Downy mildew: reading the risk and timing the spray

Downy mildew is the disease that decides many vintages. In a wet spring it can strip a canopy and a crop within weeks, and it is almost entirely preventive — once you can see it, you are already behind. The whole game is timing the protection to the infection risk, and that risk is driven by weather, not by the calendar.

How it infects

Plasmopara viticola overwinters as spores (oospores) in leaf litter and soil. In spring, rain splashes them onto low leaves to cause the first, primary infections. From those lesions the fungus sporulates and launches secondary infections that spread explosively through the canopy whenever it stays warm and wet.

The "3-10 rule" for primary infection

A long-standing rule of thumb for the risk of primary infection combines three thresholds:

• Shoots at least ~10 cm long (susceptible green tissue present),
• Air temperature at or above ~10 °C,
• At least ~10 mm of rain within 24–48 hours.

The "3-10" is a screening heuristic, not a guarantee. Modern models refine it with leaf wetness duration, temperature and an incubation clock — but the intuition holds: warm, wet, growing tissue means risk.

Why incubation makes the calendar useless

After infection the fungus incubates invisibly for days — shorter when warm, longer when cool — before symptoms (oil spots, then white sporulation) appear. By the time you see them, several more infection cycles may be underway. This is exactly why a fixed spray calendar wastes product in dry spells and arrives too late in wet ones. The decision has to track the weather.

From forecast to spray decision

• Before a wet period, make sure a protectant (contact) fungicide is on the susceptible tissue — protection has to precede the rain.
• After a high-risk infection event, a systemic or curative material within the kick-back window can still help.
• Match the interval to growth and rain: fast growth and washing rain shorten the protection.
• Respect pre-harvest intervals and, in organic viticulture, the copper limits.

GrapeFlow’s disease models turn the weather at each block into a downy-mildew risk read, and every application you make is logged against the block — feeding the field notebook and the pre-harvest-interval check automatically.

Guidance only — follow product labels, local advisories and your own scouting. Models inform decisions; they do not replace walking the vineyard.