Plant Growth Regulation 31: 1–16, 2000. 1 © 2000 Kluwer Academic Publishers. Printed in the Netherlands.

The history of fruit thinning

F.G. Dennis, Jr. Department of Horticulture, Michigan State University, E. Lansing, MI 48824-1325, USA

Key words: apple, application methods, caustic sprays, economics, flowering, growth regulators, Malus xdomestics Borkh., modeling, photosynthesis, review

Abstract The history of fruit thinning is reviewed, beginning with hand removal of fruits, the effects upon subsequent flowering, and current attempts to develop mechanical methods of fruit removal. Early experiments with caustic compounds and growth regulators and their subsequent development as commercial practices are discussed, as well as the modes of action of growth regulators. Brief reviews of methods of application, factors affecting response, modeling to improve efficiency, and the economic value of thinning to the grower are also included.

1. Introduction Thinning was recommended by several early Englishhorticulturists.Cotton[42]wrote,“...tohave Fruit thinning has been practised for thousands of [pears] very large and fair, you are again to take away years, and serves a number of purposes. Too many the greater number, and leave no more than one or fruits per tree can result in small fruit size and poor two at most” (p. 70). Excessive fruiting “is exceed- quality, breakage of limbs, exhaustion of tree reserves, ingly hurtful to Espaliers, ...the Trees will neither and reduced cold hardiness. In some species/cultivars, be long preserved, nor fair and large fruit gathered heavy fruiting can also partially or completely inhibit from them, if care be not taken to take away a great flower bud initiation. Many review articles have been number so soon as they are knotted, and before they written on thinning, or on flower/fruit development, have taken much nourishment from the Tree” (p. 72). where the effects of thinning are discussed [10, 39, 58, Similarly, Lawson [96] advised, “If your trees be over- 59, 123, 137, 144, 145, 146, 147, 150, 156] Given the laden ...I like better of pulling some off, (though wealth of information available on thinning of apple, I they be not ripe) ...than of propping by much, the will focus on that species, with emphasis on the North rest shall be better fed” (Book 3, p. 50). Langley American literature. [95] gave directions for thinning apricots, peaches, and pears, whereas “plumbs” are seldom thinned. He notes that little growth of stone fruits occurs from 2. Mechanical removal of fruit late May to late June, that considerable fruit drop occurs at this time, and that “all our late Authors 2.1 Hand thinning on Gardening take Notice, and therefore advise, that the thinning of Fruits be omitted, until it appears 2.1.1 Effects on size. Theophrastus [61], in discussing thatthisgreatFallisover...Hestates, “’tisevident, “Why Wild Trees Fail to Ripen their Fruit”, stated, that their Fall is caused by some Decay most prob- “...alongwithaheavyyieldgoesafailuretoconcoct able.” When this drop ceases, “...we should now it all, which is why some growers remove some thin them for good, taking away the least promising fruiting parts when there are too many of them” (Book ones, and preserving the best.” Langley advises that, I, p. 125). He also advised growers to thin flower “...sometimes a tree becomes weak by bearing too clusters of grapes (Book III, p. 120). plentifully.” He continues with detailed directions on 2 when to thin (no later than the month of May), which questioned whether the same effect on size could fruits to leave, and how far apart they should be. Hitt be obtained by removing a given proportion of the [80] also believed that “sometimes a tree becomes fruit, regardless of position on the tree. He therefore weak by bearing too plentifully,” and that “All fruits thinned major branches, leaving 1/6, 1/3, or 1/2 ought to be thinned according to their natural size” of the fruits, either by removing whole clusters or no later than the month of May; otherwise “they rob by leaving 1 or 2 fruits per cluster. Thinning fruits one another” and “never come to the size they would within clusters was most effective in increasing otherwisedo...” fruit size, indicating that fruit distribution, as well Numerous research bulletins dealing with thinning as total number of fruits, is crucial in determining of apple, peach, plum, and pear were available in the size. early decades of the 20th century (see [4] p. 429). Nevertheless, Gourley [67, 68], writing in 1922, stated 2.1.2 Effects on flowering. (See also [136].) Although that,“...manysuccessful commercial growers do not positive effects on annual bearing of apple were include it among their operations because its neces- claimed by Downing [54], most results were negative. sity has not been so apparent as that of pruning or For example, Beach [15] concluded from 4 years of spraying.” He notes, however, that “The western workthatthinning“...willnot...materiallyinfluence [U.S.] growers have been pioneers in this work from the regularity of production or the amount of fruit a commercial standpoint probably because their prac- setting for subsequent crops. The profit, if there be tice of packing fruit in small packages, notably the any, must come from the crop thinned.” Based upon 7 standard box, has made thinning not only necessary years of research with soil management and fertilizer but profitable.” He predicted that “thinning of fruit will applications, Gourley [67] observed that thinning did doubtless become still more general” once all fruit- not induce regular bearing in ‘Baldwin’ “...asreadily producing states had packing and grading laws. This as might be supposed.” The basis of his statement is prediction proved to be accurate. obscure, for he applied no thinning treatments. Several early papers provide data on the benefi- Considerable research was devoted to thinning cial effects of fruit thinning on size of apple. Walker by American horticulturists in the 1920s and 1930s. [141] commented on the considerable increase in the Auchter [2] published a report on apple thinning in value of the crop from thinning peaches, “...to say West Virginia, from which he concluded that flowering nothing of the great saving to the strength of the could not be affected by the practice. Shortly there- tree in reducing the number of pits borne,” Gourley after [3] he reported that thinning increased fruit size, [68] recommended thinning at about he time of June but did not affect annual bearing in ‘Rome Beauty’, drop because this would (1) improve fruit size more ‘York Imperial’, ‘Baldwin’, ‘Grimes Golden’, ‘Ben than would later thinning, (2) prevent the draining Davis’, ‘Northern Spy’, or ‘Delicious’. He noted of the energy of the tree by seeds and pits, and that spurs that set fruit generally did not flower the (3) avoid removal of fruits that fall of their own next year, whereas those that blossomed but did accord. Powell [113] reported that, although thinning not set fruit might or might not flower, and that ‘Burbank’ plums reduced the percentage of diseased blossom thinning would induce many spurs to form fruits, it reduced