Duane Greene’s key contribution to fruit thinning was the discovering that apples that will drop can be identified very early, helping growers know what to do early in the thinning window. (Courtesy Dr. Duane Greene)
Duane Greene’s key contribution to fruit thinning was the discovering that apples that will drop can be identified very early, helping growers know what to do early in the thinning window. (Courtesy Dr. Duane Greene)

Over the last few years, apple growers should have gained confidence that they can chemically thin their apples and achieve pretty good results.

The gains have come through increased understanding of tree fruit physiology—how trees shed excess fruit and why. There have been few breakthroughs in new chemistry (although some are on the horizon).

The plant growth regulators now used to thin apples were discovered more than 40 years ago, and the newest, BA (benzyladenine), was introduced as a thinner in the 1990s.

There have been several key players in developing an understanding of how to ­effectively use ­thinners. One is Dr. Duane Greene at the ­University of Massachusetts.

Greene was one of a half-dozen tree fruit physiologists who spoke during the Cornell University In-Depth Fruit School early this spring.

The school’s intent was to recognize the contributions of those who have devoted their careers to ­discovering why fruit trees act the way they do.

Greene was selected to speak based on his long career studying plant growth regulators. Over the years, he has appeared frequently in the pages of Good Fruit Grower, most recently for his development of a way to determine how well thinners already applied have worked so that additional thinning can be done if needed—without ­danger of overthinning.

Quite simply, Greene’s “fruitlet model” involves accurately measuring the growth of selected fruit.

Greene found that apples growing at half the rate, or less, of fruit that persist to harvest will fall off, and that the growth rate of apples that will fall off begins to slow down as soon as three or four days after a thinner has been applied.

He measures the diameter of selected fruits three or four days after thinners are applied, and again three or four days later.

Greene collaborated with other researchers, Drs. ­Terence Robinson and Alan Lakso at Cornell ­University and Phil Schwallier at Michigan State University, in developing the fruitlet model.

They were working together to develop a carbohydrate model for predicting when apples could be thinned most effectively.

That model shows that when apple trees are stressed by weather conditions that result in the tree producing less carbohydrate than growing fruit demands, some of that fruit will fall off the tree.

Adding more stress, like a chemical thinner, increases thinning. Weather monitoring allows prediction of periods when fruit is most susceptible to thinning, so that timing and rate of chemical application can be determined.

Thinning windows

As Greene explained during the fruit school, growers have several “windows” during which to thin apples.

The first opportunity is during bloom, when caustic materials like lime sulfur and fish oil will damage flowers, reduce pollination, and lower fruit set.

The second is from petal fall to when fruits are 6 millimeters (mm) in diameter.

Then comes the “ideal” window when fruits are 7 to 15 mm in size. A third window is when fruits are 18 to 25 mm.

When fruits are larger than 25 mm, thinning is very difficult. Hand thinning, and the labor bill that goes with it, are the final determiners of how effective chemical thinning was.

“Chemical thinning is a process that by its very nature provides different opportunities,” Greene said. “We must understand the underlying fundamentals of each to take full advantage of fruit vulnerability at different periods.”

Carbohydrate demand

Chemical thinning can be done during  about 28 days after petal fall, during which time fruits grow to about 25 mm in diameter, or just under one inch.

When fruit is small, from petal fall to 6 mm, some thinning is possible, he said. But since the fruit is small and growth is relatively slow, carbohydrate demand is not great. Fruit is more difficult to thin unless fruit is stressed. Sevin (carbaryl) and NAA (naphthaleneacetic acid) are thinners of choice, although NAD (naphthaleneacetamide) is useful on some varieties at this time.

Trees might become extremely stressed if temperatures are high, increasing carbohydrate demand, but light conditions are poor, resulting in lower photosynthesis. Excessive thinning can occur when nights are warm and days are cloudy. The carbohydrate model monitors these conditions and rates the potential for carbohydrate deficiency.

“Chemical thinning is most successful when fruits are from 7 to 14 mm,” Greene said. “Fruit growth is proceeding rapidly. At high temperatures and/or low light levels, excessive thinning is possible.”

Growers may need to lower chemical rates to prevent overthinning. Chemicals used at this time include NAA, carbaryl, and BA, usually in combination. Thinning effects are additive, he said.

Greene said of the 18 to 25 mm size, “This is the period when it is very difficult to thin, even when there is a carbohydrate deficit. We have very few thinners that work well and consistently at this fruit size.”

Fruit is physiologically different at this size, he said. Starch accumulation is going up, and ethylene production is going down. Because it is so hard to thin at this fruit size, Greene has concentrated his attention there, hoping to give growers a workable alternative to hand thinning.

Greene’s experiments showed that larger fruits are harder to thin because they have more carbohydrate reserves and are less vulnerable to stresses and because the seeds in the apples produce auxins that prevent abscission.

So, he believes, fruit thinning may be possible if auxin movement from the seeds to the stem, where abscission occurs, can be restricted. Alternatively, ethylene increases auxin destruction. Ethephon has been used as a rescue thinner, mixed with carbaryl, because it will knock larger apples off trees, but it carries some risk and uncertainty, he said.

Greene is working with some newer chemicals like metamitron, a herbicide that inhibits photosynthesis, 1-aminocyclopropane-1-carboxylic acid (ACC), which produces ethylene, and abscisic acid (ABA), which causes abscission. All of these potentially could make chemical thinning effective on larger apples.

Fruit growth model

Greene is best known for his work on the fruit growth model.

“To thin effectively, more than one thinner application is generally required,” he said. “We have had no way to tell if or how well thinners worked until after the thinning ­window of opportunity has passed. This model was developed to serve as a tool to assess the effects of previous thinners, usually within seven days of application.”

The fruitlet model is based on two observations, he said.

“The first observation is that fruit that persist will start to grow rapidly a few days after fertilization, and their growth will continue somewhat regularly and without interruption throughout the season.”

The second observation is that fruit destined to fall off would grow slower well in advance of the time they actually fall off.

“Under most circumstances, measuring the reduction in fruit growth between four and seven days after thinner application has proved sufficient to determine if a fruit will continue to grow or to abscise. All fruit that slow to a growth rate of 50 percent or less of the growth rate of the fruit that persist to harvest will ultimately stop growth and abscise.”

The measuring procedure was described in an article in the April 15, 2010, issue of Good Fruit Grower. Here is a brief summary:

• Select 10 to 20 spurs per tree on 5 to 10 trees (50 to 80 spurs). Mark and identify individual fruits on each tagged spur. Greene uses a permanent marker to write a number on each fruitlet.

• Using a caliper (digital readout is handy), measure each fruitlet starting no earlier than when it reaches 6 to 7 millimeters diameter and record each fruit’s size.

• After thinner application, measure fruit. As few as two measurements may be enough—one starting four days after application and another three to four days later.

• Predict which fruitlets will drop off—those failing to grow at least half as fast as the fastest growing. •