You may think that Summer being in full swing and everyone thinking about ferie, there would be less to do in the vineyard. Isn’t it just time for Madre Natura to do her thing, get that photosynthesis crankin’ and sweeten those grapes?
Not so. One of the things we have to be really careful about are vine diseases, peronospera, or “Downy Mildew” being one of the most common.
Copper Sulfate (mixed with lime), or “Bordeaux Mixture” as it’s often called, is the most common way of PREVENTING fungus infections in the vineyard. I stress the word prevention because the action of Copper Sulfate keeps the fungus from germinating on the leaves. It is NOT a fungus killer. Therefore, if you already HAVE downy mildew, Bordeaux mixture is not going to help you.
Think of it as sunscreen. If you put it on AFTER you get burnt, there’s not much point. And like sunscreen, it’s a topical treatment. It doesn’t get into the “blood” of the vines. It covers the leaves and protects them from the fungus. The effect lasts about 10 days. Less if it rains and gets washed off. That means we have to spray often to prevent problems before they start. Spraying stops at least 2 weeks before harvest, to be sure that the grapes are clean for vinification.
Copper Sulfate has an interesting story. Supposedly the French botanist Pierre-Marie-Alexis Milliardet was studying vine diseases imported to Europe from the New World.
(Oops. Sorry about Phylloxera… and Downy Mildew… oh, and powdery mildew. Our bad.)
He noticed that some vines near the road weren’t affected by Downy Mildew. Turns out folks were snacking on grapes along the road, so the vines were sprayed with Copper Sulfate and lime because the bluish color was a visible deterrent and the spray made the grapes taste very bitter. In 1885 he published his conclusions that the mixture was a effective at preventing Downy Mildew and recommended its use.
It’s been around ever since, so it must be doing something right. It’s even acceptable to use Copper Sulfate in Organic (Bio) vineyards.