yield and had little effect on fruit flower buds. Heavy cropping reduced spur growth, size. Similar effects were obtained with ‘Poole’s and Auchter observed that more ‘Grimes Golden’ and Pride’ plum. However, thinning prevented damage to ‘Rome Beauty’ spurs, especially non-blooming ones, branches, which “were literally broken to pieces” on bloomed in the off-year if they grew more, and had nonthinned trees. greater leaf surface, in the on-year. He suggested Despite the advances made over the past 75 years, that fertilization and pruning might overcome biennial hand thinning remains an important tool for the bearing, but acknowledged that no data were available fruit grower. “Follow-up” thinning is often required to support this proposal. to break up clusters of fruit or better distribute the Crow [44] observed that blossom cluster removal cropload following chemical thinning. The earlier was effective in stimulating flowering only until the thinning, the larger the fruit, and Batjer et al. full bloom in the biennial apple cultivars ‘Wealthy’ [11] developed a system to predict the effect of and ‘Duchess of Oldenburg’; the percentage of thinning at any given time during fruit development spurs flowering following flower cluster removal the on final fruit size of apples. Knight [88], however, previous year dropped from 95% when clusters were 3 removed at full bloom to 0% when they were removed flowering occurred following commercial thinning to 5 days later. He stated that, “We suspect that no approximately 25 leaves per fruit 52 DAFB. treatment can be applied after that time [the critical Harley et al. [75] later published the results of a period] which will change the destiny of buds falling series of experiments conducted in Washington State. in categories 1, 2, 3 and 4 [i.e., terminal or bourse They adjusted leaf/fruit ratio by defruiting or defoli- buds on spurs 1–9 mm long] on either bearing or ation and demonstrated that the period during which non-bearing trees.” flowering could be influenced was shorter in a bien- Roberts [114] measured length and leaf area of nial (‘Yellow Newtown’) than in an annual bearing spurs on biennial vs. annual ‘Wealthy’ trees and noted (‘Jonathan’) cultivar. (Note, however, that ‘Jonathan’ that spurs grew more and produced more leaves in the limbs were scored, whereas ‘Yellow Newtown’ limbs “off” than in the “on” year. He classified spurs into were not.) Furthermore, Harley et al. [76], using four categories based upon terminal or bourse shoot similar techniques, showed that thinning caused large length, and observed that flowering was negligible limbs of ‘Yellow Newtown’ to flower out-of-phase in very short and very long spurs, with the optimum with the remainder of the tree. length being 3 to 12 mm. Blossom removal increased McCormick [107] appears to have been the first spur growth and leaf number. Roberts suggested that to demonstrate that the biennial bearing habit could “the percentage of spurs blossoming can be reduced be controlled by blossom thinning. He reported that to the point where it is possible to have regular annual bearing of mature trees of the biennial cultivars bearing, without seriously affecting the crop of fruit.” ‘Yellow Newtown’ and ‘Ortley’ could be promoted by He recommended application of nitrogen and small blossom and fruit thinning. Non-thinned controls aver- pruning cuts to stimulate spur growth, but provided no aged 86 kg per tree over 4 years, whereas thinned trees data on the effects of these treatments. averaged 159 to 174 kg per tree. McCormick provided Auchter and Schrader [7], following up on no information on tree numbers or differences between Roberts’ experiments, classified spurs into five cultivars, but Bobb and Blake [20] repeated this work. categories based upon flowering/fruiting performance. Blossom clusters on a mature, biennial, ‘Wealthy’ tree They reported that blossom removal stimulated were hand-thinned to 10–12 inches (25 to 30 cm) apart flowering of ‘Stayman Winesap’, ‘Rome Beauty’, and at early pink bud for four consecutive years. Not only ‘Grimes Golden’, but that the response declined as did this increase fruit size and promote annual crop- flower/fruit removal was delayed; little response was ping, but the total yield per tree during the 4-year study obtained at “June drop” or thereafter. was increased from about 745 (control) to 1565 kg. Magness et al. [103] determined the time when Despite many advances in knowledge concerning flowering of apple could be influenced by fruit thin- the process, flower initiation in apple remains difficult ning and ringing of branches in the Shenandoah- to predict, even when conditions (cropload, weather, Cumberland area of the U.S. Atlantic Coast. They etc.) appear favorable. The timing of thinning there- defruited to 100 leaves per fruit at 15-day inter- fore may be more important in some years than in vals from late May/early June until September; these others, but no study is known where this factor has branches were also ringed. Biennial cultivars did not been critically examined. respond after mid-June, whereas some annual cultivars (York Imperial, Delicious, Jonathan) showed some 2.2 Mechanical thinning response in early July. They concluded that early thin- ning was necessary, but that response varied with Other methods of removing fruit mechanically are cultivar. available, but are generally used only with stone fruits. Harley et al. [76] were among the first to quantify Club thinning can be used to thin peaches and apricots; response to fruit thinning in terms of leaf/fruit ratio. a section of rubber hose is attached to a wooden handle They removed ‘Yellow Newtown’ apple fruits at either and fruit are knocked from the tree during the pit- 37 (early) or 73 days (late) after full bloom (DAFB), hardening state, prior to the final swell. Because fruit leaving either 70 or 50 leaves per fruit. The percentage are hard at this time, those that remain to not show of spurs blossoming the next year following early thin- bruises at harvest. High pressure spray guns can also ning was 23% when 50 leaves were left per fruit and be used to knock fruit from the tree. Mechanical trunk 47% when 70 were left. Thinning at 73 days resulted or limb shakers are sometimes used with stone fruit. in only 3 to 5% return bloom, whereas essentially no Such methods are not recommended for apple because 4 they are easily bruised, and the damage is visible on that killed all blossoms injured the twigs and leaves. the mature fruit. Menzies [108] evaluated the effects Subsequently [6] they applied these and other treat- of thinning ‘Delicious’ apples with a tree shaker at ments on the same and additional cultivars, concluding intervals from 4 to 12 weeks after full bloom. Thin- that tar oil distillate was the most satisfactory. Again, ning 4 weeks prior to “June drop” had no effect on many of the treatments were toxic and/or caused fruit final numbers of fruit per tree, but increased fruit russeting. size; all other treatments reduced fruit numbers and The use of selective chemicals for thinning was improved size. However, final fruit weight ranged not actively