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Apple fruit thinning

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Written by: 
Duane Greene, University of Massachusetts Amherst

Fruitlets at Cold Spring Orchard

The apple is a biennial bearing tree. It is characterized by heavy bloom in the 'on' year which generally leads to an overset of fruit. There are several negative consequence associated with overcropping. Many small fruit are produced, they ripen late, have poor appearance and quality, and are worth less when sold as fresh fruit. Further, flower bud formation is significantly reduced, if not totally inhibited, for the following year. Reduced cropping in the "off” year will lead to significant lower cropping, and inferior quality fruit that has a reduced postharvest storage life. In high density plantings, excessive vegetative growth may occur on lightly-cropped trees, making future management of the block difficult.

Over 50 years of experience has shown that chemical regulation of cropping is the only practical way to break the biennnial bearing cycle and reestablish more regular cropping. However, the challenges posed by chemical thinning are among the greatest obstacles fruit growers face in achieving profitable production. 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

Some of the first attempts to reduce biennial bearing involved using caustic chemicals to prevent pollination, pollen germination or pollen tube growth. Many of these caustic chemicals caused unacceptable phytotoxicity to the leaves and caused russeting of the fruit. The product sodium dinitro- ortho-cresolate (DNOC), sold commercially as Elgetol, ultimately became the material that was adopted as the blossom thinner of choice for general commercial use. DNOC was used extensively in the arid fruit growing areas in western North America, but its use never became a commercial practice in the East for several reasons. Erratic weather during the bloom period made it difficult to determine the extent of set until after the window of opportunity for application of Elgetol passed. More phytotoxicity was encountered in the East where drying times are longer due, in large part, to the higher humidity. If rain followed application, even greater phytotoxicity and excessive thinning were frequently experienced. Elgetol was removed by the manufacturer from the market in 1989 due to the high cost of reregistration. This resulted in renewed interest in identifying a safe and effective blossom thinner to replace Elgetol.

The search for replacements for Elgetol continues.   Many have been tested and several have been approved for use.  At this point lime sulfur, lime sulfur plus oil or ammonium thoiosulfate  seem to be the products of choice.  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.  It was shown in the 1940’s that a bloom spray of NAA and NAAm could reduce set. While this has been confirmed by later research, these two compounds 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 thinning that is done commercially for the past 50 years is with postbloom thinners. There is a comfort level for growers to delay thinner application until they know the weather conditions that occur during bloom and the extent of bee activity that occurred at that time.

Naphthaleneacetic acid (NAA). NAA was the first hormone-type thinner to be adopted and used commercially. 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. It is used at concentrations ranging from 3 to 20 ppm and its thinning effectiveness is linear with concentration. However, under most commercial thinning situations the concentration range used is 5 to 12 ppm. 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 varieteis. 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). Initial testing of NAD was done with NAA, but NAD was not the chemical of first choice because it is a less potent thinner than NAA. NAD has been applied at rates up to 100 ppm but the concentration range normally used is between 40 to 50 ppm due to label restrictions. 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 N AA. NAD is never used on 'Delicious' since it may result in a high percentage of pygmy fruit that persist to harvest. Like NAA, NAD is frequently combined with carbaryl in situations were more aggressive thinning is desired. In general, NAD is an underutilized chemical thinner and further research may show how this chemical can be more effectively used and integrated into a chemical thinning program.

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 appears to be most effective when applied to the fruit but under cloudy or shady conditions, carbaryl applied to the spur leaves may contribute to the thinning response. Carbaryl is an insecticide rather than a hormone so it is applied at higher concentrations than other postbloom thinners. Generally it is applied at dilute concentrations equivalent to 0.5 to 1.0 lb/100 gal.

Benzyladenine (BA). Benzyladenine is the newest thinner to be introduced. It was initially identified as a promising chemical thinner because it reduced crop load, increased fruit size, and enhanced return bloom . The effective thinning concentration is between 50 and 150 ppm. 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.

Ethephon.  Ethephon is generally not considered a mainstream postbloom thinner in most apple growing regions. However, it is recommended as a postbloom thinner in areas and 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, flowers and fruit susceptibility to ethephon varies depending upon the stage of development. Ethepon is an effective blossom thinner but application made just 7 d later appear 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.   

Time of Thinner Application

Chemical thinners may be applied over a relatively wide range of times, depending on the year, this may be from bloom until fruit reach a diameter of 25 mm.  The weather largely determines the length of the thinning window of opportunity.  If the weather following bloom remains relatively cool and sunny, fruit growth is slow, and developing fruit are place under little stress so thinning can be carried out over a long period of time.  Conversely, if the weather is warm to hot following bloom, fruit growth is rapid, fruit are stressed by competition with other fruit and growing shoots, and the time period when fruit can be thinned is shortened.

The weather following thinner application is probably the single most important factor influencing thinner efficacy and the extent of June drop. The weather can not be regulated, and an accurate forecast of the weather may not extend beyond 2 or 3 d. Increasingly, growers in the eastern part of the United States make two or more thinner applications over the thinning period. This has been done on the west coast for many years. 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.

Growth and development of fruit during the thinning period is dynamic and continuous. There are specific times during this developmental process when fruitlets may be especially vulnerable to chemicals with specific modes of action, while at other times all thinners may be effective. Each time period during the developmental process provides thinning opportunities, and strategies may be devised to thin at these times.

