Disease Forecasting Model Calculator
Predict plant disease infection risk from weather conditions, host susceptibility, and inoculum pressure using DSV-based forecasting.
About this calculator
This calculator estimates daily plant-disease infection risk using the logic behind established weather-driven forecasting systems like the Mills table for apple scab and the Tom-Cast disease severity value (DSV) system for tomato diseases. It scores temperature favorability on a curve that peaks at 70°F and falls off toward hard floors and ceilings at 40°F and 95°F — reflecting how most foliar fungal pathogens have a real optimum, not a simple "warmer is worse" relationship — and raises that ratio to the 1.5 power so favorability sharpens near the optimum rather than growing linearly. Leaf wetness duration is scored on a threshold curve too: minimal risk under 4 hours, ramping steeply between 4 and 18 hours (the window in which most fungal spores need continuous moisture to germinate and infect), and maxing out beyond 18 hours; humidity above 80% adds a bonus on top since high ambient moisture reinforces the wetness effect even between wetting events.
These two weather factors each carry 35% weight in the composite daily risk score, with host susceptibility and inoculum pressure — factors you supply based on cultivar resistance and local disease history — each weighted 15%. That daily risk score maps to a 0-4 Disease Severity Value per day, which accumulates across consecutive favorable days toward a spray threshold of 18 cumulative DSV, the level at which fungicide application timing models typically recommend action. This is a decision-support estimate built on generalized foliar-pathogen curves, not a species-calibrated model — treat its spray-timing output as a planning signal to combine with pathogen-specific extension guidance, not a replacement for it.
Inputs
Results
Daily infection risk (0-100)
74.3
Risk rating (1-5)
4
How to Use This Calculator
- Enter Average temperature (°F), Leaf wetness duration (hours), and Relative humidity (%).
- Set Host susceptibility (1-10), Inoculum pressure (1-10), and Consecutive favorable days.
- Review Daily infection risk (0-100) and Risk rating (1-5).
- Use Temperature favorability (0-1) and Moisture favorability (0-1) to inform your decision.
How the result changes with Average temperature (°F)
| Average temperature (°F) | Daily infection risk (0-100) | Risk rating (1-5) |
|---|---|---|
| 34 | 42.7 | 3 |
| 51 | 50.5 | 3 |
| 102 | 42.7 | 3 |
| 120 | 42.7 | 3 |
What each input means
- Average temperature (°F)
- Average daily temperature. Most foliar pathogens favor 60-80°F.
- Leaf wetness duration (hours)
- Hours per day leaves remain wet from dew, rain, or irrigation. Key infection driver.
- Relative humidity (%)
- Average relative humidity. >80% enhances spore germination.
- Host susceptibility (1-10)
- Cultivar susceptibility rating. 1 = highly resistant, 10 = highly susceptible.
- Inoculum pressure (1-10)
- Pathogen inoculum level based on disease history, neighboring fields, residue. 1 = very low, 10 = extreme.
- Consecutive favorable days
- Number of consecutive days with similar weather conditions (for cumulative DSV).
What each result means
- Daily infection risk (0-100)
- Composite daily disease infection risk index.
- Risk rating (1-5)
- 1 = Very Low, 2 = Low, 3 = Moderate, 4 = High, 5 = Very High.
- Temperature favorability (0-1)
- How favorable the temperature is for pathogen development.
- Moisture favorability (0-1)
- Combined leaf wetness and humidity favorability for infection.
- Disease Severity Value (DSV/day)
- Daily DSV accumulation (0-4 scale, similar to Tom-Cast system).
- Cumulative DSV
- Total accumulated DSV over consecutive favorable days.
- Days to spray threshold
- Estimated days until cumulative DSV reaches the spray threshold (18 DSV) at current rate.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersAverage temperature (°F) = 68, Leaf wetness duration (hours) = 12, Relative humidity (%) = 85, Host susceptibility (1-10) = 6 = 6 input(s) provided
- Calculate Daily infection riskDaily infection risk = min(100, max(0, dailyRisk))74.3 = 74.3
- Calculate Risk rating4 = 4
- Calculate Temperature favorabilityTemperature favorability0.902 = 0.902
- Calculate Moisture favorability0.75 = 0.75
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does temperature favorability peak at 70°F and drop off toward both 40°F and 95°F instead of just rising with heat?
Most foliar fungal pathogens have a genuine biological optimum, not a simple 'hotter is worse' relationship — they develop poorly both in cold conditions and in excessive heat. The calculator models this as a rising limb from 40°F up to the 70°F optimum and a falling limb from there to a 95°F ceiling, so entering an average temperature near either extreme correctly produces a low favorability score even though the pathogen is technically still 'active.'
What does raising temperature favorability to the 1.5 power actually do to the result?
Raising a 0-to-1 ratio to a power greater than 1 pulls values away from the midpoint and sharpens the curve near the optimum — conditions close to 70°F score noticeably higher than a straight linear scale would give them, while conditions further from optimum are pushed down more aggressively. It's a shape correction so the favorability score reflects how strongly pathogens actually cluster their development around their preferred temperature, rather than treating favorability as a flat straight-line function of distance from optimum.
Why does leaf wetness need to pass roughly 4 hours before it meaningfully affects risk?
Most fungal spores require a minimum continuous wetness period to germinate and successfully infect leaf tissue — brief dew that dries quickly generally isn't enough. The calculator's wetness curve stays low under 4 hours, ramps up steeply between 4 and 18 hours (the range where most infection actually occurs), and reaches maximum favorability beyond 18 hours, reflecting that longer continuous moisture steadily raises infection likelihood up to a point.
How does the calculator turn cumulative DSV into a 'days to spray' estimate?
Your daily risk score maps to a 0-4 Disease Severity Value per day based on which risk band it falls into, and that daily DSV is assumed to repeat across your entered number of consecutive favorable days to get cumulative DSV. Days to spray then divides the standard 18-DSV spray threshold by your current daily DSV rate, projecting forward at today's conditions — it's a planning estimate that assumes weather stays similar, not a fixed countdown, since a shift in temperature or wetness would change the daily DSV rate.
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