Call it what you will: CPPU, forchlorfenuron, or KT-30, but watch for it, hopefully someday with a catchier name, maybe in a vineyard near you.

The growth regulator, said to double the size of Thompson Seedless berries and delay crop maturity up to a couple of weeks, is getting a closer look in the field under an experimental use permit for table grapes in California.

Nick Dokoozlian, University of California viticulture specialist, talked about the product, whose marketing and distribution rights as KT-30 in California are owned by Valent Biosciences, during the recent San Joaquin Valley Table Grape Seminar.

“It's not a revolutionary compound like gibberellic acid (GA), and it won't transform grape quality overnight, but it could be a tool for many growers to use in their vineyards,” he said at the Visalia event.

Known as KT-30, it was synthesized in Japan in mid-1970s and later tested extensively on grapes by Velsicol Chemical Co., where researchers said the size of natural Thompson Seedless berries could be increased by 100 percent or more with it at fruit set. The compound was later registered for table grapes in South Africa, Chile, and Mexico.

Experimental use

Valent Biosciences recently obtained an experimental use permit for the material on 3,500 acres of grapes in California, most of which are thought to be table varieties.

The cost of the product is $30 per gram, said Dokoozlian, who with others researched the compound with support from the California Table Grape Commission.

Briefly, CPPU, an abbreviation of the long chemical name and the handle that seems to fit best for the moment, is a synthetic cytokinin. Cytokinins, in the case of grapes, are compounds that delay fruit maturation.

The product, Dokoozlian said, is about 10 times more effective than any natural cytokinin ever tested on grapes. Research thus far shows it can delay maturity by up to two weeks, depending on cultivar, rate, and season. “We can apply it as late as veraison and get a two-week delay in maturation.”

It works by significantly stimulating both cell division and cell elongation. That brings increases in berry size and berry diameter relative to length. It also increases firmness and cap stem attachment.

The range of rates for managing size and firmness is 4 grams to 8 grams per acre, but higher rates may retard color in colored varieties.

“The interesting thing is it has not effect on return fruitfulness. It can be used it improve size on GA-sensitive varieties,” said Dokoozlian, referring to the seedless Princess (formerly Melissa) and seeded Redglobe. No effects on vegetative growth have been reported.

He said it is unlikely CPPU, which does not stimulate early stages of berry development, would replace GA for Thompsons, Flame Seedless, and other standard seedless varieties.

The two can be combined for a synergistic effect on berry growth; however, the disparity between activity, and rates, of the two compounds is great. On Thompsons, Dokoozlian said, “We can do with 5 to 10 grams per acre of CPPU what it takes with about 100 grams per acre of GA.”

Lower rates are generally suggested for Flame Seedless to avoid potential loss in color development. In fact, Dokoozlian said the potentially negative effects on color are the most important things about CPPU for growers to be aware of.

CPPU can change the shape of berries, which may be detrimental to consumer acceptance of the fruit. Where GA tends to stimulate elongation, CPPU tends to expand the radial growth of berries. Thus fruit set has to be considered in determining rates for varieties having cylindrical berries.

Rates of CPPU should be lowered (or eliminated) in years where grapes have poor shatter to avoid excessive cluster compactness.

More research

Dokoozlian said additional research, particularly for more recently released cultivars such as Crimson Seedless and Autumn Royal, is needed to see how timing and rates can be manipulated to achieve delayed maturity without impairing quality.

Although CPPU is shown to improve berry firmness and cap stem removal force at harvest, some studies indicate eating quality suffers with high rates of CPPU. Rates greater than 8 grams per acre resulted in bland tasting fruit, even though soluble solids levels were adequate, Dokoozlian reported.

Questions also remain on the interaction between CPPU and ethephon relating to color in major red and black table varieties. Early research showed ethephon improves color of CPPU-treated fruit but does not completely reverse adverse effects on pigment development.

E-mail: dan_bryant@intertec.com