Damp and warm weather favour the fungus that causes the grape disease, while the unseasonable rain washes away the pesticides used in organic operations.
"I've already treated (the vines) 10 times since April, almost every three or four days," said Jerome Boutinon, clambering off his tractor fitted with a device for spraying copper sulphate -- the only organic option to fight mildew.
After losing "half the harvest" on his family vineyard in the Entre-deux-Mers region to mildew last year, fighting off the fungus is taking up most of 47-year-old Boutinon's time.
Each successive copper treatment is washed off the next time it rains.
"We used to have a year or two of mildew every decade. Since 2018, we've gone through five years under very strong pressure," said Patrick Delmarre, an independent consultant for wine growers.
He blames climate change, which has brought warmer and wetter winters and "very rainy springs" to the Bordeaux region.
"If every spring is rainy like this, we'll have to consider whether we keep going," one of Delmarre's wine-growing clients told AFP on condition of anonymity.
Mildew could be "the last nail in the coffin for the sector", said Dominique Techer, spokesman for the local branch of the Confederation Paysanne agricultural union.
"All the ingredients are there for a health and business disaster," he added.
- Tearing up vines -
Bordeaux' CIVB wine industry body told AFP that many vintners are "fed up with spraying" and "worried" about the new climate that appears to be settling in.
The CIVB is testing new vines and varieties that could be more resistant than Bordeaux' highly vulnerable mainstay, merlot grapes.
But some growers are already talking about abandoning organic farming rules to lay their hands on more effective treatments that penetrate deeper into their plants.
"We need to be able to treat (vines) with effective products, insurance companies to take diseases into account and better pay for producers," said Stephane Gabard, president of the Bordeaux Superieur designation.
Around 8,000 of Bordeaux' 103,000 hectares of vineyards are set to be torn out this year after last year's weak harvest, on top of lower wine consumption meeting overproduction from the region.
And this year's weather has not been promising, with May bringing 116 millimetres (4.6 inches) of rainfall -- twice the usual level -- in Saint-Emilion, the Mecca of Bordeaux wine.
Some remain hopeful, noting that mildew can sap the quantity of the harvest but doesn't affect quality.
They say better weather conditions could allow growers to beat back the fungus.
"We may yet have a very good year," said Jerome Boutinon.
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