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

By Vicky Boyd


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.

The data also showed similar results between orchards in the Tulare County area and orchards farther north in San Joaquin County.

In one of Grant’s 2005 whole-tree studies, he found that ReTain-treated trees had 82.6 percent flower set, whereas untreated trees had only 30.8 percent flower set. The additional flower set translated into a per-acre yield of 4,816 pounds for the treated trees and 2,972 pounds for the untreated trees.

Despite his results, Grant says he doesn’t want Serr growers to rush out and apply the product until they determine what is causing their low yields.

Grant says growers should ask themselves, “Is it PFA? Is it Phytophthora or nematodes? Before they make this big expensive application, they need to have some level of confidence that the lack of production is due to PFA. It’s usually the case, but this is just an extra step in the process.”

Although Beede says he doesn’t know how much Valent or agricultural chemical dealers will charge, he thinks the PGR will provide growers a strong return on investment.

“If these guys get a 1,000- to 1,200-pound increase in walnuts at 80 cents per pound, that’s an $800 to $1,000 improvement in income,” Beede says. “Although the product won’t be cheap, it will be worth it if they do it right.”

Timing and spray coverage is critical

To obtain results similar to theirs, Beede says you’ll need to correctly assess the bloom stage.

“If there’s anything that is the single biggest challenge in applying this product, it’s having farmers make accurate estimates of what percentage of bloom is actually present in their orchard,” Beede says. ”This product will require walnut orchardists to put on their boots and get out in the field and personally observe what’s going on relative to the bloom.”

This is complicated because the tops of some trees may be at 100 percent bloom while branches in the shade or weak spurs on the same trees haven’t begun to bloom yet.

Although Beede says growers should target 30 percent bloom, three years of field trials show there is some leeway.

Chilling hours also will affect bloom, Beede says. If trees receive less than the optimum 800 to 850 hours of chilling temperatures, blooms tend to be more strung out, making it more difficult to determine proper spray time.

During 2005, Beede says trial applications made at 20 percent to 30 percent bloom and applications made at 60 percent to 70 percent bloom yielded similar results. But 2005 also had an unusually cool, wet spring that prolonged Serr bloom.

Equally critical for best results are application rates and coverage. This season, Beede and Grant recommend one 11.7-ounce water-soluble pouch per 100 to 200 gallons of water per acre. That equates to 125 parts per million of the active ingredient.

Valent does not recommend adding an adjuvant or tankmixing ReTain with other products, such as copper for walnut blight, says Tom DeWitt, Valent field market development manager in Fresno, Calif. That may change after company researchers conduct trials this season examining the PGR’s compatibility with other chemicals.

To ensure adequate spray coverage, tractors pulling spray rigs should drive between 1.75 and 2 mph.

“All bets are off if you’re applying this product faster than 2 mph,” Beede says. “If you go slower [than 2 mph], you’ll get even better coverage and even better results. This is about this product hitting the flowers.

“It won’t pole vault in, it won’t be carried in by ants and it won’t translocate, so it has to be on target,” he says.

Beede also recommends against spraying in the wind or rain because the product has to hit the flowers.

With no data on aerial applications, Beede says neither he nor Grant is comfortable advising ReTain’s use by air. They plan this season to conduct trials involving aerial application.

In addition, Valent researchers plan to conduct trials with Beede and Grant on Chandlers, since that variety also suffers from PFA,albeit to a much lesser extent than Serrs, DeWitt says.

Changing other cultural practices

Because ReTain should significantly increase yields, DeWitt says growers may need to prune trees to avoid limb breakage caused by a heavier nut load. They also may need to modify irrigation and fertilizer because of the larger crop.

To find out how the PGR will perform under your cultural practices, the farm advisors suggest experimenting with it on a small block of Serrs with the worst PFA problems – perhaps an orchard abutting one with Chicos, Vinas or Chandlers that shed an overabundance of pollen.