pursued until 1940, when MacDaniels from only 66 (no treatment) to 96 g. Similar treat- and Hildebrand [101] reported the effects of dinitro- ments increased fruit size of ‘Williams’ (= ‘Bartlett’) o-cresylate (DNOC, or ElgetolR) in preventing pollen pear without affecting the number of fruit harvested. germination when applied to the stigmata of apple Unfortunately, no data were presented on fruit quality flowers, and Magness et al. [102] were able to reduce for either species. set of several cultivars with tar oil distillates and Rope thinners can be used for blossom thinning of DNOC, thereby reducing their alternate bearing ten- peaches. Heavy ropes are dragged over the trees as the dency. Magness et al. [102] cited another purpose frame holding the ropes is rotated above the tree. If for using such chemicals: complete prevention of too few blossoms are removed, the treatment can be set as a possible means of codling moth control. repeated. Again, such machines do not work well with Although DNOC found ready commercial acceptance apples, because too many spurs are knocked off and in the arid regions of the Western United States, it the foliage is damaged. Although some machines are was not as valuable to Eastern growers, where high being tested in Europe, where environmental restric- humidity and/or re-wetting often resulted in injury tions on the use of thinning chemicals are more severe, and/or over-thinning. Nevertheless, Hoffman [81], especially in “bio-orchards” devoted to the produc- working in New York State, duplicated the effects tion of organically-grown fruit, results to date have of hand removal of blossoms obtained by Bobb and not been commercially acceptable [17, 120, 126, 132, Blake [20] (see above) on the same cultivar (Wealthy) 133]. by applying DNOC over a 4-year period. The production of DNOC, which contains heavy metals, was discontinued in 1989 because of environ- 3. Chemical thinning mental concerns and the high cost of re-registration. This led to a search for replacements. Williams [152, 3.1 Use of caustic sprays 153] and Williams et al. [155] tested several chemicals for this purpose, the most promising of which were Bagenal et al. [8] compared several fungicide mixtures sulfcarbamide (monocarbamide dihydrogen sulfate), for control of apple scab (Venturia inaequalis). pelargonic acid, and endothall [7, oxybicyclo(2,2,2) Although Bordeaux mixture did not affect fruit heptane-2,3-dicarboxylic acid]. Among other chem- retention, sprays containing lime-sulfur induced “an icals that may become useful are ammonium nitrate, abnormal drop of fruit.” Drain [55] may have been the ammonium thiosulfate [34], and hydrogen cyanamide first to use chemicals in a conscious attempt to prevent (see [146]). Ammonium thiosulfate is currently being fruit set. Although he presented no data, he reported evaluated both in the United States and other coun- that iron sulfate sprays thinned at concentrations as tries. Urea has similar effects [159]. These compounds low as 2 gL−1. The sprays tended to under-thin apples, are safer to use on peaches than on apples, as less while over-thinning Japanese plums (Prunus salicina foliage is present at the time of flower thinning in the L.). former. Concentration of most of these chemicals is Auchter and Roberts [5] attempted to prevent fruit crucial, as all can be phytotoxic at higher concentra- set entirely with chemicals, as a means of obtaining tions. Even at concentrations that are not phytotoxic, annual cropping of orchards, if not of individual trees. fruit defects such as russeting and “dimpling” can be In response to grower requests they performed tests a problem (see [152, 153]). A second problem with involving eight cultivars in six American states. They blossom thinning is the difficulty of proper timing. used sodium and calcium polysulfide (liquid lime The stage of bloom often varies among trees within sulfur), CuSO4,NaNO3,ZnSO4, and oil emulsion, but an orchard, and even within trees. Timing is critical no treatment was completely satisfactory. Chemicals if optimum results are to be obtained whereas post- 5 bloom thinners can be used successfully over several especially with ‘Delicious’. Tromp and Wertheim days or even weeks. [137] indicated that carbaryl had superseded NAA Other natural products, including soap, waterglass and its amide, NAAm, for use in Europe, because of (sodium silicate), mineral oils, and rapeseed oil, have the adverse effects of both compounds on fruit size been tested for blossom thinning in Europe [133]. A and leaf condition. Under some conditions, NAA can combination of soap and oil was most effective in induce the formation of “pygmy” fruit or “nubbins”. reducing set, but waterglass had little effect. Most of Black, et al. [19] evaluated the effects of timing of the treatments were injurious to foliage and fruit. NAA application on fruit size, and found that, within years, neither timing of treatment nor spray volume 3.2 Use of plant bioregulators (when applying the same dosage) affected the inci- dence of small fruit. However, when three years of A major advance in fruit thinning was the report by data were “normalized” by plotting fruit size in units Burkholder and McCown [27] that naphthalene acetic of standard deviation of the mean, the maximum acid (NAA) applied at full bloom reduced fruit set of number of small fruits occurred when NAA was apple. This was serendipitous, for Gardner et al. [66] applied at 11.3 mm. king fruit diameter (relationship had reported that NAA delayed pre-harvest drop, and significant at P <= 0.05, using multiple regression). Burkholder and McCown had hoped to find a way to Position of the fruit in the cluster can also affect increase set. This chemical became the “workhorse” response to NAA. Black et al. [19] demonstrated that of the apple grower as a means of reducing cropping, the growth of neither “king” nor lateral fruit was thereby increasing both fruit size and repeat bloom. affected significantly by NAA treatment (15 mgL−1) Until the mid-1940s NAA had been applied only at when only one fruit developed per cluster. In clusters bloom. Davidson, et al. [46] tested the effects of NAA with on a “king” and one lateral fruit, growth of the as a post-bloom treatment, with positive results. From latter, but not of the former, was strongly inhibited that date, NAA has been used primarily as a post- in the absence of NAA treatment. NAA further inhib- bloom thinner. Further evaluation of timing effects ited growth of both fruits. Growth of the lateral fruit demonstrated that NAA was not effective after the was inhibited by the king fruit by about the same “June drop” [51, 52, 77, 124]. Donoho [51, 52] percentage regardless of NAA treatment. reported that it was most effective on ‘Jonathan’ when Both Burkholder and McCown [27] and Schneider fruit were 15–16 mm in length, with little effect once and Enzie [118] noted that NAAm was active in thin- fruit were 19 mm long. Tukey [138] observed that ning. However, its potential was not exploited until NAA was most effective on ‘Rome Beauty’ when fruit much later, when Hoffman [82] and Hoffman et al. diameter was 10–11 