Rootstocks with good vigor and resistance to nematodes are important for success with the new dried-on-the-vine (DOV) raisin varieties being adopted by San Joaquin Valley raisin growers, says a Fresno County farm advisor.
Stephen Vasquez has been evaluating DOVine, a variety released by USDA in 1995, in a Kingsburg vineyard established in 1996. DOVine was grafted on five, nematode-resistant rootstocks for comparison of production.
The vineyard has rootstocks Freedom, Harmony, Ramsey, USDA 10-17A, USDA 6-19B, and a rootstock developed by the University of California, RS-2, plus own-rooted DOVine. The trial was duplicated in methyl bromide-fumigated and non-fumigated portions.
In presenting data from the 2002-2004 seasons during the recent San Joaquin Valley Grape Symposium at Easton, Vasquez said RS-2 was consistently the highest producer, followed by Ramsey, own-rooted DOVine, and 1017-A. Harmony, which had erratic bud break each season, was the lowest producer.
Percentages of B and better raisins were highest for RS-2 in fumigated soil and Harmony in non-fumigated soil. Vines on 10-17A ranked lowest in both categories.
Own-rooted DOVine in fumigated plots had the lowest pruning weights at 2.9 kg, versus pruning rates ranging from 5.6 to 7.0 kg and from 5.5 to 6.9 kg in fumigated and non-fumigated plots, respectively.
Interest in DOV raisin production, including improved varieties, new trellis designs, and mechanical harvesting, is being driven by advantages of lower labor costs and higher quality.
Varieties such as Fiesta, DOVine, and Selma Pete share the earlier maturity required for cane-cutting. However, they encounter problems when grown on their own roots like the longtime standard, Thompson Seedless. One disadvantage is the severing of canes that reduces the active canopy in half. “This de-vigorating process,” Vasquez said, “can reduce the production of the vines over time.”
While DOVine and Selma Pete are considered vigorous enough to overcome this problem, planting them on their own roots can lead to another problem, nematodes active in the sandy soils where raisin varieties are usually grown. The tiny worms can cut production by 25 percent by limiting vines' uptake of nutrients and moisture.
That, he said, is why rootstocks that impart both vigor and nematode resistance will be important in establishing new DOV vineyards.
Matthew Fidelibus, University of California viticulture specialist at Parlier, told the symposium of his investigations of drying methods and varietal effects on raisin quality. Collaborating with him were Vasquez and George Leavitt, Madera County farm advisor.
The research was done in response to grower observations that DOV raisins might continue to accumulate sugars, and improve in grade, following cane-cutting. Some also asked if the DOV process might affect raisin shape, causing them to be favored by the air-stream sorter.
The researchers concluded 1) that drying method can affect raisin quality, depending on variety, and 2) that effects on quality are not due to the air-stream sorter method and are probably not related to soluble solids.
In one of their trials, they found that DOVine berries, dried either on the vine or on trays, had similar quantity and composition of sugars before and after drying. “The raisins also had similar air-stream sorter grades, as would be expected from fruit samples having similar soluble solids,” Fidelibus said.
However, in a separate study using raisins submitted to USDA for visual grading assessment, which the air-stream sorter is intended to duplicate, they found that the effect of drying method depended on variety.
Drying method, he said, had no effect on the quality of Selma Pete raisins, nearly 100 percent of which were graded B and better, but Fiesta DOV raisins graded much higher than Fiesta tray-dried raisins (97 percent versus 67 percent, respectively).
Fidelibus said choosing the best time to cut canes can be difficult in some years, particularly with Thompson Seedless. “The earlier in the season the canes are severed, the more likely it is that the fruit have not amassed enough soluble solids to make good raisins.
“However, if the canes are severed too late, the raisins might not dry sufficiently, thus forcing the farmer to deliver his fruit to a dehydrating facility and negating the cost savings expected from the DOV method.”
Earlier research showed that of fresh fruit having a reading of about 18 degrees Brix dried either by DOV or tray, the DOV raisins graded higher than the tray-dried.
“This suggests,” Fidelibus said, “that the DOV process could be initiated earlier, with fruit at lower soluble solids, than is currently recommended for tray drying.
“Whether the differences in quality were due to fruit composition or to some other variable that affects grading is unknown and should be tested.”
Fidelibus said raisin labor costs are among the highest, as a proportion of total production expenses, of any crop. Much of the costs are in labor needed for traditional harvesting, tray laying, and pick up, and the industry may need as many as 50,000 workers to complete a harvest.
He said between the 2000 and 2003 seasons, the raisin price was so low many growers could not recover harvest costs. “Even though prices improved in the last two years, the recent economic crisis stimulated interest in mechanical harvesting that is unlikely to ebb.”
The symposium heard from another researcher who has developed analytical sensory analysis guidelines, much like those used with wines, for determining consumer preferences for samples of tray-dried or DOV raisins.
Hildegard Heymann, professor and sensory scientist in the Department of Viticulture and Enology at the University of California, Davis, said those preferences have implications for raisin marketing.
Using “analytical sensory analysis,” which involves trained panelists and is more objective and reliable than “consumer sensory analysis” involving targeted consumers, she diagramed raisin flavor traits for Selma Pete and Fiesta, both DOV and tray-dried.
Among the results from sampling, she found that Fiesta DOV raisins were more moist, while Fiesta tray-dried were more sticky and chewy. Selma Pete tray-dried were more spicy, and Fiesta tray-dried had larger wrinkles.
Heymann concluded there were distinct sensory differences between both varieties and between both drying methods, as well as interaction of cultivar and drying method. On average, she added, consumers do not much care which, although some clusters of consumers prefer certain versions.
This type of information provides, she said, “the possibility of actually marketing to the person who is going to like your product better.”