Disease Infection Risk & Spray Only When the Weather Crosses the Line

Every fungal and oomycete disease needs a minimum number of hours of leaf wetness at a given temperature before it can infect. The tool looks up that requirement from a published model — the revised Mills table for apple scab, Smith Periods for late blight, the Plasmopara wetness curve for grape downy mildew and Puccinia dew-period thresholds for wheat rust — interpolates it to your exact temperature, and compares it with the wetness hours you enter to decide light, moderate or severe infection.

Mills periods are the classic apple-scab warning system: at each mean temperature there is a number of leaf-wetness hours required for Venturia inaequalis to infect. The tool uses the MacHardy and Gadoury revision of Mills (1944) used by Cornell and other scab services — for example about 9 hours at 10 °C for a light infection, falling to 6 hours in the warm 16–22 °C optimum. Below 6 °C or above 26 °C infection effectively does not occur.

A Smith Period is the UK criterion for potato and tomato late blight: two consecutive days each with a minimum temperature of at least 10 °C and at least 11 hours with relative humidity at or above 90%. This tool uses leaf-wetness hours as the field proxy for that high-humidity spell at temperature, so reaching the modelled threshold signals you are entering Smith-Period blight conditions and should be on a protectant programme.

The Cornell '3-10' rule states that primary grape downy mildew infection needs about 10 mm of rain, a mean temperature of at least 10 °C and shoots about 10 cm long. Once those primary conditions are met, secondary cycles are driven by the leaf-wetness requirement at temperature that this tool models — as little as 4 hours in the warm 18–22 °C optimum for Plasmopara viticola, rising at cooler and hotter temperatures.

Leaf-wetness hours are the duration leaves stay wet from rain, dew or fog within an infection event. The most reliable source is an on-canopy leaf-wetness sensor on a weather station; failing that, estimate from the rain duration plus the dew period before it dries, or use a local disease-warning service's figure. Enter the hours for the current wetting event — the tool then tells you how close that event came to an infection threshold.

Hold means conditions are below the infection threshold — no spray needed, just keep scouting. Watch means a light infection has occurred or you are within about a quarter of the wetness needed for the next threshold — keep cover on and be ready. Spray means a moderate or severe infection period has occurred and you should apply a protectant or curative fungicide now, then time the next application by the disease's re-spray interval.

Spores germinate and penetrate fastest in each pathogen's optimum temperature band and slowly at the cool and warm edges of its range. So the leaf-wetness hours required form a U-shape with temperature: fewest hours in the optimum, more at either extreme, and none outside the pathogen's overall range. The tool interpolates the published table to your exact temperature so the threshold is right rather than rounded to the nearest row.

A single infection event tells you whether to spray today, but disease pressure builds over a season. The tool combines this event with the prior moderate-or-worse events you have logged into a 0–100 index that climbs with each infection and saturates near 100, so a season with several missed infection periods reads as high overall risk even on a quiet day — a prompt to tighten your spray programme.

Weather-driven forecasting is generally better than a fixed calendar because it sprays when the crop is actually at risk and skips applications when it is not — saving product and slowing resistance. But it depends on accurate local wetness and temperature data and good coverage timing. Use it to refine your programme alongside scouting and a regional warning service, not as the sole decision in a high-value crop.

It means no infection occurred in the event you entered, given the temperature and wetness — but spores can be present and waiting. The next rain or heavy dew may cross the threshold, especially in the optimum temperature band where only a few wetness hours are needed. Re-check after every wetting event, and remember that a protectant fungicide must be on the leaf before the wetness, not after.

Each model carries a typical re-spray interval — about 10 days for apple scab and grape downy mildew, 7 for late blight in pressure, and 14 for wheat rust — reflecting how long a protectant stays effective and how fast new growth emerges unprotected. The tool surfaces that interval after a spray verdict, but a heavy rain that washes off the product or a fresh infection period shortens it.

The thresholds come from authoritative published models, but every field differs in canopy density, microclimate, inoculum load and variety resistance. Treat the verdict as strong guidance, confirm temperature and wetness with on-site sensors where you can, and combine it with scouting. The models are designed to err toward protection, so when the tool says spray on a high-value crop, it is usually right to act.