mm. Leuty [97] applied NAA [83] reported that NAAm was safer to use than NAA. to three cultivars of apple over 4 years, then used Application of NAA often induced epinasty and early polynomial regression to determine the fruit diameter ripening when applied during late stages of bloom or at which sensitivity was maximum. ‘Delicious’ was at petal fall, especially in early cultivars. NAAm has most sensitive at about 6–8 mm, ‘Mclntosh’ at 8–10 become a useful alternative to NAA where excessive mm and ‘Northern Spy’ at 9–10 mm. Considerable thinning may occur, as the amide is approximately 10- variability is evident in the data, however, perhaps fold less effective than the acid or sodium/potassium because of environmental conditions during treatment. salt. It is therefore less likely to overthin when small Examination of the developing seeds indicated that errors are made in concentration. However, like NAA maximum sensitivity occurred 2–3 days before onset (see above), it can induce the formation of “pigmy” of cytokinesis in the endosperm. However, Batjer fruit in ‘Delicious’. et al. [12] were unable to confirm such a precise Edgerton and Greenhalgh [60] appear to have “window” in sensitivity. Experiments conducted both been the first to evaluate the thinning activity of in Washington State and in New York indicated that ethephon (2-chloroethylphosphonic acid). This chem- NAA was equally effective over a period of 13 days ical releases ethylene on hydrolysis within the treated when ‘Delicious’, ‘Golden Delicious’, ‘Mclntosh’, tissues. Although many investigators have tested it, and ‘Rome Beauty’ fruits ranged from 5 to 13 mm in its use appears to have become a recommended prac- diameter. tice only in Tasmania [22], where extensive tests have Despite its thinning action, increase in fruit size been conducted to determine the optimum concen- following treatment with NAA is often disappointing, trations and timings of application [92, 93]. One of 6 the major problems with this chemical is the marked of these chemicals is currently approved for commer- effect of temperature on the rate of release of ethylene cial use. in the tissue. High temperatures following applica- tion could result in severe overthinning, given Olien 3.3 Use of insecticidal carbamates and Bukovac’s [112] report of the marked temper- ature effect on release of ethylene from ethephon (Q10 The thinning activity of the insecticide carbaryl (1- = 6.3–7.0) in leaves of sour cherry (Prunus cerasus naphthyl-N-methylcarbamate) was first reported by L.). Batjer and Westwood [14] and Batjer and Thomson Early experiments with benzyladenine (N6- [13] in Washington State and New South Wales, benzylaminopurine, BA) indicated that it would Australia, and confirmed by Bukovac and Mitchell stimulate cell division in young apple flowers and [26] in Michigan. Batjer and Thomson [13] compared fruits, with early applications (full bloom to petal fall) the effects of carbaryl with those of NAAm and being most effective [127, 139]. Similar effects were NAA on thinning, seed content, rate of fruit growth, obtained when BA was used in combination with harvest drop, fruit maturity and return bloom of five gibberellins [127], or in the commercial preparation cultivars. Much higher concentrations of carbaryl were R Promalin , which contains both BA and GA4/7 required but less overthinning resulted. Carbaryl was in approximately equal amounts. Stembridge and effective at concentrations of 440 to 1800 mgL−1, Morrell [127] reported a significant reduction in with little effect of dose within this range. Batjer and fruit set of ‘Delicious’ when BA was applied in Billingsley [10] reported that the effects of carbaryl combination with GA4/7 on ‘Delicious’ at full bloom were more consistent than those of NAA, and carbaryl in 1970. Although some reductions were apparent seldom overthinned. Responses to NAA and carbaryl when BA (100 mgL−1 alone was applied at full paralleled one another in several cultivars and over bloom or when BA + GA4/7 were applied together a range of fruit sizes [12]. Carbaryl is considered at 50 or (100 mgL−1 at pink, or at 25 mgL−1 at full to be a mild thinning agent, and is best used with bloom, the effects were not statistically significant. another compound, such as NAA or BA, for difficult- Little effect of similar treatments was evident in to-thin cultivars such as ‘Golden Delicious’ and spur- experiments conducted by the same authors in 1969. type strains of ‘Delicious’ [70, 71, 98]. Multiple The fruit-thinning effects of BA were not thoroughly applications of carbaryl alone were no more effective evaluated until considerably later [62, 63, 70, 73]. than single applications in Virginia [28, 35, 104] R Accel , another mixture of BA and GA4/7,but and North Carolina [140]. Because NAA and BA, with a much higher proportion of BA, also thins. applied together or sequentially, can inhibit fruit Benzyladenine is considered to be a mild thinning growth in ‘Delicious’ (see above), carbaryl is preferred agent, and combinations of BA or AccelR with for combinations with BA in this cultivar. However, NAA or carbaryl are often used by fruit growers for carbaryl is an insecticide that can kill beneficial maximum effect. With ‘Delicious’, BA and NAA, insects, including honeybees. A flowable formula- applied either simultaneously or sequentially, can tion (SevinR XLR) is less harmful to bees than is the inhibit fruit development [19, 22, 25, 74]. Under powder formulation, the particles of which are the size some conditions, BA alone can stimulate fruit growth of pollen grains. more than would be expected from its thinning effect Some other insecticidal carbamates, such as [63, 64, 157]. Wismer et al. [157] reported that BA oxamyl {methyl N,N0-dimethyl-N-[(methylcarba- stimulated cell division in the cortex of ‘Empire’ moyl)oxy]-1-thiooxamimidate; VydateR 2L} also are fruits, whereas NAA and carbaryl did not. effective in thinning apples [28, 35, 105]. Other cytokinins are also effective in fruit thinning. Elfving and Cline [63] reported that thidiazuron (N- 3.4 Use of inhibitors of photosynthesis phenyl-N0-1,2,3-thiadiazol-5-ylurea) thinned ‘Empire’ apple. Subsequently, Greene [69] observed that both As might be expected, fruit abscission can be induced thidiazuron and CPPU [N-(2-chloro-4-pyridyl)-N0- by shading individual limbs or entire trees [29, 30, 32, phenylurea] thinned both ‘Mclntosh’ and ‘Empire’ 36], as fruit growth is dependent upon carbohydrates when applied at either full bloom or at 22 days after supplied by the leaves. Fruit set and retention are full bloom, but the later treatment was more effective; generally better in regions with higher solar radiation bloom sprays on ‘Delicious’ were ineffective. Neither (e.g., the central valleys of Washington State vs. the 7 eastern United States). Chemicals that inhibit photo- Table 1. Mechanisms proposed to explain the fruit thinning action of chemicals synthesis, including the herbicide terbacil (3-tert- butyl-5-chloro-6-methyluracil), can also thin apples 1. Abortion or inhibition of growth of embryo and peaches [29, 30, 31]. Treatment of the fruit alone 2. Delay of