How the three checks work
Downy mildew (Plasmopara viticola) — the 3-10 rule says a primary infection can start once shoots are 10 cm long, at least 10 mm of rain has fallen in 24–48 hours and the mean temperature is 10 °C or more. All three must hold.
Powdery mildew (Erysiphe necator) — the UC Davis Gubler-Thomas index counts days with 6 or more hours between 21 and 30 °C. Three such days in a row start the index at 60; after that it rises by 20 on each warm day and falls by 10 on cooler days or when the vines spend 15 minutes at 35 °C or more, which harms the fungus.
Botrytis (grey rot) — infection needs bunches that stay wet for hours at mild temperatures. Bloom and the weeks from veraison to harvest are the susceptible windows; before bloom and between fruit set and bunch closure the risk is lower.
Example
Over seven days in late spring a vineyard has warm afternoons on days 1–3 (index starts at 60), another warm day (80), a cool day (70) and two warm days (90, then 100). The risk is high: the fungus can complete a cycle in about five days, so use the shortest spray interval on the label.
Frequently asked questions
Where do I get the hours at 21–30 °C?
From a weather station in or near the vineyard: count, for each day, whether there was a stretch of at least 6 hours between 21 and 30 °C. Most station software can export hourly temperatures.
Does the 3-10 rule tell me when to spray?
No. It flags weather that allows a primary infection. Whether to spray depends on the protection left from the last application, the forecast and what you see in the vineyard.
Why does rain not appear in the powdery mildew index?
Powdery mildew does not need free water to infect; it spreads in warm, shaded and humid canopies. Heavy rain can even wash spores off the leaves.
Can botrytis start before veraison?
Yes — infections at bloom can stay latent in the young berries and show up as rot near harvest. That is why bloom counts as a susceptible stage.