Have you driven down Highway 29 or up the Silverado Trail recently and wondered why so many farmers are ripping out their vineyards? And, are you crazy or does it seem like those piles of pulled out vines get bigger each year?

Well, you’re not crazy. In the last two years, grapevine nurseries have seen unprecedented business selling grapevines for replanting vineyards in Napa.

A grower considers several factors before ripping out a vineyard. It is a costly operation involving heavy equipment, engineers, permits from the county and at least two years without any income from those replanted acres.

Some growers put off the decision for years until yields are so low that farming costs exceed income. At this point, it is a good bet that it is time to replant. Other growers are proactive — rogueing out sick vines, sections of blocks, or entire blocks in a vineyard as soon as an infection starts to prevent further spread.

A grower must answer several questions to determine if the time is right to replant:

• Has productivity continued to decline in the vineyard in recent years?

• Is it time to update the vineyard infrastructure with new technology, change variety, improve your trellis system or adjust the row orientation and the vine spacing?

• Are the vines sick, and need to be replaced with clean planting material?

• Which disease(s) do the vines have?

• What percentage of the vineyard is infected?

• Does the whole vineyard need to be removed, or just the infected vines?

• How quickly does the disease spread? What are the effects of the specific disease on the vineyard?

• Does the disease cause decreased yields, compromised quality, later ripening, or a serious threat to the grape growing industry?

There has been an increase in grapevine leaf roll associated virus (GLRaV) infections in vineyards throughout the Napa Valley as well as other winegrowing regions. The majority of vines that a specific nursery I spoke with sold in the last two years are replacing vines infected with GLRaV-3. Vineyard managers also face the decision of whether or not to replace vines infected with a newly discovered virus referred to as Red Blotch.

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A variety of fungal pathogens, collectively referred to as trunk canker diseases, are also infecting vines. When the vines are pruned in the winter, wind and rain transport and release these fungal spores onto these fresh wounds, and a fungal infection start and slowly kills the vine.

The market is driving replanting decisions as well. According to the USDA’s 2012 Grape Crush Report for the State of California, Napa has the highest price per ton in the state for many cultivars, including cabernet sauvignon and chardonnay. The average price for a ton of Napa Valley cabernet sauvignon, $5,059.96, is more than double that of Napa Valley chardonnay, $2,353.72. It is no surprise that more growers choose varieties with a higher price per ton for their replantings. If grapes were widgets, this would make a lot of sense.

However, here in the Napa Valley, our grapes are anything but widgets. Matching a grape variety to a vineyard site is an important decision. Different varieties strive at specific temperatures, in varying soil types, with certain sun exposures, and have distinct water requirements. Even though only 4 percent of California’s wine grapes are grown in Napa, we are recognized not only throughout the state, but throughout the world as a leader in the industry.

With this honor and distinction comes great responsibility. Growers have set the bar high in terms of the quality that is expected out of our grapes and resulting wines. In order to maintain this well-earned reputation, growers are taking great care of their older vineyards; rogueing out diseased vines, planting cultivars that will express the highest quality that site has to offer while reacting carefully and thoughtfully to pressure from the market.

So the next time you meet a farmer who is replanting their vineyard, ask him or her why they made that decision. Ask what they are planting, and why. Chances are they will be a bit sad to tell the tale of the older, excavated vines. Most likely there will be hope, pride and excitement in their eyes when they talk about the new vines they will plant in the spring. Whatever the circumstances are, you are guaranteed an interesting story.

Happy holidays to you and yours.

Amy Warnock is a viticulturist for Orin Swift Cellars and Warnock Vineyards and a member of Napa Valley Grapegrowers.