When it comes to California table grapes, yesterday's varieties are not today's, and today's certainly won't be tomorrow's.
Aside from the venerable and versatile Thompson Seedless, table varieties have changed dramatically during the past 75 years.
That's how long USDA scientists, responding to preferences of consumers and growers, have been breeding table grapes at the Fresno Field Station.
The first crosses with classical breeding techniques were made in 1923, and the first variety, Cardinal, was released in 1946. Another 13 improved table varieties, once they passed exhaustive selection by USDA scientists, have been released since.
Notable is the Flame Seedless, bred by John Weinberger and released in 1973. It has become the most important red table variety grown throughout the world. Raisin varieties and root stocks also originated at the station.
During the recent Table Grape Seminar in Visalia, David Ramming, USDA research horticulturist at the Fresno Station, described a half-dozen new table cultivars now fruiting and in screening for viruses by the Foundation Plant Materials Service at the University of California, Davis.
He also disclosed current breeding for table and raisin varieties resistant to powdery mildew and Pierce's Disease and genetic transfers in rootstocks to create resistance to fanleaf virus.
C67-120 is a white seedless adapted to cold storage and ripening later and having berries slightly larger than Thompson Seedless.
A61-16 is a red seedless with a crisp berry and a slight Muscat flavor. It produces clusters weighing two to three pounds, ripens with Ruby Seedless, and responds to gibberellic acid. Some questions linger about its productivity.
C63-119, another seedless red that ripens with Ruby Seedless, has natural berries averaging 4.7 grams and responds to gib treatments. Initial storage trials have been encouraging, however, color can be tricky to achieve with high yield.
“Another red seedless we are excited about is B34-82, which ripens with or slightly after Ruby Seedless but before Crimson Seedless. Its real advantage is its ability to color in the shade,” Ramming said, adding that if leaf pulling is not sufficient, it can develop a dark purple color. It has a thick skin and shows promise for cold storage.
B40-66 is an early-season black that matures with Flame Seedless and has a high count of clusters. Crisp and firm with a slight Muscat flavor, it is being closely observed for its natural berry size, relatively modest at 3 grams, and tendency to tough skin.
The sixth selection is another mid-season black seedless, B32-94, which can throw a large crop with either spur or cane pruning. Berry size is large, its clusters are ideally sized at about one pound, and it has good eating qualities.
Ramming has two other promising entries. B40-97 is a white seedless, Muscat-flavored, mid-season variety, being considered as a replacement to Italia. With gib sprays, it can develop berries at least three-quarters the size of Italia. Like Italia, it may amber if exposed to sunlight. It is sensitive to gib sprays for return crop.
A specialty with a Concord flavor fancied by consumers in the Midwest and East, A29-67 is purple and seedless, with small berries. It is being considered as a potential replacement for Niabell.
A cross of Concord with Thompson Seedless, A29-67 occurred not in a search for a potential new variety but as a serendipitous discovery during a study on an embryo-rescue technique developed and refined by Ramming and his staff.
Other 1999 selections to be added to Ramming's advanced trials are three early to mid-season whites and a large red seedless. All have appealing large berry size.
Ramming is also searching for new varieties able to fend off powdery mildew and Pierce's Disease. “In 1999,” he said, “California grape growers applied more than 3.5 million pounds of sulfur to table grapes (to control powdery mildew).
“The cost of chemical control and encroaching urbanization, which increases exposure of adults and children to pesticides, are of great concern.”
Since 1994 he has found several selections and cultivars that remained free of mildew in a plot not treated with fungicide. He has made several crosses and plants are being screened. One mildew-tolerant selection has traits similar to Flame Seedless.
In yet another project, a long-term venture in cooperation with Andrew Walker, geneticist at the UC, Davis, Ramming last year made crosses of table and raisin cultivars for resistance to Pierce's Disease.
Prospects of genetically transformed grape rootstocks also appeared on the vineyard horizon in 1994 and 1995 as Ramming and a USDA colleague in West Virginia introduced test genes and the tomato ringspot virus coat protein gene into grape.
Another current project with geneticist George Bruening of UC, Davis saw gene insertion in grape for fanleaf resistance. This research centers on genetic transformation to introduce useful genes not present in grape germplasm or found in grape species that are difficult to hybridize with Vitis vinifera.
Ramming's breeding program, supported by the California Table Grape Commission, starts with crosses made with his timesaving embryo-rescue technique. Even so, the process, from selection among hundreds of crosses to promising selections for 25-vine test plots and on to release for commercial production, typically consumes 11 to 12 years.
Ramming noted that the variety Melissa, released in 1999, has been renamed Princess due to a trademark conflict.
Objectives of the breeding program are guided jointly by consumer preferences as determined by commission surveys and potential for reduced production costs for growers. Selections of new varieties are made on the basis of berries, clusters, production, and vigor.
Since 1993 Ramming and his staff have evaluated 2,300 seedlings per year. During the same time, they have grown 16,000 seedlings from seedless by seedless crosses.
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