Scientists try to boost consistency of apple thinning with prediction model and chemical combinations

By Renee Stern

Contributing Editor

Thinning sprays are the most cost-effective tool now available to manage apple crop loads and gain desired fruit size.

New chemical formulations, such as 6-benzyladenine alone or mixed with carbaryl, and renewed interest in older tools, such as a lime sulfur and fish oil combination, give growers more opportunities to thin their crop without resorting to expensive hand labor.

"We've never had a better and broader range of materials," says Jim McFerson, manager of the Washington state Tree Fruit Research Commission in Wenatchee.

But greater consistency and predictability remains a goal.

"Many time growers put them on and wait and pray," says Duane Greene, professor of pomology at the University of Massachusetts in Amherst.

A second chance

Greene is part of a research team developing a reliable predictive model that would give growers a second chance at thinning if they didn't achieve initial results don't match expectations.

The model, which focuses on growth rates in sample fruit, could be ready for release within a year.

Growers would measure fruit size in a sample of 75 to 100 spurs when they apply a thinning spray, then measure the same fruit three days later and again after another week.Because growth slows before fruit drops from the tree, the comparison, when run through the model, pinpoints how well the thinner worked, Greene says.

But before using the model, growers must have a goal in mind: the desired amount of fruit to remove.

Except for Sevin (carbaryl), "All these materials are rate-dependent to get a response," says Phil Schwallier, district horticulture agent at Michigan State University's Clarksville Horticultural Experiment Station."So the challenge is to determine how much you want to thin and then figure out how much to apply."

Adding to the difficulty are environmental factors that can blur how much of the results stem from thinner sprays.Greene and Steve McArtney, associate professor at North Carolina State University in Fletcher, suggest leaving some unsprayed trees as a control.

"The amount of natural drop differs each year," McArtney says.These check trees help gauge a thinning program's effectiveness and whether additional sprays are needed.

"The biggest problem with chemical thinning is predictability," says Terence Robinson, a Cornell University horticulture professor in Geneva, N.Y."There's a huge cost to over-thinning, and a big cost to under-thinning in fruit size and return bloom."

But hand-thinning can correct a too-light drop from thinning sprays, Robinson says.

"We rarely see over-thinning," he says."More often we see under-thinning, so we encourage growers to be aggressive."

Two are better than one

The 6-benzyladenine thinners, such as MaxCel and Exilis, tend to increase cell division, boosting fruit size beyond the typical response seen from other thinners, Schwallier says.

Another thinner, naphthaleneacetic acid, provokes harsher reactions, decreasing fruit size for a short period before rebounding, he says.And some varieties, including Fuji and red delicious, are sensitive to NAA; the thinner can produce pygmy fruit, especially in red delicious.

Combining either thinner with Sevin produces a more aggressive response.Thinners also work best on 8- to 12-millimeter fruit, after which results begin to diminish, Schwallier says.

McArtney says growers appreciate the more predictable response they see when using MaxCel and Sevin together, as well as the longer application range that permits more flexibility with weather.The combination is effective on 5- to 15-millimeter fruit and creates a 12- to 14-day window.

And while Fujis otherwise pose thinning difficulties, he says the variety responds well to the MaxCel-Sevin combination—with an added bonus of tremendous size gains.

How does Sevin work?

Carbaryl promotes leaf absorption for greater and more consistent thinning, Robinson says.Concerns over carbamate use have spurred a hunt for a Sevin substitute; NAA can substitute, but not with all varieties, he says.

Because carbaryl isn't completely water-soluble, some remains on leaf surfaces after application.Subsequent rains or even heavy dew may lead to additional absorption and thinning.

NAA, however, is sensitive to ultraviolet radiation, which degrades any leftover material.

Sunlight is another part of the equation.Intense cloudy periods immediately after spraying tend to increase fruit drop.

"It's hard to get a handle on," Robinson says."It's hard to measure (the amount of light) by the human eye the way a tree's photosynthesis mechanism can."

Chemical thinners are also temperature-sensitive, working best at temperatures of at least 70 degrees Fahrenheit and preferably in the 80s, Schwallier says.

Western growers have embraced bloom thinners, a risky practice in other areas where cold, wet springs are the norm.

"Bloom-thinning is a requirement here," McFerson says.

An organic alternative

Lime sulfur combined with fish oil is an organic bloom thinner being adopted by a broader range of growers, he says. But it may not suit every thinning program, and too much could disrupt other parts of an integrated pest management program.

Although North Carolina growers are reluctant to use bloom thinners, McArtney is researching other oil emulsions, with promising results from corn and soybean oils.