurrence during summer drought periods.
Turf Management Errors That Increase Susceptibility
The final factor in the disease triangle is the host plant, and poor turf management practices weaken the grass, making it highly vulnerable to the activated fungus. The single greatest cultural factor increasing susceptibility to Dollar Spot is a deficiency in nitrogen fertility. Turfgrass that is starving for nitrogen exhibits thin cell walls and generally slow growth, which significantly limits its ability to repair and “grow out” of the damage caused by the fungus.
Improper watering techniques create the perfect microclimate for the pathogen to thrive. Frequent, shallow irrigation, especially when timed late in the afternoon or evening, is highly detrimental. This practice artificially prolongs the necessary leaf wetness period, often extending it well past the required 8 to 12 hours, essentially inviting the fungus to infect the grass blades. Watering deeply and infrequently, while allowing the surface to dry, is necessary to avoid this error.
Another set of errors involves the physical condition of the turfgrass stand, particularly excessive thatch and soil compaction. A thick layer of thatch—the accumulation of dead and living organic matter between the soil surface and the green grass blades—acts as a protective reservoir for the dormant fungus. This organic layer retains moisture near the soil surface, providing a continuous source of humidity for the pathogen.
Soil compaction restricts root growth and limits the plant’s ability to absorb water and nutrients, which places the grass under stress. This stress weakens the plant’s natural defenses and slows down its recovery rate, leaving the turf more susceptible to infection. These common management deficiencies collectively transform a resilient lawn into a vulnerable host, completing the conditions necessary for a Dollar Spot outbreak.