abscission, increasing competition among fruits is ineffective in inducing abscission, indicating an for nutrients indirect effect. Some evidence suggests that at least 3. Inhibition of phloem transport to fruit part of the activity of NAA and NAAm may be the 4. Reduction of sink strength of fruit/stimulation of sink activity in bourse shoot result of inhibition of photosynthesis. Schumacher et 5. Inhibition of auxin synthesis by seed al. [121] observed a slight reduction in carbon assimi- 6. Inhibition of auxin transport from seed lation in ‘Golden Delicious’ leaves following applica- 7. Stimulation of ethylene biosynthesis tion of NAAm. Stopar et al. [128] reported that NAA 8. Inhibition of photosynthesis/stimulation of dark respira- application inhibited carbon assimilation by as much tion as25%forupto48hin‘Delicious’and‘Empire’ leaves, and inhibition continued for as long as 2 weeks. In this case, NAA was applied too late to affect fruit abscission, hence no relationship between assimilation 4.Modesofactionofgrowthregulating rate and thinning was reported. In general, however, compounds and insecticidal carbamates (see also given the low concentrations of NAA that are effective [9, 10, 21, 48, 50, 58, 59]) in thinning under some conditions (2–5 mgL−1) [105], inhibition of photosynthesis is unlikely to be a major The mechanisms by which growth regulators thin factor contributing to this response. (Table 1) have been investigated since the effects of NAA were first discovered, but remain obscure. 3.5 Use of combinations of chemicals, multiple applications, and surfactants 4.1 Naphthaleneacetic acid (NAA)

Use of combinations of chemicals (BA plus GAs, Struckmeyer and Roberts [134], noting that NAA NAA, or carbaryl plus NAA or BA) has been treatment delayed fruitlet abscission, proposed that mentioned above, and will be discussed only briefly this delay increased competition for nutrients, thereby here. Combinations of two chemicals are recom- increasing the final number of fruits that abscised. mended for difficult-to-thin cultivars, where high However, several facts make this hypothesis unten- concentrations of NAA might lead to detrimental side- able. The fruit that eventually drop usually cease effects (e.g., small fruit), or where one chemical alone growth soon after treatment, and therefore should not is not sufficiently effective. Because both BA and attract nutrients from persisting fruits [135]. Further- carbaryl are considered “mild” thinning agents, they more, NAA does not always delay abscission [10]. are often used together or with NAA (note deleter- Luckwill [99], nothing that seed abortion was ious effects of BA plus NAA on ‘Delicious’ mentioned common in fruitlets sprayed with NAA, proposed that above). This also permits the use of lower rates of this response reduced the ability of some fruits to expensive chemicals. Multiple applications of single compete for nutrients, leading to abscission. Murneek chemicals or mixtures are sometimes used to adjust and Teubner [110] reported that embryos were smaller fruit load when the initial rate used does not remove in NAA-treated fruits of ‘Wealthy’. If NAA induces enough fruit. In general, recommendations call for low abscission by causing seed abortion, application rates near bloom, with the rates increasing with fruit directly to the seeds should be effective. Abbott [1] size. cut ‘Crawley Beauty’ fruits transversely, and treated Because the thinning chemical must penetrate the cut surface with lanolin pastes containing NAA at the leaf to be effective, surfactants can be used to intervals from 1 to 13 weeks after petal fall. Seeds reduce surface tension and improve contact angle, were removed from some fruits, left intact in others. thus increasing penetration through the cuticle. Some Only 10 fruits were used per treatment per date, but commercially-prepared thinning agents contain a NAA appeared to promote abscission of seeded fruits surfactant for this purpose. during the first 2 weeks and inhibit it during the next 3 weeks. It had no effect thereafter. The results are there- fore difficult to interpret. Although NAA-induced seed 8 abortion is common in some cultivars, e.g., ‘Delicious’ finally, fruitlet abscission. However, Teubner and [13], ‘Mclntosh’, ‘Golden Delicious’, ‘Jonathan’, and Murneek [135] were unable to find a significant rela- ‘Winesap’ fruits can be thinned without affecting seed tionship between the amount of ethylene generated number [13, 106]. Even in ‘Delicious’, there is little following NAA treatment and the extent of fruit relationship between the degree of seed abortion and abscission. Walsh et al. [142] reported that NAA stim- thinning response [10]. Furthermore, NAA will thin ulated both fruit abscission and ethylene production in seedless apple fruits [47]. Luckwill and Lloyd-Jones ‘Golden Delicious’. Ebert and Bangerth [56, 57] (see [100] applied 14C-NAA (50 mgL−1)toleavesof above) compared the effects of three thinning agents ‘Miller’s Seedling’ apple. Only 0.2% of the radio- on (a) apple fruitlet abscission, (b) ethylene content activity applied (= 0.7% of that recovered from all of the fruitlets, and (c) both extractable and diffus- tissues) was found in seeds 5 days later, none was ible IAA, ABA, and GAs. High concentrations of recovered as NAA. However, Donoho et al. [53] ethylene were found in the gas extracted from the fruit- reported that 14C moved from the spur leaves to the lets treated with ethephon. Application of NAAm had fruit of the same cultivar within 1 h of application much less effect, whereas carbaryl had none. Based of ring-labeled 14C-NAA,andwasdistributedinboth upon these data and the fact that ethephon had little fruit and seeds after 4 d. In peach, 14C was found in thinning effect in 1977 despite high concentrations of both fruit and seeds 6 days after treatment (no earlier ethylene in the fruitlets, they concluded that ethylene samples were taken). evolution of itself was not sufficient to induce abscis- Thinning chemicals may interfere with transloca- sion. Greene, et al. (71) (72) also reported a stimula- tion of nutrients from leaf to fruit, or of endogenous tion of ethylene production in Rogers ‘Mclntosh’ auxins down the pedicel. Crowe [45] reported that the fruits and leaves following application of both NAA amount of diffusible auxin (measured by Avena meso- and BA (see below), but thinning response again was cotyl assay) recovered from the pedicels of ‘Golden not proportional to the rate of ethylene evolution. Delicious’ apple fruitlets was reduced within 24 hours The possible effects of NAA in inhibiting photo- of application of NAAm, and remained low for 2.5 synthesis have been discussed above. weeks. The amount of auxin recovered from small fruits was lower than that from large fruits. Ebert 4.2 Benzyladenine (BA) and Bangerth [57] measured both extractable and diffusible auxin content of apple fruitlets following BA has been used as a thinner for only a short time application of carbaryl, ethephon, and NAAm. They relative to the use of NAA, and therefore its mech- concluded that reduced auxin transport was the major anism of action has not been studied extensively. factor responsible for the thinning action of all three Greene et al. [72] found it more effective when applied compounds. However, inconsistencies in their data to the leaves alone than to the fruit alone (Table make this conclusion questionable. 