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Princess table grape: Poor fruit-set, other problems

Princess, a white, mid-season, muscat-flavored, table grape cultivar released in 1999, showed poor fruit-set and other problems in trials with grower-cooperators, according to University of California viticulturist Nick Dokoozlian.

Dokoozlian, who is based at Parlier, reported his observations aimed at finding the best cultural practices for several new table varieties during the recent San Joaquin Valley Table Grape Seminar at Visalia.

Veteran breeder David Ramming of the USDA/ARS Horticultural Crops Laboratory at Parlier developed the new varieties, using both classical and gene-splicing methods.

The goal of the industry-supported breeding research is to come up with white, red, and black varieties that will extend the market period for California table grapes.

As with traditional table grape cultivars, Dookozlian noted, "each of these new cultivars has distinct horticultural traits and requires a unique set of cultural practices or manipulations to maximize fruit productivity and quality."

The Princess variety’s large berry size, late maturity, and appealing flavor caught the notice of researchers and growers alike in evaluations before its release.

Princess was originally released under the name Melissa, which was later changed due to copyright issues with a grocery chain of that name. The variety has been planted on about 1,000 acres in the SJV.


Despite its early promise, it has shown susceptibility to poor fruit-set and inflorescence necrosis, or early clusters turning dark and dying prior to, or soon after, bloom. Remaining clusters have severe shatter.

Dokoozlian said the disorder, most severe in third- and fourth-leaf vines, usually occurs when daytime temperatures are below normal during the bloom period. Its exact cause is unknown, and little can be done to correct it.

Another problem with Princess observed during the past two years was the decline of many new vineyards using bench-grafted Freedom and Harmony rootstocks, while own-rooted vines showed no symptoms of disease. The problem was linked to non-certified Princess wood infected with Grapevine Leafroll Virus II. Dokoozlian recommends that only certified scion-wood be used with rootstocks.

Crimson Seedless, the popular, red cultivar released in 1989, requires special attention to crop load and use of plant growth regulators to achieve adequate color development, according to Dokoozlian. The grape now occupies 9,000 acres in the SJV, and more is being planted.

Among vineyards surveyed in the trials, yields varied from 900 to 1,400, 22-pound boxes per acre on the open-gable trellis system.

"Excessive crop loads," Dokoozlian said, "delay maturity and decrease color accumulation, and clusters rarely exceed 50 per vine. Due to the moderate cluster size and architecture, little tipping or shoulder removal is typically necessary."

Coloring caution

Nearly all Crimson Seedless vineyards in California need Ethrel (ethephon) applications to achieve optimum color development, but he urged caution, since seasonal temperature variations during ripening generally have the greatest impact on berry color development.

"Ethrel," he said, "reduces berry firmness compared to untreated fruit, but the reduction can be minimized by making the initial application three or four weeks after berry softening, typically mid-August, rather than at the initial stages of berry softening.

"However, the delayed application also delays color accumulation and harvest. In contrast to most cultivars, few correlations between cultural practices and color development have been found, including crop load and berry-sizing treatments."

Autumn Royal, the late-season black boasting 8-gram or larger berries (vs. typical girdled and gibberellic acid-treated Thompson Seedless berries of about 6 grams), has been planted to about 2,000 acres since its release in 1996, but data collected in recent years show yields in the same vineyard can fluctuate from 750 to 1,200 boxes per acre within a three- or four-year period.

Dokoozlian attributes that variability to Autumn Royal’s requirements for relatively high temperatures and large amounts of sunlight during fruit bud differentiation to produce maximum bud fruitfulness.

Controls problems

Applications of gibberellic acid can control two special problems in Autumn Royal: excessive tight clusters in a heavy-set year and seed traces.

Its relatively weak cluster framework and tendency of berries to separate from the rachis can be managed by packing clusters in plastic bags.

The skin of Autumn Royal is also relatively more sensitive to cracking caused by exposure to prolonged rainy weather. That can be moderated by the use of open canopies near harvest to promote air circulation after a rain.

Only a few hundred acres of the mid-season black seedless, Summer Royal, have been planted since its release in 1999. Quadrilateral cordon training, on either standard California "T" or open-gable trellising, and spur pruning are recommended for maximum yields and fruit quality.

"Bilateral cordon training," Dokoozlian explained, "is not recommended for this variety due to inadequate productivity caused by too few spurs retained per vine."

Trials with growers thus far reveal that Summer Royal, which has a yield range of 900 to 1,200 boxes per acre on an open-gable system, suffered no color development problems with the higher crop loads.

Thinning sprays of gibberellic acid are not necessary because of the variety’s naturally loose clusters, but the moderate size of the berries, about 5 grams, makes sizing treatments with the growth regulator desirable.

Another speaker at the seminar was Jocelyn Millar, professor of entomology at the University of California, Riverside and authority on pheromones, the chemical cues that insects use to communicate.

Pheromones are employed to sample for pests and track their populations, to detect exotic, quarantine pests, to disrupt mating, or as lures to trap pests.

Millar said it takes many years of laboratory effort before a new synthetic pheromone becomes available to commercial agriculture. One example is a navel orangeworm pheromone that he has been working on for 15 years.

Steps in process

Major steps in the process are monitoring the target insect species to determine how it uses a pheromone, determining the pheromone’s chemical structure, reconstructing a synthetic duplicate, and testing the synthetic formulation in the field. At that point it can be considered for commercialization.

"Structure identification is still somewhat of an art, and most new structures are worked out by combining fragments of data from different methods, until you have narrowed the possibilities down to one or a few structures," he said.

"Unfortunately, technology has not yet advanced to the point where you can feed a chemical into an instrument and it will tell you exactly what the structure is."

Commercialization, he added, ultimately depends on the size of the potential market for the pheromone, as well as the cost and difficulty of manufacturing it.

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