Microdochium Patch — Model Thresholds and Predictive Indices

Purpose of this entry

This entry is a model-reference supplement to the main microdochium-snow-mould entry, which covers the qualitative disease profile, cultural management, and communication doctrine. This entry focuses specifically on the quantitative environmental thresholds that Eagle's predictive model uses to score Microdochium risk.

The existing snowMold.ts model (v1) uses a simple temperature-range guard. This entry documents the v2 parameters: leaf wetness duration, temperature window, plant-stress multipliers, and the basis for each in published sources.

Temperature window (Tmean 5–14°C)

Microdochium patch is active in a well-documented narrow temperature band. The R&A GC2030 ITM of Parkland Greens states the disease "favours leaf wetness and mild temperatures of 5–14°C to spread" [r-and-a:gc2030-itm-parkland]. This is the primary model gate:

| Parameter | Value | Basis | |---|---|---| | Lower temp guard | 5°C (air Tmean) | R&A GC2030 ITM — below this, disease spread ceases | | Upper temp guard | 14°C (air Tmean) | R&A GC2030 ITM — above this, drying/heat suppresses spread | | Optimal Tmean | ~8–10°C | Inferred from UK autumn-season coincidence; no single-paper source; T2-PENDING |

The R&A's climate-change framing makes the temperature guard operationally critical: as the UK's average of frost nights falls (15% fewer air-frost days vs 1961–1990 baseline per R&A UKCP18 data), the 5–14°C window now occupies more calendar weeks per year, extending the Microdochium risk season into what was historically protected winter [r-and-a:gc2030-itm-parkland].

Leaf wetness duration

Leaf wetness is the co-trigger alongside temperature. Without sustained surface moisture, spore germination and infection cannot proceed even when temperature is in-window.

The 2025 BIGGA "perfect storm" report cites weather analyst Mark Hunt (Prodata Weather Systems): "Some sites recorded up to 16 hours of unbroken leaf wetness… Microdochium typically only needs about six hours to become pathogenic" [bigga:2025-perfect-storm].

Source note: the 6-hour pathogenicity threshold is attributed to a commercial weather service (Prodata), not to a STRI or USGA research trial. Eagle model treats this as a T2 indicator — directionally correct but not a peer-reviewed threshold. The model version grades this component MEDIUM pending STRI laboratory confirmation.

| Parameter | Value | Source tier | |---|---|---| | Minimum leaf wetness for infection | ~6 hours continuous | T2 (BIGGA/Prodata) | | High-risk leaf wetness | ≥12 hours | T2 (same source) | | Dew-reform after removal | As fast as 1–2 hours in still, humid air | T2 (BIGGA/Prodata) |

Plant stress multiplier

Plant-stress state amplifies infection success within the temperature+wetness window. BIGGA's 2025 report identifies the dry summer → stressed plant → autumn Microdochium cascade: "We've come out of an extremely dry spring and summer, and that's left many plants weakened and under stress" [bigga:2025-perfect-storm].

The v2 model applies a stress multiplier when:

• Summer drought stress recorded (SWC below STRI parkland minimum of 20% VWC in the preceding 6-week window)
• Plant not recovered (no rainfall relief + no irrigation supplementation)

This stress-state input is Eagle-data derived (club SWC sensor or Eagle weather model) rather than a published threshold; it is marked as an Eagle-proprietary heuristic in model outputs, not a library-grounded value.

Combined risk scoring — v2 model logic

The Microdochium v2 model fires a risk elevation when ALL of:

1. Tmean 5–14°C (published R&A threshold — T1)
2. Cumulative leaf wetness ≥ 6 hours in the prior 24h (T2)
3. 7-day Tmean does not exceed 15°C (sustained window, not brief dip)

Risk grades:

| Risk grade | Condition | |---|---| | LOW | Tmean in-window but leaf wetness <4h | | MEDIUM | Tmean in-window AND leaf wetness 4–8h | | HIGH | Tmean in-window AND leaf wetness >8h | | VERY HIGH | All three conditions + plant-stress flag |

Model confidence note: v2 grades MEDIUM overall (improved from v1's temperature-only guard, but leaf wetness threshold is T2-sourced and UK trial validation is pending).

Deviation from defaults

The supe's agronomist advice and local disease history override model outputs:

• Sheltered greens with poor air circulation — leaf wetness accumulates faster than the station sensor reads; raise risk grade one level on protected sites
• Poa-dominant vs bent-dominant greens — poa swards carry higher disease load at equivalent risk scores; the model's surface-level risk output does not distinguish species composition (Eagle v2 limit)
• Iprodione-era benchmarks — supes who historically sprayed at first visual sign should recalibrate expectations; modern chemistry slows rather than stops Microdochium, so model thresholds should trigger cultural intervention first, not an immediate fungicide call

Related

• microdochium-snow-mould — primary disease entry: qualitative profile, cultural management, UK climate context
• leaf-wetness-and-disease-pressure — how Eagle models leaf wetness; the 6-hour threshold here draws on the same physical process described there
• degree-day-definitions-uk — HDD base 5°C is the winter-stress context that defines when Microdochium's lower temp guard becomes the operative constraint
• fungicide-resistance-management-uk — mode-of-action rotation when the risk model triggers a fungicide decision
• dollar-spot-management — Smith-Kerns dollar spot model uses a parallel two-parameter structure (temp + RH); model architecture cross-reference