Yield reduction can range from zero to some 30 percent in wine grapes left on the vine to accumulate sugars, but how much hinges on variety, trellis design, site, irrigation, and year, according to studies by Paul Verdegaal, San Joaquin County farm advisor.
Verdegaal did the field trials near Lodi, Calif., in 2005 and 2006 to chart the effects of the controversial “hang time” sought by wineries to enhance wine flavors but criticized by growers because of tonnage lost as grapes mature beyond 25 degrees Brix.
During the recent San Joaquin Valley Grape Symposium at Easton, he said his data showed when fruit is left until it reaches 25 to 29 degrees Brix, weight loss can range from 5 percent to 35 percent, plus or minus 5 percent.
Variations were observed on Syrah, Merlot, and Cabernet Sauvignon vines between standard bilateral cordon (T), vertical bilateral cordon (VSP), and quadrilateral cordon (HDC) trellises at plots near Woodbridge and near the Gallo Liberty winery.
“Cluster weight and berry weight were definitely affected, but loss of entire clusters was not evident at all in 2005 and only slightly so in 2006. Generally, the HDC showed more losses than the T trellis,” he said. Although intermediate in response, the VSP was the most widely variable.
“Total acid levels were more affected than pH, but both were negatively affected with delayed harvested. These negative effects are assumed to be countered by better and more complex flavors in resulting wines with less or no vegetal notes.”
Key factors in vineyards tolerating delayed harvest are the health of the vines, sufficient leaf area for the crop load, sufficient time to recover before leaf fall, and no excessive water stress prior to harvest.
This trial and earlier research, he said, suggest the interaction of irrigation levels, crop levels, and delayed harvest might alleviate nearly half the weight loss under mild harvest conditions seen in 2005 and 2006.
He went on to say that fruit from early harvest tasted different from fruit picked later, and although these were not measured objectively, they could indicate potential for wine differences.
There are both costs and benefits from delayed harvest to manage fruit maturity for quality or style of wine. However, more field trials and wine evaluations will be needed to reveal answers about how to provide quality wines at reasonable cost, Verdegaal said.
Regardless of the management strategy used, he added, “vine balance is the key. And we also still see anecdotally, and to some extent experimentally, that for any given variety, the first third of the harvest of that variety is usually from the best vineyards.”
Hang time can make a difference, but it does not automatically mean high quality. In any case, close communication between the grower and the winery about the practice is important, Verdegaal said.
The symposium also heard from Matt Fidelibus, University of California viticulturist at the Kearney Agricultural Center, Parlier, on wine grape clonal selections for improved yield and quality.
The project by Fidelibus and Pete Christensen, emeritus viticulture specialist, evaluated clones — or cuttings from a single mother vine — of Cabernet Sauvignon, Chardonnay, Merlot, and Zinfandel/Primitivo at Parlier to find those on par or better than the industry standard clones.
All the Foundation Plant Materials clonal selections were planted in 1997, and ratings were made in 2000 to 2003.
Among the six Cabernets, they found that selections 8, 21, and 22, when harvested at the same date, had higher yields because of heavier berries, larger or more clusters, or combinations thereof. Since selection 22 and was one of the earliest maturing, growers may want to consider it instead of clone 8 if earliness is desired.
Although consistent differences among six Chardonnay clones were not found, clone 4, the current industry standard, generally had fewer and heavier clusters and clones 6 and 15 generally had more and lighter clusters.
Clone 4 had higher soluble solids, lower pH and similar or higher acidity, and although clone 4 had similar or more sour rot than the others, clone 15 had less sour rot, due to its smaller berries and open clusters, than the group.
“Results,” Fidelibus reported, “suggest that growers in the San Joaquin Valley, or similar warm-climate areas, might consider planting clone 15 instead of clone 4.”
The trial also looked at a half-dozen clones of Merlot. Plantings in the SJV make up 45 percent of Merlot's tonnage and 20 percent of its acreage in the state.
Selection 10 produced consistently higher yields than the others, along with some of the lowest pH and highest acidity levels. Selection 3, the current industry standard, was similar to selections 1 and 9, but was not superior to selection 10.
Also evaluated were three clones of Zinfandel and three of Primitivo, now both considered to be selections of an ancient Croatian grape. Few differences were seen between the three Zinfandel clones.
Fidelibus said the Primitivo selections, particularly 3 and 6, were generally superior to the Zinfandels because of earlier maturity, similar or higher yield, and similar or lower sour rot susceptibility.
The clonal studies continue with evaluations of several varieties, including reports on Barbera and Syrah scheduled to be released soon, Fidelibus said.
Grape vine canker disorders caused by at least nine species of Botryosphaeria make it the most prevalent fungal disease of vines in California, said George Leavitt, emeritus farm advisor for Madera County, who offered tips on how to manage them.
Botryosphaeria typically enters grape vines through fresh pruning wounds and grows from the entry site toward the roots. It can be found with another fungus, Eutypa, and the two cause losses estimated at more than $260 million annually in California.
Since no complete chemical control is available, not enough is known about how the diseases spread, and hosts are so numerous, total eradication is impossible.
Thus, Leavitt said, “the best recommendation for control is vineyard sanitation keeping the inoculum level as low as possible.
“Cultural practices such as pruning all diseased wood, removing pruned wood from the vineyard, and destroying it are highly recommended,” he said.
Pruning cuts should be as small as possible, since when a wound is doubled in diameter, the total area exposed is quadrupled. Although perhaps difficult to do in typical vineyards, cuts should ideally be made where possible so the wound is facing downward to reduce exposure to airborne spores.
Pruning during the period from about Thanksgiving to Christmas creates ideal conditions for infection with four to six weeks of exposure, but pruning during late February, when the wounds heal more rapidly, reduces susceptibility to about 10 days, Leavitt said.