USDA breeders, aiming to improve the bottom line of the California raisin industry, are developing a new generation of grape cultivars having lower production costs and greater consumer health benefits.
David Ramming, breeder at USDA’s Parlier, Calif., center, and his associates are pursuing four objectives: natural drying on the vine without cane-cutting; resistance to powdery mildew; resistance to Pierce’s disease; and high anthocyanins content.
Key in the crossing and selection of new seedless cultivars for all four objectives is the embryo rescue technique, developed by Ramming and his colleagues in the early 1980s, which allows hybridization of seedless by seedless grapes.
In detailing progress with the projects during the annual San Joaquin Valley Grape Symposium at Easton, Ramming said previous work on early-maturing seedless raisin types resulted in the Fiesta, DOVine, and Selma Pete varieties.
These are adapted to mechanical harvesting, but they still require cane-cutting for drying on trellises. The next improvement was to harness traits for new varieties that dry naturally without the need for cane-cutting.
Ramming explained that during the breeding process that led to Selma Pete, they noticed selections that dried on the vine without canes being cut.
Sources of that trait include an Italian variety, Primus, and early-ripening table grape selections. Crosses have been made for the past several years and evaluated in plots for production, moisture, and fruit quality.
The most prominent advanced selection is one that last fall had fruit ready to be harvested by early October. Yield at Parlier for two years has been 3.4 to 3.8 tons per acre, and production on spurs has been about equal to that of cane-pruned vines. It has been rated with commercial potential.
“The moisture at harvest,” Ramming said, “has ranged from 12 percent to a high of almost 17 percent, and the B or better fruit has ranged from 100 down to 90 percent. We hope this selection will be ready to release in a couple of years.”
Additional hybrids are made each year in the quest for selections with resistance to powdery mildew, the ancient foe of grape vines. Breeding for the resistance began in 1994 after cultivars with the trait were discovered in a no-fungicide spray plot.
Seedlings are first screened in the greenhouse with exposure to the fungus, Ramming said. Those that do not show the disease are planted to field plots, and about half of them make it to the plot stage.
Sources include North American grape species, a species from China, and even some examples of V. vinifera, which, he noted, have shown resistance. Molecular markers have been developed to identify resistant plants.
“Molecular markers,” Ramming explained, “allow us to know which gene controls the resistance so we can combine a number of sources into a single plant for a very durable source of resistance.”
Mildew-resistant plants have a diversity of traits, including Thompson Seedless types, large clusters, black- or white-skinned Zante Currant types, and types naturally suited to drying on the vine.
In work on resistance to Pierce’s disease, Ramming is collaborating with the University of California, Davis Viticulture Department, mainly with resistance carried by V. arizonica. Breeding has been underway since 2000.
Backcrosses to raisin grapes have been made for five generations to build fruit quality equal to Thompson Seedless while maintaining resistance to the bacterium that causes the disease.
Ramming said molecular markers enable segregation of susceptible and resistant selections in test tubes before screening in the greenhouse. This past year selections were made from crosses having up to 97 percent V. vinifera in their background for potential for high fruit quality.
The work to achieve raisin grapes having high content of anthocyanins, Ramming said, stems from other research pointing to the compounds having antioxidants for health benefits to consumers.
Anthocyanins, which provide the color for various fruits, vegetables and flowers, have been linked to protection of human nervous and circulatory systems and eyesight.
The wine variety Rubired is being used as a source of the red flesh to prevent off-flavors found in wild grape species.
“Our goal is to add red flesh, the anthocyanins trait, to raisin grapes,” he noted, adding that they are using molecular markers while seedlings are in test tubes, along with another sophisticated procedure using a leaf sample to predict red flesh in crosses, all to speed the process.
Tests of the trait in the first generation of seedlings showed amounts equal to those in Rubired.
The embryo rescue technique refers to the tiny, soft seeds of seedless grapes that under natural conditions would abort. However, when embryos are surgically rescued, or removed, by hand in the laboratory, they can develop into seedlings and later be hybridized.
According to Ramming, this practice makes the effort three to four times more efficient and does it in half the time. When a seedless grape is hybridized with a seeded grape, only about 15 percent of the offspring are seedless. However, with seedless by seedless parents, the offspring can be up to 100 percent seedless.
In addition to improved raisin varieties, the technique has been successful in development of several table grape varieties produced in California to provide more alternatives for consumers while lengthening the growing season for growers.
In another presentation before the symposium, Allison Beadle, vice president and nutrition specialist of Fleishman-Hillard, Inc. in Austin, Texas, told of studies that show nutritional values of California raisins.
During the past two years, the California Raisin Marketing Board has funded three efforts. A University of Illinois-Chicago investigation showed that California raisins contain phytochemicals that inhibit some of the bacteria which cause cavities and gum disease.
A study at the University of Connecticut revealed that daily consumption of one cup of raisins and walking lowered harmful cholesterol in the blood stream.
Another study pointed to the value of intake of raisins and other dried fruit in decreasing overweight and obesity in adults.
Beadle’s agency also coordinated the promotion of California raisins in magazines and other media dealing with proper nutrition, as well as endorsement by dietitians to food service conferences and retail outlets.