People unfamiliar with the San Francisco Bay Area's micro-climates find it hard to believe that there is a wide range of diurnal temperatures in the Napa Valley also. A friend, who grows Cabernet sauvignon (Clone 7) up in St. Helena, sent me this photograph last weekend of what he originally thought was a grapevine disease. It turns out that his vineyard, that on any given day can be 5-10 degrees warmer than Vinoland, had just experienced a major hiccup in the transition from flower to berry known as shatter.
Fruit set is the stage of berry development 1 to 3 weeks after bloom when most flowers have fallen and those that remain, about 30-60%, develop into berries. Shatter is the detachment of berries from the cluster at the pedicel base (the dark, necrotic tissue in photograph), after the fruit set stage. It appears that the poor fruit set, in this particular vineyard, was the result of dry conditions and high temperatures at bloom. However, shatter can also be exacerbated by variety (Merlot is particularly susceptible), vine vigour, and disease.
What does all this mean? Well, shatter causes widespread yield losses, which in this instance means a diminished crop from which to make wine at harvest time. The fruit set in Vinoland looks solid so Vinomaker will not be without plenty of grape juice to vinify. But he is mourning the loss of those Clone 7 grapes that fell victim to shatter, especially since the 2009 crop surrendered up the best Cabernet of the vintage.