Sonoma County wine grape growers have been thrown many curves this year. The results of nasty weather have reduced the opportunity to harvest a normal crop. Yet the latest pitch, wildfires, has actually provided some limited benefits to the vineyards.

“Smoke from the Mendocino fire complex that has filled the air in Sonoma County for several weeks is actually protecting the vines by decreasing the impact of the hot temperature spikes,” says John Clendenen, who with his wife Kathy owns and operates Clendenen Vineyard Management, LLC, Healdsburg, Calif.

The smoke has acted as a screen protecting leaves from the burning effects caused by extreme heat, Clendenen says. It’s also reducing evapotranspiration. James King, viticulturalist at Clendenen, says leaf pressure bomb tests have shown no water stress in the leaves from the smoke.

Ironically Clendenen knows a lot about fires; he’s a former California firefighter with the U.S. Forest Service. The Clendenen family manages 600 acres of wine grapes in northern Sonoma County for Sonoma and Napa wineries.

“Freezing temperatures in Healdsburg dipped to 23 degrees in the spring followed by searing heat during bloom and three weeks of high winds that combined for the loss of one-quarter to one-third of the wine grape crop our company manages,” Clendenen says.

Cold temperatures in some vineyards killed vine shoots, stunting plant development. While the wine grape bloom in Sonoma typically occurs over a two to three week period, severe heat spikes caused some vines to progress through bloom in as few as four days. High winds for three consecutive weeks took their toll on the blooms.

The triple atmospheric attack hammered Chardonnay vines, as damage varied from block to block.

“We have a number of disappointed clients,” Clendenen says. “We’re currently conducting bunch counts to get a hard estimate on this year’s production.”

Clendenen grew apples in Humboldt County and worked as a vineyard manager for Preston Vineyards in Sonoma County’s Dry Creek Valley for 18 years. He formed his own company in 1992.

About 20 vineyard management companies operate in Sonoma County. Clendenen manages and develops vineyards for 20-plus clients including Bella Winery, Fritz Winery, Kathryn Hall Winery, Delectus Winery, Stryker Sonoma Winery, Martorana Family Winery, and Gann Cellars. Clendenen grows four acres of his own grapes which are sold to the Simi Winery.

About 75 percent of Clendenen’s grapes are grown conventionally with the remaining 25 percent organic. His organic customers include Kathryn Hall and Martorana.

“There is a great interest in high-end clients going organic,” Clendenen says. “Organic wine grape production costs up to 25 percent more than conventional.”

The 16 varieties Clendenen manages are Chardonnay, Pinot Noir, Zinfandel, Cabernet, Sirah, Petite Sirah, Grenache, Sauvignon Blanc, Merlot, Cabernet Franc, Primitivo, plus Nebbiolo, an Italian variety grown specifically for the Bella Winery.

“Our goal is producing very high quality fruit that commands a premium price for our clients,” Clendenen says. “When we’re establishing a vineyard, we seriously consider various issues including spacing, row orientation, and planting the best rootstock in specific soils and locations.”

Block sizes range from one-half to 20 acres. Most soils in Sonoma County’s valley floor are well-drained loams; perfect for white wine grape production. Gravelly clay soils on the hillsides best suit red wine grape production.

Most of Clendenen’s vineyard rows are spaced 6 to 8 feet apart with 3 to 6 feet between the vines. While some high-end Zinfandel vineyards are trellised without wire on a single stake, most vineyards have a single vertical trellis system.

“We’re learning that too much light exposure can come from an overly tight vertical trellis system so we’re experimenting with different kinds of cross arms on trellises,” Clendenen says. “Instead of having the wire press the canes tight together, there’s a slightly wider panel of foliage that goes above the fruit which provides more shade for the grapes.”

Ninety percent of the grapes are hand-harvested. The rest are mechanically harvested with the Korvan 3000 harvester from the Oxbo International Corporation. Clendenen has 90 employees, including 40 full-time workers.

While a few vineyards are dry farmed, most are drip irrigated. Annual water use equals about one-half acre-foot per acre. Sprinklers provide heat and frost protection during the growing season. While sprinkling twice is usually enough to combat normal spring frost, the heavy frosts this year required up to 20 nights of sprinklers.

On average, 1,000 vines are planted per acre. Clendenen’s preferred rootstocks include 110R, 420A, 1616 C, 5C, 101-14, and St. George. With the assistance of the University of California, Davis, Clendenen is investigating possible problems on two sites with the 101-14 rootstock that shows signs of substantial vigor decline and large nematode numbers on the rootstock.

Major disease challenges include powdery mildew that can be more challenging in organic vineyards. In the conventionally farmed vineyards, King uses the fungicides Pristine, Elite, JMS Stylet-Oil, and sulfur for mildew control. Though it is less expensive, dusting sulfur use has been reduced due to occasional burn on the fruit and winery concerns over possible sulfite formation in wines from these vineyards.

Pristine, Elevate, and Vanguard are used to control botrytis bunch rot, a fruit rot that also can produce early-season shoot blight after frequent spring rains.

Due to dry and dusty conditions this summer, Pacific and Willamette spider mite populations have increased. King prefers JMS Stylet-Oil for early season mite control and Acramite for later season control.

While the grape leafhopper, whitefly, and root knot and ring nematodes also pressure wine grapes, Clendenen and King have their eyes focused on a much larger threat; the vine mealybug. The insect infests grape vines producing large amounts of honeydew that damages the fruit and foliage resulting in unharvestable fruit.

The insect was first found in California in 1994 in the Coachella Valley, then moved north, and was found in Sonoma and Napa counties among others in 2002.

“The vine mealybug is a huge new headache for wine grape growers,” Clendenen says. “Fortunately, we don’t have it in any of our parcels yet, but we expect to see it at some point.”

Clendenen is also very grateful that the glassy-winged sharpshooter (GWSS) has not become established in the North Coast. The insect is a half-inch long leafhopper that spreads Pierce’s Disease, which kills grapevines by clogging the water-conducting vessels or xylem. He credits the California Department of Food and Agriculture, the local agricultural commissioner, and grape grower organizations for effective GWSS control efforts.

Clendenen’s outlay annually for wine grape pest and disease control is about $1,000 per acre, including equipment and products. While labor is his No. 1 cost, he is struggling with spiraling fuel costs. He spent about $100,000 on diesel and gasoline last year, up 25 percent from a year earlier.

The wine grape harvest in Sonoma County begins in late August to early September and concludes in late October.

email: cblake@farmpress.com