Powdery Mildew Risk in Napa Valley
Erysiphe necator · Napa Valley AVA · CA
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Updated Jul 19, 2026, 8:48 AM UTC
Modelled forecast
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Days in red are the strongest candidates for a protective spray. Index from GrapeFlow’s operational model on Open-Meteo weather — a planning aid, not a substitute for scouting.
About Napa Valley
Napa Valley has a warm Mediterranean climate — hot, effectively rainless summers moderated by morning fog and afternoon breezes off San Pablo Bay, with nearly all rain falling in winter. The dry growing season keeps downy mildew rare, while the warm days make powdery mildew a season-long concern.
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The biology of powdery mildew
Powdery mildew is caused by the ascomycete fungus Erysiphe necator (formerly Uncinula necator), an obligate biotroph that grows only on living grapevine tissue. It overwinters in two ways: as cleistothecia (chasmothecia) lodged in bark crevices, and as dormant mycelium inside infected buds that unfold the next spring as heavily colonised "flag shoots". A few millimetres of spring rain at temperatures above roughly 10°C trigger the cleistothecia to release ascospores, which start the season's primary infections on the first green tissue.
Unlike downy mildew, E. necator does not need free water to infect — its conidia germinate using moisture drawn from humid air — so it is favoured by warm, overcast, moderately humid weather rather than by rain. The optimum is about 21–30°C; growth slows below 15°C, and sustained heat above roughly 35°C kills conidia. Prolonged leaf wetness and heavy rain actually suppress the disease, washing conidia from the surface. From those first infections a fast, polycyclic secondary cycle spreads white, powdery colonies across leaves, shoots and — most damagingly — the berries, where early infection can split the skins.
Because the fungus lives on the tissue surface, susceptibility runs highest from just before bloom through berry set and the following weeks; clusters gain considerable "ontogenic" resistance a few weeks after fruit set as sugars rise.
How the risk index is calculated
The live risk is an operational adaptation of the Gubler–Thomas / UC Davis powdery mildew risk index, applied to hourly Open-Meteo weather for the region's centroid. The published index tracks runs of consecutive days with six or more hours in the pathogen's ideal 21–30°C band: three such days in a row drive the index to its ceiling, while each day that falls short pulls it back down — the point being that sustained warmth, not any single warm afternoon, is what powers an epidemic.
GrapeFlow scores each day 0–100 from three signals in the hourly series — how many hours sit in the ideal (21–30°C) or marginal (15–21°C and 30–35°C) temperature ranges, how many hours have moderate humidity (40–85%), and the longest unbroken run of favourable hours — then subtracts penalties for lethal heat (hours above 35°C) and for heavy rain (over ~5 mm) that washes conidia off the leaf, plus a modifier for prolonged leaf wetness. The result is a planning index for spray timing and scouting, not a measurement of infection; because it runs on modelled weather with no on-site sensors, confidence is capped at medium and lower for forecast days.
Frequently asked questions
What weather favours powdery mildew?
Warm, moderately humid, often overcast weather. The fungus is most active between roughly 21 and 30°C and — unlike downy mildew — does not need rain or free water to infect, since humid air is enough. Long runs of favourable temperature matter more than any single warm day, which is why the risk index rewards several consecutive days in the ideal band.
Does rain increase powdery mildew risk?
No — if anything the opposite. Heavy rain washes conidia off the leaf surface, and prolonged leaf wetness suppresses the disease and favours its natural antagonists; free water is required by downy mildew, not powdery. That said, the mild, humid spells around unsettled weather can still favour powdery mildew between the rain events.
How is this risk index calculated?
It is an operational adaptation of the Gubler–Thomas (UC Davis) powdery mildew index, computed from hourly Open-Meteo weather: hours in the ideal 21–30°C range, moderate humidity and unbroken runs of favourable conditions raise the score, while heat above 35°C and heavy rain lower it. It is a decision aid for timing sprays and scouting — not a substitute for field observation.
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