What Causes Dollar Spot in Lawns? The Fungus, the Weather and the Turf
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What causes dollar spot in lawns? The disease is caused by a fungus that infects the leaf blades of turfgrass, leaving small spots about the size of a silver dollar. The fungus is most active in warm, humid weather with heavy dew, and it spreads through foot traffic, mowers and infected clippings. This article explains the cause, the symptoms, the weather and lawn conditions that favor it, and what the extension sources say about managing it.
Quick answer
- Dollar spot is caused by the fungus Sclerotinia homoeocarpa, which the Texas handbook notes was recently renamed Clarireedia jacksonii.
- The fungus is most active during warm, humid days and cool nights that produce heavy dew.
- It spreads through foot traffic, mowers and infected plant material.
- The sources list adequate nitrogen, steady moisture, and aeration for compaction and thatch as ways to reduce occurrence and severity.
The pathogen and its name
The Nebraska publication identifies the cause in one line: “Cause: Sclerotinia homoeocarpa.” The Texas A&M AgriLife handbook notes that the causal fungus is “Sclerotinia homoeocarpa, recently renamed as Clarireedia jacksonii.” The Iowa State article describes dollar spot as “a disease of turfgrass caused by the fungus” of this name. The same organism appears under both names in these sources.
The Texas handbook places its symptom descriptions on leaf blades, and the symptoms are discussed in the next section. The Iowa State article states that “Accurate diagnosis is the first step in successful disease management.”
Which grasses are affected
The Nebraska publication says that “All cool-season turfgrasses can be affected, but the disease is most common on bentgrass and bluegrass.” The Iowa State article lists “Kentucky bluegrass, annual bluegrass, bentgrass, fescue, perennial ryegrass, and zoysiagrass” among the grasses affected.
The Texas handbook gives a more detailed ranking. It states that “Highly managed cool-season turfgrasses such as bentgrass and annual bluegrass are the most susceptible.” It adds that “Warm-season turfgrasses can be infected by dollar spot, but highly managed bermudagrasses and zoysiagrass are more susceptible.” Susceptibility depends on how intensively the turf is managed.
What the spots look like
The Nebraska publication describes the symptoms by mowing height. For closely cut turf, it says “Greens and fairway height: Small, round, bleached spots approximately the size of a silver dollar (1-2 inches in diameter).” On taller turf, the Nebraska publication describes “Four- to six-inch, straw-colored patches of blighted turf.”
The leaf lesion is distinctive. According to the Nebraska publication, “A bleached lesion in the shape of an hour glass is present on the leaf blade. The lesion has a characteristic reddish-brown margin.” The Texas handbook describes the same shape: “Characteristic symptoms for dollar spot on leaf blades are white to tan leaf spots with a dark brown margin at the beginning of the infection; these leaf spots later cover the width of the leaf, taking on an hourglass shape.”
A visible sign can appear in the morning. The Nebraska publication states that “In the early, dewy mornings a cobweb-like mycelium is visible in the affected area.” The Iowa State article says the same growth is “white cobweb-like fungus growing on dew covered blades.” The mycelium is the fungal growth itself.
The Texas handbook also notes that on coarser or taller turf the spots look different. It says that “In coarse textured or high-cut turf, the symptomatic spots are larger and more diffuse. This leads to dollar spot being confused with other diseases such as brown patch.” This is one reason diagnosis matters.
Weather and timing
Weather drives the disease. The Texas handbook states that “Dollar spot occurs from late spring through fall and is most active during warm and humid days” with cool nights “which leads to heavy dew formation.” The Iowa State article makes the same point in shorter form: “Warm days and cool nights are prime conditions for dollar spot development.”
The Nebraska publication gives the active season as “Occurrence: June – September.” Symptoms in the Iowa State article “may begin to appear late spring through the summer months.” Timing therefore varies with the region and the season.
How the fungus spreads
The fungus moves physically across a lawn. The Iowa State article states that the mycelium “can be spread by foot traffic and mowers.” The Texas handbook adds that “Infected plant materials are a primary source of new outbreaks.” It advises that clippings from an infected area be disposed of properly and that equipment be cleaned “before using it in other non-infected sites.”
Traffic on wet grass increases the risk. The Nebraska publication recommends that you “Limit traffic on wet turf in the early morning.” For more on how clippings are handled in a garden, see whether grass clippings belong in the garden.
