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Apple Fruit Thinning

Written by: 
Duane Greene, University of Massachusetts Amherst
Revised by Renae Moran, Univ. of Maine

Fruit thinning is an essential practice for producing commercial quality fruit and for getting consistent yield from year to year.  It increases the overall value of fruit because the reduction in crop load increases the size of fruit that remain.  For some varieties, there is an improvement in red color, as well. 

 Apple trees are prone to biennial bearing or the condition in which trees produce abundantly one year and poorly the next.  Chemical thinning in one season will increase the amount of bloom in the following season (return or repeat bloom).  Hand thinning is not as effective as chemical thinning for promoting return bloom.

The chemicals and concentrations a grower chooses, the timing of their application, and the environmental factors encountered before, during, and after application all influence the ultimate thinning response. This section of the Guide will discuss the chemicals most frequently used, the circumstances when they are used and the precautions associated with their use. Also discussed will be the timing of the applications and the environmental factors one must be mindful of when applying chemical thinners.

Thinning Chemicals

Blossom thinners

12 mm is equivalent to ½ inch

Some of the first attempts to thin involve using caustic chemicals to prevent pollination, pollen germination or pollen tube growth. Many of these caustic chemicals can cause phytotoxicity to the leaves and russeting of the fruit. If poor pollination occurs during bloom, thinning should be postponed until early fruit set. 

Lime sulfur, lime sulfur plus oil or ammonium thoiosulfate are mild blossom thinners.  Although none of these have label approval for thinning they may be applied legally on apples for other reasons. 

While less frequently used, hormone sprays can thin when applied at bloom. A bloom spray of NAA and NAAm can reduce fruit set, but are generally not applied commercially during bloom because of grower uncertainty about the extent of initial set and the desire to assess initial set before attempting to adjust crop load. Further, the most effective time to apply NAA as a chemical thinner is when fruit diameter is 7 to 12 mm.

Ethephon may also thin when applied at bloom or even several days earlier, at the balloon stage or red stage. The response appears to be quite cultivar and temperature sensitive. The use of ethephon as a blossom thinner has not been widely adopted except in locations where chemical thinning with other compounds is difficult and satisfactory results using other thinners is generally inadequate.

Postbloom thinners

The majority of chemical thinning is done with postbloom thinners. There is a comfort level for growers to delay thinner application until they know the extent of bee activity and pollination.

Naphthaleneacetic acid (NAA). It is probably the most potent of the general-use thinners on the market today, and it is the preferred material for cultivars that are difficult to thin. Its thinning effectiveness is concentration dependent. Lower concentration may cause only modest thinning while higher concentrations may overthin and reduce fruit size or not increase fruit size even though the crop load is substantially reduced. Over application of NAA may also lead to pygmy fruit formation and severe leaf epinasty on some varieties. Therefore, NAA is often combined with another thinner, especially carbaryl, and used at lower and “safer” rates. The thinning action of NAA is sometimes not immediately apparent since fruit abscission following NAA application if often delayed by as much as one or two weeks relative to untreated trees.

Naphthaleneacetamide (NAAm, NAD). NAD is a useful thinner that is frequently applied as a petal-fall spray. It is considered safer to use than NAA and it does not cause as severe leaf epinasty following application that is often experienced with NAA. NAD should be avoided on 'Delicious' since it may result in a high percentage of pygmy fruit that persist to harvest. NAD can be combined with carbaryl in situations were more aggressive thinning is desired.

Carbaryl.  Carbaryl is the most versatile thinner in general use. It is a mild thinner, and since the thinning is not rate responsive, overthinning is rarely observed. It can be used effectively over a wide range of developmental stages from petal fall until fruit grow to 18 mm in diameter. It is very toxic to bees so practically speaking the earliest time of application is at petal fall after the bees have been removed from the orchard. The Sevin XLR Plus formulation may be less of a problem near bloom since the particle size is less like pollen thus it is less likely to be transported by bees back to the hive. One of the most important characteristics of carbaryl is that it can break up fruit clusters. Carbaryl is an insecticide rather than a hormone so it is applied at higher concentrations than other postbloom thinners. 

Benzyladenine (BA). BA is a mild thinner when used by itself, but when combined with carbaryl it is a potent thinner combination that can overthin. The combination of BA with NAA for use on 'Delicious' and 'Fuji' is not recommended since pygmy fruit may form in some circumstances. Unlike other thinners, BA can increase fruit size beyond that attributed to a reduction in crop load.

Protone® (S-Abscisic Acid, ABA). ABA is a naturally occurring compound that plays a critical role in regulation of several physiological processes in a plant, especially those related to stress. If a plant is stressed or if it is sprayed with ABA stomata close, resulting in a significant reduction in photosynthesis. This in turn results in a carbon deficit in the plant. In the case with pome fruit, this happens for a long enough period during the time when developing fruit are competing for photosynthate (7-15 mm) fruit abscission will be initiated. Special attention should be paid to the weather conditions that occur especially the three days following application. If the weather is cloudy and or the temperatures are warm to hot, thinning will be favored because these conditions will increase the carbon deficit within the tree. Protone® is OMRI organic certified.

