Dollar Spot Risk

What is dollar spot?

Dollar spot (Clarireedia jacksonii and related species) is one of the most widespread turf diseases in North America, affecting virtually every cool-season and warm-season grass species. It gets its name from the size of its lesions on putting greens — roughly the diameter of a silver dollar. On home lawns mowed at taller heights, the bleached, straw-colored patches run several inches to a few feet across and can merge into large irregular dead areas when conditions remain favorable.

The disease is caused by a necrotrophic fungal pathogen that overwinters in infected thatch and soil. It does not need a wound to infect — it colonizes healthy leaf tissue directly when the environmental conditions are right. Spread occurs via infected clippings, equipment, foot traffic, and water movement.

Why weather predicts it before symptoms appear

Dollar spot infection follows a predictable weather pattern: warm temperatures, high relative humidity, and prolonged leaf wetness create the conditions the pathogen needs to infect leaf tissue. By the time the characteristic hourglass-shaped lesions appear on individual blades — or the bleached patches are visible in the turf — the infection cycle is already several days advanced.

A weather-based model can flag favorable conditions 2–5 days before visible symptoms, giving you time to modify irrigation timing, raise mowing height, or prepare a management response before a flush of disease expression. That lead time is the reason weather models exist: they turn an after-the-fact symptom into a before-the-fact signal.

The Smith-Kerns model

This tool uses the no-fungicide form of the Smith-Kerns (2018) logistic regression model, published in PLOS ONE. The model was developed and validated across multiple turfgrass research sites using weather station data paired with daily dollar spot disease severity ratings.

The equation:

Logit = −11.4041 + (0.0894 × MEANRH) + (0.1932 × MEANAT)
Probability (%) = 100 × e^Logit / (1 + e^Logit)

Where MEANRH is the 5-day moving average of daily mean relative humidity (%) and MEANAT is the 5-day moving average of daily mean air temperature (°C). Using a 5-day average rather than a single-day reading smooths out day-to-day variation and captures the sustained weather patterns that favor disease establishment.

The model has a fungicide-history term that lowers predicted risk when a fungicide has recently been applied. This tool uses the no-fungicide form — appropriate for homeowners who are not on a preventive fungicide program.

The 20% action threshold

The Smith-Kerns paper identifies 20% as the published action threshold — the probability above which conditions are considered favorable enough to warrant a management response. This is the only validated threshold from the study.

The tool uses plain-language labels (“Low,” “Elevated,” “High”) to give directional emphasis, but only the 20% boundary has a published scientific basis. The “High” label above 50% is a UI convention for communicating stronger pressure — it is not a second validated cutoff from the research.

When the model is inactive

Dollar spot is a warm-weather disease. The model is gated off when the 5-day average air temperature falls below 10°C (50°F) or exceeds 35°C (95°F). Outside this range, high relative humidity alone doesn't drive meaningful infection risk, and running the probability equation would produce misleading results. In shoulder seasons — early spring or late fall — the tool will show “inactive” until temperatures enter the active range. The 7-day forecast still displays, so you can see when the temperature window is expected to open.

How the tool works

1. Your ZIP code is geocoded to a latitude/longitude using Zippopotam.us (free, no account required).
2. Hourly relative humidity and 2-meter air temperature for the past 10 days and the next 7 days are fetched from Open-Meteo (free, no account required).
3. Hourly readings are averaged into daily means for each variable.
4. A trailing 5-day moving average is computed for each daily mean, producing MEANRH and MEANAT for each date.
5. The Smith-Kerns logit equation is applied to each date where the temperature gate is satisfied, converting the logit value to a probability via the logistic function.
6. Today's probability and pressure band are shown as the headline readout; the next 7 days are displayed as a forecast bar chart.

What to do when pressure is elevated

For most homeowners, cultural practices are the first line of defense — both because they reduce conditions that favor infection and because they don't require product purchasing or application timing decisions:

• Raise your mowing height. Taller turf has a more open canopy with better air circulation, reducing the leaf wetness duration that favors infection. Dollar spot is most aggressive at low mowing heights; raising the deck 25–50% during high-pressure periods is one of the most effective cultural responses.
• Water in the early morning, not the evening. Evening irrigation extends the period leaves stay wet overnight — exactly the conditions the pathogen prefers. Early-morning watering gives leaves the full day to dry. University extension programs consistently cite irrigation timing as a primary cultural control for dollar spot.
• Avoid drought stress. Drought-stressed turf is significantly more susceptible to dollar spot. Keep your lawn consistently watered — deeply and infrequently — so it isn't entering a disease-favorable window already stressed.
• Hold off on heavy nitrogen during high-pressure windows. This one is nuanced: very low nitrogen promotes dollar spot susceptibility, but pushing heavy nitrogen during active disease pressure can stimulate lush, succulent growth that is more easily infected. Maintain adequate — not excessive — fertility.
• Remove morning dew. Dragging a hose or pole across the turf in the early morning disrupts dew and guttation fluid, reducing the leaf wetness window. This is standard practice on golf courses during high-pressure periods and works just as well on home lawns.

Fungicide applications are an option when cultural practices aren't sufficient and disease is actively spreading. However, fungicide timing and product selection require reading the product label carefully and understanding your turf species and disease history — this tool does not provide fungicide recommendations.

Honest caveats

• The model was validated on golf-course creeping bentgrass, not home lawns. The Smith-Kerns paper used putting green and fairway research plots mowed at 2.5–12.7 mm. Dollar spot on Kentucky bluegrass, tall fescue, bermudagrass, zoysia, and other home-lawn species behaves differently — different susceptibility levels, different canopy microclimate, different inoculum dynamics. The model reliably identifies weather patterns that favor dollar spot; the absolute probability value and the 20% threshold were calibrated for bentgrass, not home-lawn species. Treat this as a weather-pressure awareness tool, not a home-lawn fungicide trigger.
• Regional weather is not your microclimate. Open-Meteo provides model-based weather for a grid cell centered on your ZIP code's coordinates. Your actual lawn microclimate depends on shade (which reduces air movement and extends dew periods), low spots that collect cold air and extend leaf wetness, compacted or poorly draining areas that stress the turf, and proximity to water features. Real local risk can be meaningfully higher or lower than the modeled probability.
• The tool assumes inoculum is present. Dollar spot requires the pathogen to be present and active in your thatch and soil. If your lawn has never had dollar spot, risk is lower even when conditions are favorable — the disease pressure reading is still valid as a weather trend, but the infection likelihood depends on whether a viable pathogen population exists.
• Forecast uncertainty compounds beyond 3–4 days. The 5-day moving average means today's probability reflects the past 4 days plus today — anchored in observed data. Forecast-day probabilities inherit the uncertainty of the numerical weather prediction model, which grows meaningfully beyond 3–4 days. Treat the latter half of the 7-day bar chart as directional, not precise.

Sources

Smith DL, Kerns JP, Walker NR, Payne AF, Bonos S, Buckley WE, Meyer WA, Dickson WK, Burrus PB. (2018). “Development and validation of a weather-based warning system to advise fungicide applications to control dollar spot on turfgrass.” PLOS ONE 13(3): e0194216. https://doi.org/10.1371/journal.pone.0194216 — Primary source; equation, coefficients, and 20% action threshold.

UW-Madison Turfgrass Diagnostic Lab. Dollar Spot Model. https://tdl.wisc.edu/dollar-spot-model/ — Published implementation reference confirming the no-fungicide equation form.

Open-Meteo (https://open-meteo.com) — Free, keyless weather API providing hourly relative humidity and air temperature.

Zippopotam.us — Free, keyless ZIP code to latitude/longitude geocoding.