Leaf Wetness & Read the Infection Risk

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.

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.

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.

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.

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.

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.

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.

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.

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.