Dollar Spot Management (IPM)

What it is

Dollar spot is a foliar disease caused by the fungus Clarireedia jacksonii [psu-turf:landschoot-2023]. Symptoms on closely mown putting greens appear as spots "about the size of a silver dollar" in white or tan; on individual leaf blades the classic sign is an hourglass-shaped lesion — yellow-green blotches that progress to "a tan-colored lesion with reddish-brown borders" [psu-turf:landschoot-2023].

Why UK pressure is rising

Glenn Kirby — identified in the BIGGA article as Research Director at the STRI Group (and concurrently technical manager for sports turf and landscape at Syngenta) — frames dollar spot as "the turf disease thriving on climate change": "the climate is moving in a direction that massively favours dollar spot," driven by "high humidity in the UK" combined with a "generally warmer climate." Milder winters mean "we don't get the cold breaks that used to reduce spore populations," so populations carry over into spring [bigga:2024-dollar-spot].

Climate context — UK seasonal shift

The operationally important shift for UK supes is when in the season the disease now hits. BIGGA's 2024 piece is explicit that dollar spot breaks the historical pattern of UK turf-disease scheduling: "Other diseases we've traditionally suffered have tended to be active during the cooler winter months, but dollar spot is at its most active in the months of June, July and August" [bigga:2024-dollar-spot]. The practical consequence is that the disease's peak window now sits directly on top of the UK main playing season, so cultural-management levers (dew removal, irrigation regime, mowing/rolling cadence) have to be running at full intensity exactly when course traffic and member-event pressure are also at their highest. Combined with the milder-winter spore-carryover noted above, this is the canonical "no longer a warm-US-states-only problem" expansion that R&A's 2025 ITRI program (see companion entry dollar-spot-itri-koch-program) funds Koch et al. to address with a sustainability-first modelling+robotics+alternative-suppressant playbook [r-and-a:2025-itri].

Environmental conditions favouring infection

Per Penn State Extension [psu-turf:landschoot-2023], dollar spot peaks "in late summer under high humidity and air temperatures ranging from 60° to 85°F" (≈15–29°C), most severely on "turf receiving closely spaced summer irrigation" and when "humidity is high from prolonged muggy summer weather." BIGGA's framing is consistent: "leaf wetness" is "the main driver for disease," triggered by "warm temperatures and wet leaves" [bigga:2024-dollar-spot].

Smith-Kerns prediction model (operational thresholds)

The Smith-Kerns model (Smith & Kerns 2018) is the canonical weather-driven decision-support layer for dollar-spot programmes. The University of Wisconsin-Madison Turfgrass Diagnostic Lab publishes the operational summary [wisc-turf:smith-kerns-model].

Inputs. Two daily weather metrics, both 5-day moving averages:

• Daily relative humidity (mean RH)
• Daily air temperature (°C, mean AT)

Model. Logistic regression producing a daily probability of disease activity:

Logit(μ) = -11.4041 + (0.0894 × MEANRH) + (0.1932 × MEANAT)

Inactivity bounds. TDL flags the model as inactive — output should be ignored — when the 5-day average temperature is below 10°C or above 35°C, regardless of high-humidity exceptions [wisc-turf:smith-kerns-model]. The operational consequence: in UK/IE shoulder-season windows where 5-day mean temperatures fall below the 10°C floor, the supe falls back on cultural levers (dew removal, irrigation regime, mowing/rolling cadence per §"Cultural management" below) without the model as decision support.

Action threshold (first preventive spray). 20% probability is the documented threshold for first preventive fungicide application [wisc-turf:smith-kerns-model]. TDL notes the threshold is a starting point — supes adjust based on grass type, course disease history, and fungicide-efficacy duration.

High-pressure / re-apply interval. The 60%-probability threshold sometimes cited as the "shorten re-apply interval" trigger is industry-common operational practice, not a TDL-published figure. Use as a supe-judgment escalation point during sustained high-humidity warm-temperature windows; the canonical TDL spec remains the 20% first-spray threshold plus the temperature-bounded inactivity rule.

Provenance. The peer-reviewed model paper is Smith & Kerns (2018), published in PLOS ONE (DOI 10.1371/journal.pone.0194216); the wisc-turf operational summary is the practitioner-facing surface cited here.

Cultural management (IPM)

Both sources converge on the same IPM playbook:

• Daily dew removal — BIGGA: "daily dew removal, mowing and rolling" is central to non-pesticidal management [bigga:2024-dollar-spot]. PSU: research demonstrated "significant reductions of dollar spot through removal of dew and guttation fluid from turf foliage by dragging fairways" [psu-turf:landschoot-2023].
• Nitrogen — "Late spring nitrogen fertilizer applications can help to minimize dollar spot severity" [psu-turf:landschoot-2023].
• Irrigation regime — apply "more water less frequently, thereby wetting the entire rootzone" rather than closely spaced light cycles [psu-turf:landschoot-2023].
• Rolling — "Rolling creeping bentgrass greens five days per week with a lightweight roller following morning mowing generally reduced dollar spot" [psu-turf:landschoot-2023]; BIGGA echoes "mowing and rolling" as paired cultural levers [bigga:2024-dollar-spot].
• Aeration, topdressing, organic-matter management — BIGGA: "organic matter management reduces disease pathogen pressure" [bigga:2024-dollar-spot].

Fungicide-resistance discipline

PSU is explicit: "resistance to penetrant fungicides has occurred with C. jacksonii," so the standing advice is "alternating penetrant fungicides with different modes of action" [psu-turf:landschoot-2023]. Mode-of-action rotation is the discipline; a single class used repeatedly is the failure mode.

When to deviate

Supe's own dollar-spot action threshold always overrides. Reasons to escalate beyond the cultural-practice baseline:

• Forecasted high pressure (warm humid week, recent infection history) → tighten dew removal, consider preventive fungicide rotated by mode of action
• Resistance suspected at the course → contact diagnostic lab; do not re-apply the failing class
• Tournament window approaching → prioritise preventive cover over curative response

Related

• leaf-wetness-and-disease-pressure — leaf wetness drives multiple cool-season pathogens, dollar spot being the canonical case
• dollar-spot-itri-koch-program — R&A / USGA / STERF ITRI 2026–2028 funded program led by Paul Koch (UW-Madison) on sustainable dollar-spot management (disease modelling + robotics + alternative fungicidal suppressants); roadmap not yet-published findings
• microdochium-snow-mould — winter / cool-season pathogen that historically occupied the "main UK turf disease" slot; dollar spot's June–Aug shift overlaps the opposite end of the calendar. The microdochium entry's §"Climate context — UK winter shift" is the deliberate parallel to this entry's §"Climate context — UK seasonal shift": same climate-shift story, opposite calendar (frost-day decline opens the winter shoulder for microdochium / warm-humid expansion opens the summer for dollar spot)
• fungicide-resistance-management-uk — the UK doctrine entry on FRAC-group rotation discipline; this entry's §"Fungicide-resistance discipline" is one specific application of that broader stewardship framework, and the asymmetric-cost-of-no-rotation framing applies directly to the Clarireedia jacksonii resistance evidence cited here
• withdrawn-actives-uk-reference — the chemistry-loss baseline that makes the rotation discipline above more critical than it was a decade ago; iprodione (Microdochium contact-fungicide workhorse) withdrew in 2018, raising the operational weight of the remaining penetrant classes