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Research trials comparing various protective films for reducing sunburn in apples show variable results, which is typical of what growers have been finding, says Tom Auvil, research horticulturist with the Washington Tree Fruit Research Commission.

In trials last summer, the commission compared four products: Surround (kaolin clay) from Engelhard Corporation; Raynox (a wax-based product) developed at Washington State University and sold by Pace International, LLC; Eclipse, a calcium and boron fertilizer sold by D&M Chem; and Fruit Shield, an experimental product made of black particles from G.S. Long.

The first application was made on June 29, and the second a week later, to create a good initial cover. The products were then reapplied every seven to ten days until August 29.

Surround was used at 50 pounds per acre for the first application, and 25 pounds per acre for subsequent applications. Raynox was applied at the recommended rate of 2.5 gallons per acre. Eclipse was applied at 3 gallons per acre with a surfactant, and Fruit Shield was applied at 10 pounds per acre. Auvil said Fruit Shield goes into solution very easily so the rates used are fairly low.

Doug Anyan at G.S. Long said Fruit Shield is manufactured in California and the manufacturer has not disclosed the ingredients, though they are food-grade products. G.S. Long has not yet decided whether to commercialize it.

Trial results

When fruit was evaluated in the trials, the various degrees of sunburn were rated as yellow 1 (the least severe), yellow 2, yellow 3, tan, or black (the most severe).

In Fuji trees planted on Malling 26 rootstocks at Azwell Orchard in north central Washington, there was very little sunburn even in the control, and no statistical differences between the treatments. When sun-exposed fruit on the same trees were evaluated, there were no statistical differences between the amount of clean or slightly sunburned fruit, but in the Raynox treatment, there were fewer classified as yellow 3. There was more tan sunburn in the Surround treatment than in the control.

In Fuji apples on M.9 at the Lucky Bohemian Orchard in Quincy, there was only 7 percent sunburn overall in the untreated fruit and no statistical differences in the amount or severity of sunburn in any of the treatments. But in sun-exposed fruit on the same trees, all treatments increased the amount of clean fruit. Raynox fared best, with 86 percent clean fruit, up from 73 percent in the control.

In a Golden Delicious block at the KTW Ranch in Othello, all the films tested reduced sunburn, but there was still more than 60 percent sunburn in the best treatments. Auvil said the grower had tied up the tree limbs with string before harvest, exposing fruit during the August heat. Fruit Shield worked best, with 36 percent of treated fruit being free of sunburn, compared with only 15 percent clean fruit in the control. All films cut the amount of yellow sunburn. However, the proportion of fruit with tan sunburn increased with all film treatments except Raynox, going from 1 percent in the control and Raynox treatments up to 8 percent with Surround.

Severe conditions

“One of the fascinating things about sunburn suppression is that under very severe conditions where the trees have a heavy crop load, little leaf area, and a lot of heat exposure, none of the products seemed to do a really acceptable job of suppressing sunburn,” Auvil remarked.

However, the films can be a good tool, he stressed, and some producers have been able to make them work. Packers have analyzed packout figures and evaluated the revenue per bin or per acre, and have shown that the products pay their way. But Auvil said he’s not sure that the industry as a whole has found the best techniques for creating a protective film and maintaining the residue through the season.

With all the products, coverage and timing are very important, he said, and the commission has been discussing with the manufacturers of the products how much material and how many applications are needed to keep a protective film on the fruit. Growers tend to want to get more with less, he noted.

The commission also looked at how easily the particle films could be removed. Auvil said some growers are using particle films early in the season, but then switching to the wax-based Raynox after they start overhead cooling, because the water tends to wash off the other products. There’s been some concern that applying Raynox on top of the particle products might seal the residues so they can’t be removed.

But Auvil said the research showed that wasn’t the case. Fruit Shield, which is difficult to see in any case, was particularly easy to wash off. The white products (Surround and Eclipse) left persistent residues in the stem bowls. After the fruit came off the brush bed and the stem bowls and calyx area were wet, they looked clean, but as they dried, the residues became very visible again.

Hard water used in cooling or irrigation can also leave mineral residues on the fruit, independently of any particle films applied, he noted. A residue starts to build up from the time water is applied, so while it sometimes looks as if sunburn products are creating an impervious film that can’t be removed, part of that can be due to the water quality and natural wax. Although some products can make the residue worse, in some situations, by the end of the year, control fruit can be as difficult to clean as treated fruit.