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There’s more to chemical thinning agents than just their value in adjusting crop load to grow bigger fruit. Thinners are becoming an important tool used to encourage return bloom on bearing trees, especially for growers in eastern United States, says Dr. Duane Greene, University of Massachusetts horticulturist.

Greene, who has studied crop load and fruit set for years, was part of a special fruit thinning and return bloom symposium held during the Great Lakes Fruit, Vegetable, and Farm Market Expo in Grand Rapids, Michigan. During the one-day seminar, he shared apple-thinning strategies and ways to enhance return bloom under eastern United States conditions.

Years of research have shown that cultivar, leaf area, light, crop load, and chemical thinning agents all influence return bloom. The earlier the crop is thinned, the better the return bloom the following year. Also, the more leaves per spur, the greater the return bloom. Trees receiving 60 to 100 percent sunlight produce more flowers, larger fruit, and better fruit set than those receiving less sunlight.

But of the many factors that influence flowering on bearing trees, Greene said that chemical thinners, with the exception of ethephon, are the most efficient and effective way to encourage return bloom on bearing trees.

However, using thinners effectively is not always easy. Timing and weather greatly affect the influence of thinners—it’s usually not a one-shot deal.

Greene believes in the "nibble approach" when it comes to a thinning program. "You take a little bit off here, a little bit there. The concept of getting perfect results with thinning sprays are rare," he said, advising growers to plan for multiple thinning applications.

Postbloom thinners

Generally, blossom thinners, such as ATS (ammonium thiosulfate), Wilthin, and lime sulfur, are not used in eastern apple production because of unpredictable weather. "Many of you growers are uncomfortable putting on thinners until you know if you have a crop to thin off," he said. Moreover, blossom thinners tend to be caustic and result in phytotoxicity problems if there is excessive dew or rain. He noted that blossom thinners are used more in arid regions, like the Pacific Northwest, where weather is more predictable and the humidity is lower.

The majority of growers in the East apply postbloom thinners, he said, noting that growers can make an assessment of what kind of pollination and fruit set their orchard had during bloom and adjust their thinning program accordingly.

Sevin (carbaryl) is a commonly used postbloom thinner, and is also used in combination with other chemical thinners like NAA (napthaleneacetic acid); BA (benzyladenine), sold under tradename MaxCel; and Ethrel (ethephon).

Nibble approach

In using the multiple thinning or nibble approach, Greene suggested that orchardists start their program with a petal fall application. "Don’t wait for perfect weather, but when the bees have been taken out of the orchard, and it’s not raining or too windy, make your first application. The philosophy is that you always have another chance, but you want to get off to a good start."

Growers should be able to see what type of fruit set they have about seven to eight days after petal fall as fruit goes into the 7- to 12-mm stage, the stage at which the young fruit are most vulnerable to chemical thinners.

As fruit approaches the 6- to 7-mm stage, growers should start to look for a three-day period of nice weather when they can apply the next thinner. "It’s the weather conditions following the thinning application that are important," he emphasized. Research has shown that response is seen from thinning applications when temperatures are in the mid-70s°F, but drastic change in abscission and fruit drop occurs when temperatures reach the mid-80s.

"The red flag goes up when temperatures are 84 to 85°F," he said. "There’s far less of a gamble when you put thinners on at temperatures in the low 70s."

Low light conditions after thinning applications can also mean a dramatic response to the thinner, he said. "If you have cloudy conditions followed by warm temperatures immediately after thinning application, that equals fruit drop and you could overthin. But if you have cloudy and cool temperatures, don’t worry."

Greene conducted research on McIntosh apples to measure thinning response to eight different shade treatments, using shade cloths over trees six days before and after thinning treatments of carbaryl and NAA. Applying a thinner that was followed by six days of cloudy weather nearly defruited the trees, he said, compared to six days of shade before the thinner application, which resulted in targeted fruit set levels.