Using plant growth regulators (PGRs) is as much an art form as a science. When they work, they work great. When they don't, farmers and researchers often scratch their heads in bewilderment.

Nevertheless, PGRs continue to capture the imagination of growers and pest control advisors alike for the potential they hold to produce more and better quality crops like grapes.

Prestige is a PGR from Valent that has proven effective on table grapes and kiwi-fruit to increase fruit size.

In table grapes, forchlorfenuron — the active ingredient in the synthetic cytokinin product — increases berry size, uniformity and firmness, as well as delays ambering in green grapes. It can also delay harvest for bigger grapes and better prices. Increased berry size allows for higher yield at pack out, giving the growers the potential to have more boxes in the premium grade category.

Wine grape growers are forever fascinated by PGRs.

Rhonda Smith, Sonoma County, University of California Cooperative Extension farm advisor, tried her hand at seeing if she could make CPPU — as it is commonly called — increase fruit set, berry size and composition of Sonoma County premium wine grapes.

Not surprising, she got mixed results as she reported at the recent Sonoma County grape day in Santa Rosa, Calif.

The only consistent result was that where it was used in four trials over a two-year period, harvest was delayed. That is not surprising considering the product increased fruit size in many cases.

In the lone 2006 trial in a 10-year-old block of Merlot, two gallons and four gallons of CPPU per acre (8 and 16 ounces of Prestige respectively) were applied in 100 gallons of water per acre at 90 percent bloom.

Vines treated with the four gallon rate set a significantly larger number of berries compared to the two-gallon rate and the untreated vines.

Besides delaying ripening, the four-gallon rate also reduced juice pH and total acidity. The two gallon rate produced more fruit color than the untreated fruit or the grapes treated with the higher rate of CPPU.

In one of Smith's two 2007 trials, an 11-year-old Pinot Noir vineyard on the Sonoma Coast was treated at bloom on May 25 with the two gallon CPPU rate.

The result was no difference in average weight or average cluster weight at the two-gallon rate. The treatment did not affect juice soluble solids, pH or titratable acidity.

The average yield of the five-row plot was slightly greater, but not significant.

Very cool and foggy conditions during bloom on the Sonoma Coast can reduce berry set, so Smith tested the PGR on a nine-year-old Pinot Noir vineyard with a variety of treatments; two and four ounces at early (20 percent) bloom (May 25) and the same two rates at a late bloom, June 3 when the bloom ranged in clusters from 50 percent to 100 percent bloom with an occasional after bloom cluster set.

The average cluster weight was increased, regardless of when or how much PGR was applied. However, clusters from untreated vines had fewer small-sized berries and more tiny-sized berries than clusters treated with CPPU.

No differences in berry weight were recorded across treatments.

Clusters from vines treated with the two-gallon rate at early bloom were slightly lighter than clusters treated at either rate at late bloom.

Clusters from untreated vines had higher soluble solids and pH than any of the treatment clusters. However, the yield from treated vines was significantly higher than untreated vines.

A pair of growers conducted their own CPPU trials last year and Smith monitored the results.

The first was in a block of young Malbec vines treated with the two gallon rate on May 18 at bloom. Clusters from treated vines weighed more than the average weight of berries from untreated vines. The average number of berries per cluster was greater. There were no differences in berry composition.

The other grower treated two small areas of a Sauvignon Blanc vineyard where yields had been reduced due to fanleaf virus.

Where the virus was severe in the vineyards, yields were increased significantly where CPPU was applied. Where it was not a major problem, yields were far greater in the untreated area.