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03/01/2006 02:00:00 AM

Apple PGR reduces yield-robbing Serr drop in walnuts, but you can’t cut corners

By Vicky Boyd
Editor

An apple plant growth regulator recently registered for use on walnuts may not be the silver bullet growers of the Serr variety are seeking, but it should provide some much-needed relief from yield-robbing pistillate flower abscission.

“We’re confident that we can tell growers that ReTain will improve yields at least 33 percent, and in some of our trials, we’ve actually doubled the yields,” says Bob Beede, a University of California Cooperative Extension farm advisor for Kings and Tulare counties.

But Beede is quick to point out that to be successful, growers should not cut corners. He recommends applying the at 5 percent to30 percent bloom, using one pouch in 100 to 200 gallons of water per acre and a sprayer speed of 1.75 to 2 mph.

What is PFA?
PFA, also known as Serr drop, occurs when walnut flowers get too many pollen grains on the sticky surface of the pistil. During the process, excessive ethylene is produced, causing flowers to abort and fall from the tree about 10 days after bloom. This is different from the typical nut drop, which occurs four to six weeks after bloom.

Although Serrsare the most severely affected, other walnut varieties—such as Chandler—are affected to a lesser extent. Under the worst conditions, Serr yields may drop to 500 pounds or less per acre – not even enough to pay for production costs.

ReTain, a plant growth regulator manufactured by Walnut Creek, Calif.-based Valent USA, helps control PFA by reducing ethylene production in the pistillate flowers after pollination. The California Department of Pesticide Regulation registered it for use on walnuts in February.

Research results stir excitement
During the past three years, Beede and his colleague, San Joaquin County farm advisor Joe Grant, conducted small-scale trials looking at ReTain’s effects on Serrs.

Although the trials involved only a limited number of trees, Beede says he’s confident of the results because of the scientific procedures they followed.

All of the trial data showed that ReTain, when applied at the proper time and concentration, significantly increased yields, Beede says. And no results showed a carryover effect of ReTain depressing yields the following year.

In one 2004 whole-tree study, for example, Beede found that ReTain-treated trees had 84.7 percent flower set compared to untreated trees with only 52.9 percent set. Trees sprayed with water had 48.9 percent flower set.


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