2), but fruit size was increased only when BA was Schneider and Lasheen [119] observed that NAA applied to the fruit and occurred even in the absence thinned ‘Golden Delicious’ and ‘Stayman’ fruits of significant thinning. Application of BA stimulated equally well whether applied to the leaves alone or ethylene production in both leaves and fruits, and to both leaves and fruits. The treatment reduced sugar the rate of evolution increased with the concentra- content per fruit and movement of sugars from the leaf tion applied. Nevertheless, although the amount of to the fruit. In a parallel study [117] NAA did not ethylene evolved was greater than that induced by affect the transport of IAA or sucrose in the pedicel NAA, the thinning response was less. Greene, et al. in either direction. [72] suggested that BA might reduce the supply of Auxin applications stimulate ethylene production sugar to the fruit. Yuan [158] observed that trans- in many plant tissues, and this increase in the concen- port of 14C-sorbitol from leaf to fruit was increased tration of ethylene could induce abscission. Ethephon, when BA was applied to the fruit, but not when it was as noted above, can be used for thinning. As early as applied to the leaf. Net photosynthesis was inhibited 1954 Murneek [109] proposed that NAA uncouples 10–15% by BA application (50 or 100 mgL−1), and oxidative phosphorylation, leading to a sequence of leaf carbohydrate levels were also reduced. Applic- events, including reduced hormone content of the ation of BA inhibited the growth of single fruits on endosperm, reduced fruit growth, reduced embryo girdled branches only if leaf number was less than growth, an increase in ethylene production, and, 16, and induced abscission only when less than two 9

Table 2. Results of experiments to determine sites of He suggested that carbaryl might favor sink activity in action of thinning chemicals the bourse shoots, thereby limiting export of essential Chemical Site of action Reference metabolites to the fruit. However, this does not explain its direct action on the fruit. NAA Seed Early – No [1] Byers et al. [29] applied a solution of carbaryl Late – Yes by spraying leaves only or fruits plus leaves, or by Leaf = both leaf and fruit [119] dipping the fruits in carbaryl. In well-exposed spurs, BA Leaf > fruit [72] abscission occurred only when the fruits were treated. Carbaryl Leaf << fruit [154] However, in spurs that were shaded for 4 days, begin- Bourse leaf = fruit [89] ning on the day of treatment, all treatments were >>> rosette leaf effective. Thus, they confirmed that carbaryl is more Leaf < fruit (not shaded) [30] effective when applied to the fruit, but is partially Leaf = fruit (shaded) effective on the bourse leaf, as well, especially when Ethephon ??? branches are shaded. Terbacil Leaf [36] Schneider and Lasheen [119] were unable to detect any effect of carbaryl on translocation of sugars, transport of IAA in the pedicel, or ethylene evolution. Chiba and Kubota [38] measured ethylene produc- leaves were present. Yuan concluded that BA thinned tion by apple fruitlets following application of carbaryl by stimulating dark respiration, thereby reducing the 3 weeks after full bloom. Thinning response was supply of carbohydrates available for fruit growth. greatest in Starking ‘Delicious’, intermediate in ‘Jonathan’, and negligible in ‘Ralls’. Ethylene produc- 4.3 Carbaryl tion was stimulated in ‘Delicious’ fruitlets, but not in those of the other two cultivars. However, most Carbaryl is not a hormone by definition, as much investigators have observed little effect of carbaryl on higher concentrations are required for activity than ethylene evolution (e.g., Ebert and Bangerth [57]). is the case with NAA (1000 mgL−1 vs. 5–15 for Although Ebert and Bangerth [57] concluded from NAA). Nevertheless, much effort has be devoted to its their work that carbaryl reduced IAA transport, the mechanism of action. Way [143] noted that carbaryl results were variable, as noted above for the effects retarded fruit growth for a brief period following of NAA. application, but did not study the mechanism of action. Williams and Batjer [154] treated only spur 4.4 Ethephon leaves, only fruits, or both (Table 2). Fruit drop following treatment, averaged over three orchards, Ethephon, once absorbed by the tissues, is hydrolyzed was 30% for leaf treatment, 53% for fruit treatment, to release ethylene, and this gas can induce abscission and 17% for no treatment. Thus treating only fruits of leaves in plants held within containers. Thus, this was about three times as effective as treating only spur could be the basis for its thinning effect. The role leaves. Although carbaryl induced some seed abor- of ethylene in the thinning action of other chemicals tion, no relationship was found between abortion and remains controversial, as noted above. fruit abscission. Based upon experiments with 14C- carbaryl, they concluded that the site of action was the vascular tissue of the fruit. Knight [89] did a similar 5. Methods of application of chemical thinners experiment, applying carbaryl to the spur (“rosette”) leaves, to the bourse leaves, to the pedicel of the fruit, Ecological and cost concerns aside, the concentra- or to the fruit itself. All except the first treatment were tion of pesticides applied is not important as long as effective (Table 2). When carbaryl was applied to the coverage is adequate. However, responses to growth pedicel, it moved to the fruit, but when applied to the regulating compounds are dose-dependent. Chem- bourse or spur leaves, it did not. Carbaryl applied to ical thinners have traditionally been applied as dilute the fruit remained there and did not move to the seeds. sprays, using air blast equipment. Early models of Knight [89] concluded that action in the pedicel, i.e., such sprayers deposited much more chemical on the directly on the abscission zone, or seeds was unlikely. lower than on the upper portion of full-sized trees. 10