Lawn conditions that favor the disease
Nutrition plays a role. The Iowa State article states that “Dollar spot tends to be most serious in under-nourished lawns.” The Nebraska publication gives a specific rule: “Provide sufficient nitrogen to maintain moderate growth throughout the season. Higher rates of nitrogen fertilizer will reduce dollar spot severity.” The Texas handbook says that “Regular amounts of nitrogen fertility will help damaged turf to recover from this disease.”
Water and moisture also matter. The Nebraska publication advises that you “Irrigate to maintain plant vigor and avoid drought stress.” The Texas handbook explains that “Maintaining adequate soil moisture is important for dollar spot management since drought stress increases disease severity.” Watering timing matters as well. The Nebraska publication advises to “Irrigate in the early morning hours” and to “Remove dew from the turf.”
Soil structure and thatch are factors. The Texas handbook advises that you “Aerate soils to reduce compaction and remove thatch.” It notes that “Excess thatch can be removed by vertical mowing or aerification with hollow tines.” The Iowa State article lists “core aeration, balanced fertilization, and proper irrigation” as practices that “may help reduce dollar spot.” For a related guide on soil structure, see how to keep garden soil healthy.
Mowing and airflow
Dry grass is a practical detail. The Iowa State article states that “Mowing when the grass is dry can reduce the spread of the fungus.” The Texas handbook recommends that you “Remove morning dew by poling, irrigating or mowing” and “Allow consistent air flow in the area by pruning or removing shrubs or trees as needed.”
Fungicides and resistance
Fungicides are one option, not the first step. The Nebraska publication states that “Cultural management is the first choice in managing dollar spot in turf, however, in many instances fungicides also will be needed.” The Iowa State article says that fungicides “are available and can be applied at the first appearance of the disease.”
Resistance is a concern. The Nebraska publication states that “Resistance to benzimidazole and DMI fungicides has been found with this disease.” It recommends limiting the number of applications, rotating or tank-mixing with other modes of action, and “Not using reduced product rates.” The Texas handbook adds that “The best preventative approach is to choose a less susceptible turfgrass.”
How we chose
The three sources listed at the end were selected from university extension pages on dollar spot that were reachable on 2026-10-27. Each factual sentence in this article is tied to a quoted passage in the source notes. Sentences that could not be tied to a quote were removed.
What we could not verify
The sources do not give a single weather threshold that starts the disease in every region. They do not compare fungicide efficacy for specific cultivars of grass.
What the sources do not say
The sources do not give exact nitrogen rates for a particular lawn. They do not say how long the fungus survives in clippings or soil once they are removed. The sources give a broad temperature range rather than a precise trigger.
Frequently asked questions
Can dollar spot be confused with brown patch?
The Texas handbook says so. It states that the disease can lead to “dollar spot being confused with other diseases such as brown patch (caused by Rhizoctonia solani).” Its symptoms on coarse or high-cut turf are larger and more diffuse.
Why does dollar spot appear after dew?
The sources link the disease to heavy dew. The Texas handbook says the disease is most active in warm, humid conditions with cool nights “which leads to heavy dew formation.” The Nebraska publication notes that the cobweb-like mycelium is visible “In the early, dewy mornings.”
Can dollar spot spread on a mower?
Yes. The Iowa State article states that the mycelium “can be spread by foot traffic and mowers.” The Texas handbook recommends cleaning equipment used in an infected area before using it elsewhere.
Does fertilizing help prevent dollar spot?
Sufficient nitrogen helps. The Nebraska publication says that “Higher rates of nitrogen fertilizer will reduce dollar spot severity.” Its advice is to maintain moderate growth, and the Iowa State article links the disease to under-nourished lawns.
Should I use fungicide on my lawn?
The sources say fungicides are sometimes needed, but cultural practices come first. The Nebraska publication states that “Cultural management is the first choice in managing dollar spot in turf.” Its disclaimer advises that users “Read and follow all product label directions for mixing and application.”
Sources
- University of Nebraska-Lincoln Extension, NebGuide G1910 “Dollar Spot Disease in Turfgrass”: https://extensionpubs.unl.edu/publication/g1910/2009/html/view (opened 2026-10-27)
- Iowa State University Yard and Garden, “Dollar Spot”: https://yardandgarden.extension.iastate.edu/encyclopedia/dollar-spot (opened 2026-10-27)
- Texas A&M AgriLife Extension, Texas Plant Disease Handbook, “Dollar Spot”: https://plantdiseasehandbook.tamu.edu/landscaping/lawn-turf/sorted-by-names-of-diseases/dollar-spot/ (opened 2026-10-27)
