Leaf Wetness & Read the Infection Risk
Forecasts late blight
A fungal disease infects when leaves stay wet past its minimum hours and the temperature sits near its optimum — enter the wet hours and temperature to get a low, moderate or high infection risk.
Read your leaf-wetness hours
Next: conditions favour Late Blight — apply a protectant fungicide now and tighten your scouting and spray interval.
Leaf-wetness models are a guide; combine with cultivar susceptibility, local disease history and forecast before spraying.
Leaf wetness risk — key facts
- Drivers
- wet hours + temperature
- Needs
- free water to infect
- Risk
- low / moderate / high
- Min hours
- set by the disease
- Optimum temp
- fastest infection band
- Use for
- spray timing, scouting
- Measure with
- leaf-wetness sensor
- Privacy
- Runs in your browser; nothing uploaded
Wet hours and warmth decide when a crop gets infected
Foliar fungi cannot start without free water on the leaf, so an infection event needs leaves to stay wet long enough — and a temperature near the pathogen's sweet spot decides how few hours that takes. Disease-forecasting models from Mills for apple scab to BLITECAST for late blight all turn on this pair. Read the event and you know whether to hold off, get ready, or spray before damage is done.
This tool gives the risk level, the disease's minimum wet hours and its optimum temperature window from your wet hours and temperature. Use it to time protectant sprays, prioritise scouting and avoid wasted passes. Pair it with the Pesticide Residue Decay, Trap Catch Threshold and Spray Mix tools for a full crop-protection plan.
Time the spray
Protect before an infection period, not after damage.
Skip dry windows
Hold off when wet hours stay below the threshold.
Combine the drivers
Read wet hours and temperature together.
Prioritise scouting
Focus walks on high-risk events.
Frequently Asked Questions
How is the infection risk worked out?+
Each fungal disease has an infection period — a minimum number of hours leaves must stay wet, and a temperature range where the fungus is most active. The calculator compares your leaf-wetness hours and temperature against the disease's minimum hours and optimum window: well below the minimum is low risk, around the minimum with a near-optimum temperature is moderate, and comfortably past the minimum inside the optimum band is high risk.
Why does leaf wetness drive fungal disease?+
Most foliar fungi and oomycetes need free water on the leaf for spores to germinate and infect — they cannot start in dry conditions. The longer leaves stay wet from dew, rain or irrigation, the more time spores have to germinate and penetrate. That is why leaf-wetness duration, not just rainfall, is the key variable in nearly every disease-forecasting model.
Why does temperature matter as well as wet hours?+
Wetness sets whether infection can happen; temperature sets how fast. Each pathogen has an optimum band where it needs the fewest wet hours to infect — outside that band it needs far longer leaf wetness, or cannot infect at all. Combining the two is what models like the Mills table for apple scab do: short wet periods are dangerous at the optimum temperature but harmless when it is too cold or too hot.
What counts as a leaf-wetness hour?+
An hour during which there is a visible film of free water on the leaf surface — from dew, rain, fog or overhead irrigation. Leaf-wetness sensors log it directly; otherwise it is estimated from humidity, rainfall and dew-point. Wetness periods broken by short dry spells may or may not reset the clock depending on the pathogen, so treat borderline cases cautiously.
What do low, moderate and high risk mean for spraying?+
Low risk means an infection event is unlikely and a protectant spray can usually wait. Moderate means conditions are building — check the forecast and be ready. High risk means an infection period has likely occurred, so a protectant before the event or a curative shortly after is justified. The tool flags the window; your product choice and label set the action.
Does this replace a full disease model?+
No — it is a quick first check on a single wetness-and-temperature event. Full models like Mills (apple scab), TOM-CAST (tomato early blight) or BLITECAST (potato late blight) accumulate risk over days and account for spore loads and crop stage. Use this calculator to read one event fast, and a validated model or advisory service for season-long spray scheduling.
Which diseases follow this wet-hours pattern?+
Most foliar fungal and oomycete diseases — apple scab, potato and tomato late blight, grape downy mildew, septoria, early blight and many leaf spots — all need a minimum leaf-wetness duration at a favourable temperature. The exact minimum hours and optimum temperature differ by disease, so use the figures for the specific pathogen you are watching.
How can I reduce leaf-wetness hours?+
Anything that dries the canopy faster cuts risk: wider plant spacing and pruning for airflow, drip rather than overhead irrigation, watering early so leaves dry by night, and orienting rows to prevailing wind. Less wet time means fewer infection periods, which means fewer sprays. The calculator shows how close a given event sits to the threshold.
Are the figures exact?+
They're a sound guide for a single event, not a guarantee. Microclimate inside a canopy can be far wetter than a weather station, spore pressure varies, and resistant varieties shift the thresholds. Use the risk level to prioritise scouting and spray decisions, then confirm with field observation and your local advisory.