Bloom. Blossom thinning offers several advantages over thinning at other times. Early flower/fruit removal usually results in the largest fruit at harvest and the greatest return bloom the following year.  Efficacy of blossom thinners is less influenced by the weather than hormone-type thinners, and to be effective, it may not be necessary to have specific physiological conditions exist within the fruit.

The strategy used in applying caustic thinners at bloom is to allow pollination of the king flower, and then apply a blossom thinner to prevent pollination or to inhibit pollen tube growth in the style of side flowers. Blossom thinners are often applied at 70% to 80% bloom . A delay in application until full bloom or later may have two consequences. First, thinners are less effective when applied at this time. Second, the likelihood of russet or fruit marking at harvest is increased. In some areas chilling is less intense, and flowers open over an extended period of time. In these situations, two applications of a blossom thinner may be more effective than one.

Petal Fall to 5 mm. On anyone cultivar petal fall is rather easily defined event associated with the dropping of the petals. However, from a chemical thinning standpoint petal fall is much more nebulous time because petal fall in an orchard may occur over several days because of microclimate and differing bloom times of cultivars usually found in an orchard. From a practical and chemical thinning standpoint, petal fall should be considered the time when the bees are removed from the orchard.

Thinners have been successfully applied at petal-fall for many years. There are many reasons why this may be a preferred time to apply chemical thinners. This is the first time a grower can apply a thinner with some knowledge about the bloom period, and thus insight for the potential crop for the year. Early applications of thinners generally mean larger fruit at harvest and greater return bloom . This is especially true for early maturing cultivars, and those that have a strong biennial bearing tendency. In areas where blossom thinning is not routinely used, this is generally considered the earliest opportunity growers have to chemically thin

Several chemicals can be used at petal-fall. Carbaryl can thin over a relatively long period. It is especially useful as a petal fall spray. Petal fall application of carbaryl is considered the corner- stone treatment in the "nibble" approach to chemical thinning recommended in many places in the United States. Petal fall sprays of NAA are effective but at effective concentrations they may be more erratic than carbaryl. When applied at petal fall NAA may cause severe leaf epinasty on susceptible cultivars. For this reason, NAD is sometimes used at petal fall instead of NAA. Where aggressive thinning seems appropriate, carbaryl with NAA has been successful. In general, BA when used as a stand-along thinner has not been especially effective when applied at petal fall.

Seven to 14 mm. Historically, this is the time when most chemical thinners have been applied. This usually occurs 7 to 21 d after bloom. It is the time when most chemical thinners are effective and the developmental stage where fruit are most vulnerable to thinners.  During this period of time, shoots and fruit are rapidly growing and there is active competition among these for the available carbohydrate. Demand for carbohydrate may exceed supply, especially on warm days when respirational losses are high and photosynthesis is occurring at less than an optimal rate due to cloudy conditions.

Therefore, peak thinning activity of the major chemical thinners used at this time (NAA, carbaryl, and BA) is consistent with the suggestion that they exert their thinning response by reducing available carbohydrate thus intensifying competition among the competing sinks including developing fruit.

It is generally accepted that the fruit size increase realized by the application of chemical thinners is due primarily to a reduction in crop load. BA like other thinners increases fruit size in this manner. However, BA is a cytokinin, and it does increase cell division and it is especially effective when applied during this time.  It has been shown that BA increased fruit size by increasing the number of cortical cells, and the optimum time of application coincided with a very active period of cell division in apples.

Fifteen to 18 mm. There is general agreement that this is a marginal time to apply a chemical thinner. Thinners diminish in effectiveness as fruit size increases to above 15 mm stage. Some June drop has occurred and growth of other fruit destined to drop has slowed or stopped, indicating that subsequent fruit drop has been at least partially predetermined. Successful fruit thinning at this stage of fruit development with NAA, BA, and possibly carbaryl, is generally restricted to years where the weather has been sunny and cool, and fruit to this point have not been subjected to stress. However, It has been reported that application at this time with ethephon or ethephon with carbaryl may result in good and successful thinning. NAA if applied at this time may result in a reduction in fruit size. This situation can be further exacerbated if NAA is applied at relatively high concentrations in warm temperatures.

Twenty millimeters and larger. When fruit reach this size, only ethephon or ethephon with carbaryl will consistently thin apples. Experience in New England at this stage of fruit development indicates that concentrations between 250 and 500 mg.L-1 may or may not thin, but there is the possibility on some early maturing cultivars of advancing ripening and advancing preharvest drop might occur.  Cloudy warm weather during this period of fruit development may make trees more susceptible to a thinning treatments containing ethephon and carbaryl.

Environmental Factors

Environmental influences on chemical thinning are the most difficult to document since we have no control over the weather. Control of all environmental factors under field conditions is challenging and frequently not possible. Alternatively, numerous observations must be made under widely differing conditions in the field, and this is a very time-consuming and haphazardous process.

Before application. Even in the infancy of chemical thinning researchers and orchardists recognized that weather conditions prior to spray application influenced the thinning response. 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 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.

Environmental conditions at the time of application. 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 the penetration.

Weather conditions following application. 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 d until warm temperatures are forecast to occur after application than to apply a thinner when cool conditions prevail immediately after application.

It is not uncommon to have several days of cloudy weather during the bloom period where incoming solar radiation is reduced to 10% to 15% of full sun.  This lack of sun 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.