Winemakers want motorists driving on dirt roads through or along their vineyards to proceed slowly so they don't stir up dust. They have all sorts of ways to express their request. This modified traffic sign on the approach to Naggiar Vineyards in Nevada County, however, is one of the more arresting and effective I've seen.
More often, vintners appeal to drivers by posting signs cautioning that "dust hurts vines." But just how does dust hurt vines?
"It has been our observation that vines nearest to dirt roadways are more susceptible to outbreaks of damaging spider mites," says Dr. James Wolpert, viticulture extension specialist in the department of viticulture and enology at UC Davis.
Spider mites feed on grape leaves, and as the population of the pest increases the damage from their feasting weakens the foliage and hampers the production of sugars for the grapes.
"Spider mites like a dry environment, and dust contributes to that," says Dr. Matthew Fidelibus, another viticulture extension specialist with UC Davis. "If the infestation is high, you get defoliation problems and damage to the (leaf) canopy. The leaves will look almost like they've been burned." And a damaged canopy can lead to problems like sunburned and inadequately ripened fruit.
"It is not clear (at least to me)," notes Wolpert, "whether dust somehow encourages spider mites themselves or discourages the beneficial mites and other predators that otherwise keep the spider-mite populations in check. My guess is the latter."
Either way, ease off on the throttle as you head out to tasting rooms during this most popular time of the year to visit wineries.