In more than 200 chemical thinning trials that the Washington Tree Fruit Research Commission has conducted on apples over the past eight years, Crocker’s Fish Oil and lime sulfur has outperformed almost every other material in terms of reducing the crop load, increasing fruit size, and promoting return bloom.

Tory Schmidt, horticultural associate, said the commission has tested a wide range of products on about ten different apple cultivars in 80 orchards in Washington State and has a broad base of information from which to draw some conclusions. However, he said results have been very variable. Some trials were great successes and others failures. What ­happens in one orchard may not happen in another orchard, and what happens in an orchard one year may not happen the following year.

The commission has tested several prebloom materials, including vegetable oil emulsion.

It has also tested about 40 materials for bloom thinning (including herbicides, fertilizers, fungicides, and oils), and 20 carriers or surfactants (including various oils and fish emulsions).

More recently, it has been testing postbloom thinners, which are plant hormones or hormone mimics that are applied between petal fall and when the fruit measures 15 millimeters in diameter.

Thinning goals

Schmidt said a successful thinning program should minimize production costs by reducing the crop load and the need for hand thinning. The fruit left behind should be large and high quality, and return bloom should
be promoted by maintaining a balance of fruit and ­vegetative growth in the tree.

Schmidt analyzed the data from all the trials to find out how often results were statistically different from the untreated controls.

ATS (ammonium thiosulfate), a fertilizer product that many growers apply during bloom, reduced the crop load in only 27 percent of the trials where it was tested. It increased fruit size 17 percent of the time, and enhanced return bloom in only 7 percent of the trials.

In comparison, lime sulfur with Crocker’s Fish Oil reduced the crop load in 60 percent of the trials where it was tested, increased fruit size in 28 percent of the trials, and promoted return bloom in 24 percent of the tests.

"Crocker’s Fish Oil and lime sulfur tends to outperform almost everything else we look at," Schmidt said.

The commission has tested lime sulfur with different oils, but fish oil gives slightly better results, possibly because the trees tolerate higher rates, he said.

Postbloom

The commission has also tested a range of postbloom thinners, which are mostly plant growth regulators, though they include some essential plant oils developed by Dr. Curt Rom at the University of Arkansas.

Overall, trees treated with a combination of Sevin (carbaryl) and Maxcel (benzyladenine or BA) reduced the crop load in 35 percent of the trials that the commission has conducted, compared with only 11 percent for BA alone. Carbaryl with NAA (napthaleneacetic acid) was effective just over 20 ­percent of the time.

Schmidt said one of the priorities for the future is to look at postbloom thinning programs without the use of carbaryl. Carbaryl is under review by the U.S. Environmental Protection Agency, and Schmidt said although no major label changes are expected for the time being, some export markets do not look favorably on the ­product.

A combination of NAA with BA has been as good as carbaryl in tests so far.

Fruit size

BA is a cytokinin that is supposed to increase fruit size by promoting cell division. In order to try to quantify that, the commission treated Gala apples with BA at four sites in Washington and shipped the fruit to the laboratory of Dr. Peter Hirst at Purdue University, Indiana, where the fruit cells were counted and measured. No significant difference was found in the number of cells in fruit on treated or untreated trees. "We were not able to establish that BA was increasing cell division," Schmidt said.

When cell size was compared in the four trials, cells were larger in BA-treated fruit in one of the trials, smaller in another, and in two trials, the cells were the same size as in untreated fruit. However, in the commission’s trials overall, BA did result in larger fruit, he said.

Early thinning

Schmidt said the commission has learned that consistent improvements in fruit size and return bloom require early, aggressive thinning. Trees that are not thinned during bloom carry much more fruit for one or two weeks longer. As a result, resources and nutrients are pumped into fruit that will eventually be shed, and the remaining fruit is smaller at harvest. There’s also a great improvement in return bloom on trees that receive both bloom and postbloom thinning compared with those that just received a postbloom treatment, he said.

The commission has also run trials to find out if the time of day when the thinning spray is applied makes any difference to the results. Sprays were applied in the ­morning, at noon, or in the evening.

"We don’t have a consistent story there," Schmidt said. "That’s kind of disappointing."

Alternate bearing

The commission also tested summer applications of NAA or Ethrel to promote return bloom, but did not have as much success with NAA as researchers in the eastern United States have had, Schmidt reported.

It is also testing applications of gibberellic acid (GA) during light-crop years to moderate the crop the following year when the trees would normally have a snowball bloom. "We’re trying to attack the problem by inhibiting return bloom with a GA product," he explained.

Applications were made this year, and results will be evaluated next spring. Fine Agrochemicals, Ltd., of the United Kingdom makes GA products and has expressed interest in perhaps registering a GA product to inhibit return bloom on apple, Schmidt said. This would be approved for organic production, whereas NAA and Ethrel are not.