It happens every year, and it is one of the most painful discoveries of all. In spite of our herculean efforts to open up the canopy of the grapevines and maintain a sanitary growing environment, we have discovered a few mildew strikes throughout the vineyard and, in particular, in our Cabernet Sauvignon. Powdery Mildew, or Uncinula Necator, overwinters as fruiting bodies in the vineyard and can strike tender young shoots and leaves during the growing season or, in this case, developing green berries.
Insofar as we are organic, we are somewhat limited in our options for powdery mildew control and eradication. Our first line of defense is proper canopy management, a process by which, through handwork, we manipulate the shoots and leaves of the grapevines into an upward and open orientation. Judicious leaf pulling in the fruit zone of the grapevine also helps increase airflow and light penetration, aiding the prevention of mildew.
From a chemical perspective, elemental sulfur is an excellent and organically acceptable method of prevention. We apply sulfur in both a dust and a wettable form from roughly six inches of shoot growth to a week or two beyond fruit set—early July, approximately. Other products that we use for early season prevention are Bacillus Subtilus—a biological control agent—and Potassium Bicarbonate—essentially baking soda. Both are fairly effective products, are organically acceptable and are very easy on the environment.
Finally, as a late season method of control and, most importantly, eradication, we employ Stylet Oil. A paraffin-based, extremely refined, light oil, Stylet Oil is an amazingly effective means of eradication of existing powdery mildew infections. Applied as a 2% solution at a rate of 100 gallons per acre, the oil is both smothers the active infections and serves as a barrier against further infection. Organically registered, Stylet Oil is so effective that even conventional growers, with extensive hard chemical resources at their disposal, will use it to handle existing mildew strikes.
The good news in all this is that as sugar levels increase in the grapes a natural immunity develops that prevents further mildew infection. At a about 12 degrees brix (or, roughly 12% sugar by weight) mildew is inhibited, and this level corresponds with veraison, the period during which grapes turn from hard and green to soft and dark. Veraison is almost upon us (in fact I have already seen signs of veraison in some neighboring vineyards), and so our concern of further mildew development is abating. We are applying what might be our final powdery mildew spray as I write (three o’clock in the morning!!!).
Nick (18 July 2012)