This was the opposite of the desired distribution. et al. [19] used a single sprayer to apply the same Because the upper branches of trees are better exposed dosageofNAA(15mgL−1, dilute equivalent = 31.5 to sunlight, they are more vigorous and therefore a gha−1), without surfactant, to Redchief ‘Delicious’ higher dosage of thinning chemical is required for the trees at four volumes (250, 500, 1000, 2000 L ha−1). same response. With advances in technology, methods Neither thinning response nor the percentage of small of application and formulations of the chemicals also fruits (<64 mm. diameter) was affected significantly changed (see [17, 23, 24] for more detail). Part of by spray volume over the 2 years of the study. the problem of distribution was overcome by appro- The volume of solution applied obviously must be priate adjustments in fan speed and nozzle size and adjusted when concentrates are used. The question location so that more of the spray was delivered to remains as to how much solution to apply. Dilute solu- the top of the tree. Following the shift from dilute tions were originally applied until “run-off”, but little sprays to concentrate sprays, growers were cautioned or no run-off occurs with concentrate sprays. Distri- to continue using dilute rates to avoid overthinning bution of sprays can be evaluated experimentally by and/or phytotoxicity. However, this requires changing using fluorescent dyes or other chemicals (e.g., [37]), nozzle size and reducing tractor speed; thus growers but this is of little use to the grower. The concept of prefer to apply NAA and other thinning chemicals “tree row volume” provides a guideline for the volume as concentrates. Rogers and Thompson [115] were to be applied [33]. Unrath [140] and Herrera-Aquirre among the first to compare the effects of concentrate and Unrath [79] evaluated the response of ‘Delicious’ vs. dilute sprays of carbaryl. They applied concentra- apple to NAA, ethephon, and carbaryl over several tions of 1X to 33X to ‘Rome Beauty’ trees over four years and locations. Although hand-gun applications growing seasons. No significant thinning occurred provided adequate thinning, airblast applications at the in one year, but all concentrations were effective in same rates did not. Only when the spray volume was the other 3 years. Similar results were obtained for adjusted for tree row volume was sufficient thinning ‘Jonathan’, and ‘Golden Delicious’ (one year’s data). achieved. Although their conclusions were probably They noted that carbaryl was effective at 33X (19.8 justified, no comparisons of “regular” airblast appli- gL−1 even though its solubility is only about 450 cations vs. similar applications adjusted for tree row mgL−1. volume were made in the same year. Looney and McKellar [98] reported no pronounced effect of the volume of NAA spray applied (560 to 4400 Lha−1) in thinning apple fruits, as long as 6. Factors affecting tree response to thinning chem- the dose remained constant; occasionally, response icals was greater with lower volumes. Knight [90], using carbaryl, also noted that dose was more important Many factors affect response to thinning chemicals. than volume applied, but presented no data to support Those that can be best controlled by the grower are this conclusion. Koen et al. [94] tested the effects of concentration and method of application. However, several factors, including volume applied, on response orchard conditions are also extremely important: these of ‘Golden Delicious’ to ethephon; Tween 20R was can be divided into the effects of cultivar, tree condi- included as a surfactant. They observed that response tion, and weather (Table 3). They have been discussed increased with both dosage and (independently of by many reviewers (e.g., [65, 156]), and there- dosage) with volume of solution applied. Jones et al. fore will be covered only briefly here. Williams and [86], using both 10 and 20 mgL−1 NAA, reported Edgerton [155] provide a table of 15 “conditions that 4000 Lha−1 was less effective in thinning ‘Golden affecting ease of fruit thinning with chemicals”, listing Delicious’ than was 7000 Lha−1. Oakford, et al. [111] those that reduce response opposite those that increase evaluated response of ‘Golden Delicious’ to both NAA response. Many of these conditions are difficult for and carbaryl at constant dosages while varying spray the grower to control, given their dependence on volumes from 100 to 6000 Lha−1. They reported weather conditions, and/or are observational, rather essentially no differences in subsequent cropload. than the results of controlled studies; nevertheless, Different spray application techniques were used they provide a basis for decisions as to rates of chem- in these studies to adjust volume, resulting in differ- icals to apply. Hennerty and Forshey [78] attempted ences in dosage of NAA or of surfactant that may have to obtain experimental evidence for the effects of affected response. To resolve these problems, Black physiological condition of the tree on response of 11

Table 3. Orchard factors that affect response to fruit-thinning chem- concentration of chemical – accounted for 80% of icals (adapted from [156] and other sources) the observed variation in response. A more recent Cultivar model [84] incorporates six major factors (chemical Biennial vs. annual, spur-type vs. non spur-type, fruits set in used, cultivar, rootstock, tree age, tree size, fruit clusters vs. singles or doubles, extent of “June” drop size desired) and three modifying ones (previous crop, Fruit characteristics rate of vegetative growth, pruning severity). Computer Stage of development, position in cluster programs (“expert systems”) have also been developed Tree condition to assist growers in making decisions as to what Tree and leaf age, vigor, previous crop, bloom and crop concentrations of chemicals to use (e.g., [43, 116, 148, density, stress (disease, nutrient, etc.), condition of root system 149]). Such systems allow for inclusion of weather Weather conditions prior to, during, and immediately after conditions, and permit the grower to provide informa- application tion on additional factors (see Table 3) that may affect Temperature, solar radiation, relative humidity, rain, frosts, response in a particular orchard. No studies are known water supply in which recommendations based upon such systems have been evaluated experimentally, and the effects of the recommended dosage compared with those of higher and lower ones. ‘Golden Delicious’ apple to NAA, but their results were disappointing from the standpoint of useful- ness in predicting response. Using several treatments, 8. Economic value of thinning including scoring, shading, defoliation and defruiting, they were unable to establish significant correlations Thinning has obvious beneficial effects, particularly between carbohydrate content of spurs and fruit set or with biennial cultivars that set very heavily and would effectiveness of NAA. The only significant relation- yield many small fruits one year and only a few large ship was a positive correlation between the number of fruits the next if not thinned. Thinning of annual- fruits removed by NAA and the percentage fruit set of bearing cultivars is widely practiced by growers, and the controls. One criticism of this study is that thinning has similar, though less dramatic, effects on fruit size response was measured by the number, rather than the and annual bearing. Such thinning usually reduces proportion, of fruits removed. total yield. Exceptions can occur in some cases, as Time of day could affect response to thinning when very early thinning, e.g., blossom thinning, agents, given the changes in relative humidity and reduces the severity of the ‘June’ drop, and final fruit temperature that occur, and the known effects of these number is little affected. In most cases, however, yield factors on absorption of chemicals. Jones, et al. [86] of large fruits is increased while that of small and reported that application of NAA at 0800 HR was medium-sized