Editor's Note: So called “Serr drop” has been a nettlesome problem for California walnut growers virtually since the variety was released three decades ago. It has been studied from many angles to find the cause and solution. Bob Beede tried a new approach with a plant grow regulator initially developed for apples and stone fruit. Below is a report Beede developed about his discovery along with information provided by Valent BioSciences for the use of its product to reduce drop.

University of California field research funded by Valent BioSciences and the Walnut Board suggests a practical and effective solution is on the horizon for long troublesome Serr walnut variety drop.

The University of California Pomology Department faculty, Cooperative Extension farm advisors, and the Walnut Board have collectively sought a practical solution to pistillate flower abortion (PFA) in walnut for more than 30 years. Recent UC field studies with the new Valent BioSciences plant growth regulator, ReTain, suggest it may be the “silver bullet” for this complex and economically damaging cultural problem that primarily affects the Serr variety, but is also found to a lesser degree in other major walnut varieties.

PFA is the loss of nut-producing pistillate flowers 2 to 3 weeks after bloom. It was first noted in the Serr cultivar soon after the earliest plantings came into production in the 1970s.

Originally referred to as Serr drop, flower loss due to this phenomenon sometimes exceeded 90 percent in certain orchards and years. Determining the cause of the disorder proved extremely difficult.

By the late 1980s UC research lead by Peter Catlin (emeritus), Gayle McGranahan, and Vito Polito had eliminated mites, walnut blight, numerous nutritional deficiencies (including nitrogen, calcium and boron), tree age, shading, pruning practices, water stress, tree competition, incompatible pollen and lack of pollination. Cherry Leafroll virus, the cause of Blackline, was also studied and eliminated.

At one point, frustrated growers believed that nitrogen application up to 400 pounds per acre was the solution. The severity of PFA in northern California Serr orchards forced many growers to either remove them in their prime or suffer costly and complicated scaffold grafting to another variety.

Excessive pollen

In the 1990s, researchers in Hungary and California tested the possibility that excessive pollen might be the cause. UC Davis Pomology faculty and farm advisors confirmed this from detailed tagging of flowers and collection of yields from individual trees varying in distance from a pollen source. Reduction of the pollen load in test orchards by catkin removal also decreased PFA and increased yield. Tests on cultivars other than Serr (Chandler, Vina and Chico) showed the presence of PFA but not at levels that typically resulted in economic loss.

Additional research by Polito showed that the excessive pollen tubes (produced by germinating pollen grains that carry the sperm to the egg contained in the ovule at the base of the female flower) growing down the style of the female walnut flower produce excessive amounts of ethylene, a natural plant hormone associated with organ senescence. Elevated ethylene is therefore the most likely cause of walnut flower abortion.

Preliminary field studies in 2003 with ReTain a new, commercially-available ethylene inhibitor developed by Valent BioSciences, supported Polito's hypothesis by increasing fruit set in a Serr orchard from 21 percent to 89 percent. Continued field studies in 2004 and 2005 by me and Joseph Grant, UCCE farm advisor, San Joaquin County, have focused on assessment of yield improvement and development of time and rate of application recommendations under commercial speed sprayer conditions. Doctoral candidate Holly Johnson, in cooperation with Polito and Grant, has also studied any negative effects from repeated ReTain application.

‘All systems go’

Results from these intensive projects conducted with local walnut growers (Barton Ranches, Blain Farms, Hester Orchards, Doug Verboon, and Corny Warmerdam) suggest, “All systems go!”

Applications performed at 125 ppm (one soluble bag/100 gpa) at about 30 percent bloom have consistently provided dramatic reductions in PFA and corresponding increases in yield.

ReTain reduced Serr drop by from 40 to 14 percent in Kings County trials; from 69 to 16 percent in Tulare County and from 73 to 36 in San Joaquin Country trials. These reductions in drop resulted in from 33 to 87 pound more walnuts per tree.

ReTain is registered for use on walnuts in California for this season.

At grower meetings held this spring, Valent representatives were recommending ReTain be applied when 5 to 30 percent of the female flowers bloom in the upper half to two-thirds of the tree canopy. It requires adequate coverage by a sprayer traveling no more than 1.5 to 2 miles per hour.

Since this represented such a dramatic result, Valent conducted trials in Chile last season to verify what had been discovered in California and establish application criteria.

Since the product is so new, Valent is not recommending tank mixes with other products since no compatibility studies have been completed.

Valent reports none of the trials in the U.S. and Chile have indicated an impact on size or quality of the walnuts treated with ReTain.

This exciting discovery represents the culmination of decades of difficult field and laboratory research by UC personnel in concert with growers and the Walnut Marketing Board.