  • Apples. Label recommendations for the use of Protone® on apples include using 1 to 2 applications from 5-12 mm fruit size. Starting out I would recommend using mid- concentration rates at the 10-12 mm stage.  A good nonionic surfactant is recommended for use with this product. In the past we used Regulaid with ABA with good success. This may be used with MaxCel (not NAA) for added thinning.  Protone® may cause some leaf yellowing and leaf abscission. The severity of this is  cultivar dependent. A small amount of 6-BA (MaxCel) may help reduce leaf yellowing and abscission.
  • Pears. Protone® is cleared for use on pears for thinning. Pears are more sensitive to ABA than are apples. Everything being equal, a greater thinning response may be expected when used on pears. Some leaf abscission and leaf yellowing when used on pear can be seen. Based upon experience with Bartlett pears, 6-BA (MaxCel) was unable to reverse this yellowing and leaf abscission effect when used in conjunction with Protone®.

Ethephon.  Ethephon can be used as a postbloom thinner in situations where other chemical thinners are less effective or have undesirable side effects. Ethephon has a reputation for being an erratic thinner. Part of this can be attributed to a dramatic increase in thinning response with increasing temperature following application. Also, bloom and fruit susceptibility to ethephon varies depending upon the stage of development. Ethepon is an effective blossom thinner but application made just 7 days later appears to be much less effective. Fruit redevelop thinning sensitivity to ethephon at diameters between 16 and 22 mm. Since larger fruit are sensitive to ethephon, unlike most other chemical thinners, ethephon may have a place in a normal chemical thinning program as a "last chance" thinner where other thinners do not work or where a grower made a miscalculation early in the season, and failed to apply a thinner at the normal time.   

Timing Thinner Application

The weather following thinner application is probably the single most important factor influencing thinner efficacy. The weather can not be regulated, and an accurate forecast of the weather may not extend beyond 2 or 3 days. Increasingly, growers in the eastern part of the United States make two or more thinner applications over the thinning period. This is a good strategy since it spreads thinning out over time, and increases the possibility that thinner application will coincide with favorable thinning weather. It also tends to be safer since less aggressive thinning treatments are generally used, and the chance of overthinning is reduced.

Before application. Cool, cloudy, wet periods preceding thinner application generally mean that thinning will be easier. Part of this is attributed to altered epicuticular wax and cuticle development on leaves which predisposes leaves to absorb more thinning chemical.  These conditions during and immediately after bloom may also lead to less vigorous fruit set, characterized by fruit that are not growing vigorously and have few seeds, increased seed abortion, and reduced carbohydrate reserves. Regardless of thinner absorption, these fruit will be easier to thin. Frost injury to spur leaves also will make fruit easier to thin. NAA penetration is greater into frost injured leaves and the markedly distorted spur leaves undoubtedly have a reduced photosynthetic capacity.

The two most important environmental factors that influence foliar penetration of a chemical thinner are temperature and drying time. Warm temperatures enhance uptake of NAA by apple leaves. The longer the drying time of a thinning spray the greater the penetration into the leaf. In foliar penetration studies it has been reported that the penetration of NAD increased steadily over time as long as the spray droplet was prevented from drying. During the drying process uptake into the leaf was accelerated, presumably due to the concentration effect caused by the drying. Once the droplet dried, little additional penetration occurred. Therefore, the longer the time before droplet drying, the greater chemical uptake by foliage and fruit.

Temperature following thinner application is the dominant factor influencing the response to a chemical thinner.  Elevated temperatures provides the stress required for thinners to work. Warm temperatures intensify competition among competing sinks at a time when metabolic demand is highest in the tree. If cool weather follows thinner application, thinning results are frequently disappointing. It is often better to wait 2 or 3 days until warm temperatures are forecast to occur after application than to apply a thinner when cool conditions (< 65 °F) prevail immediately after application. However, temperatures above 85 °F can lead to excessive thinning. 

Several days of cloudy weather during the bloom period where incoming solar radiation is reduced to 10% to 15% of full sun can  can intensify fruit abscission.  Applying chemical thinning sprays at the beginning of a cloudy period probably will enhance thinning. Therefore, it may be advisable to delay thinner application under circumstances where trees may be exposed to several days of cloudy, warm weather to avoid overthinning. One or two days of sun following shading partially reversed the abscission-promoting effect of shading.

Orchardists generally try to apply thinners well in advance of rain, but occasionally this cannot be avoided.  A good rule-of-thumb is if a chemical thinner dries on the leaf prior to the onset of rain, one can anticipate getting at least 80% of the thinning effect.