fruit is reduced. Because large fruits more effective in thinning ‘Golden Delicious’ than command higher prices, economic benefits accrue. was application at 1200, 1500 or 2000 HR. However, As apple production and international trade have R. Byers (personal communication) has found little or increased and customers have become more partic- no effect of time of day on response to such chemicals. ular, prices for small and medium-sized fruits have remained constant or declined. Fruits between 57 and 64 mm (2.25 and 2.5 in.) in diameter once were a 7. Modeling to improve response to thinning major product on grocers’ shelves in the United States; chemicals (see also [85]) today most of them are used for juice. Nevertheless, few good studies are known in which the economic Within the last 20 years modeling of various physio- returns from thinning have been carefully evaluated. logical processes has been attempted, either to develop Walker [141] stated that the gain in value of a better understanding of them, or to allow more peaches was “1000% of the cost” of hand thinning, effective control. Jones, et al. [87] and Koen and Jones but presented no data. Southwick et al. [125] esti- [92] developed a preliminary model for the effects mated that inhibiting flowering of peach with GA of ethephon on thinning of ‘Golden Delicious’, and reduced the cost of hand thinning the following have made improvement with time. Koen et al. [94] year by $185 to $310 per ha, after deducting the reported that two variables – time of application and cost of the treatment. Some data are available on 12 economic returns from thinning pear [129] and apple (75 mgL−1 at 5 mm) increased fruit size, grower (see below). All estimates are based upon extrapola- returns were not improved. The authors stressed the tion from relatively small plots. In a study with need for additional information as to what conditions Starkrimson ‘Delicious’/seedling rootstock apple in favor the promotive effect of AccelR on fruit size Michigan over several years, Cook [40, 41] esti- (environmental effects, timing, etc.) mated that thinning with NAA increased net income Based on these and other data, Stover [130] by 10 to 20%, whereas hand thinning alone or suggested that for ‘Empire’/M. 7 in their 6th leaf the subsequent to NAA treatment reduced it by 23 to 29% fruit load providing maximum returns without major relative to respective controls. However, no allowance reductions in return bloom was seven fruits/cm2 trunk was made for any improvement in grade other than cross-sectional area. This fruit load produced a mean effects on size. Bergh [16], working in South Africa, fruit weight of 159 g (114 fruit per 18 kg box). He attempted to develop a model for optimizing crop level estimated that each reduction of 1 fruit/cm2 from this to maximize return per hectare. He concluded that optimum would reduce income by $740 to $2000 income could be maximized by thinning ‘Golden Deli- per ha. Stover and Robinson (unpublished data), cious’ to optimum levels, but that prices for large fruit using these and similar data, found values of 7–10 of ‘Granny Smith’ and of Starking and Starkrimson fruit/cm2 provided maximum grower returns for both ‘Delicious’ were too low to warrant the necessary ‘Empire’ and ‘Delicious’. Many ‘Mclntosh’ blocks reductions in yield. Silsby, et al. [122] evaluated the yielded optimum returns without thinning; reductions effects of hand thinning ‘Empire’ trees that had been in cropload therefore reduced crop value. previously treated with NAA, but cropload was still Given the limited data available, studies are too heavy. Fruits were removed at four intervals from needed, similar to those of Bobb and Blake [20] and 25 to 100 days after full bloom (DAFB) at two sites Hoffman [81] with a biennial cultivar, to evaluate in Western New York State. Color was dramatically economic returns from thinning annual cultivars over improved by all thinning treatments, but neither this a period of at least 4 years. nor increased fruit size compensated for crop reduc- tion, resulting in net losses in all treatments in one orchard and in three of the four treatments in the other. 9. The future of fruit thinning Estimated losses ranged from $313 to $4693/ha, while the one gain ($2455/ha) was achieved only by thin- Although the title of this chapter is the history of ning 25 DAFB – considered “too early to be practical”. fruit thinning, a look to the future is in order. Fruit Nevertheless, the authors concluded that hand thinning crops occupy much smaller surface areas than do many could be profitable, provided that it was performed of the primary agronomic crops, such as maize and early enough and that fruit removal was not excessive. beans, and the market for chemicals to control fruit Stover, et al. [131] evaluated grower return, again set and development and fruit maturation is therefore by extrapolation from plot data, from the use of NAA much smaller than that for herbicides, fertilizers, and R and Accel (BA:GA4+7, 9:1) on ‘Empire’, Redchief other chemicals used for a wider spectrum of crops. ‘Delicious’ and ‘Mclntosh’ in the Hudson Valley of Concern for food safety has led to limitations on New York. In one study neither NAA nor AccelR what chemicals can be used, and chemical manufac- reduced cropload of ‘Empire’ significantly when turers cannot afford the testing procedures necessary applied when fruit diameter was 10 mm, but AccelR for registration or re-registration if the market for the (75 mgL−1) stimulated fruit growth (109% of control product is too small. Thus DNOC has been withdrawn fruit weight at harvest), resulting in an estimated gain from the market. Carbaryl can no longer be used in of $2440/ha (value = 144% of control). However, some areas of Europe and will soon be unavailable in NAA increased neither fruit size nor economic value. the United States. Therefore, research is underway to In another study with ‘Empire’, AccelR (75 mgL−1) find naturally-occurring chemicals that are effective in reduced cropload, increased fruit size, and increased thinning. The most promising of these are blossom grower return regardless of timing from full bloom thinners, which are toxic when used at sufficiently to 10 mm fruit diameter. Treatment at 15 mm. fruit high concentrations. Ethephon remains a possibility diameter increased grower return without reducing for post-bloom thinning, and is used commercially in cropload significantly, as fruit size was increased. Tasmania, but is considered to be erratic in effect by In ‘Delicious’, even though one AccelR treatment most investigators, Mechanical methods of thinning 13 provide a last resort as an alternative to chemicals. 13. Batjer LP and Thomson BJ (1962) Effect of 1-naphthyl N- Hand thinning is generally too expensive where labor methylcarbamate (Sevin) on thinning apples. Proc Amer Soc costs are high, but other methods have been tested, Hort Sci 77: 1–8 14. Batjer LP and Westwood MN (1960) 1-Naphthyl N- including dragging rakes or ropes through trees to methylcarbamate, a new chemical for thinning apples. Proc reduce the numbers of blossoms, and shaking them Amer Soc Hort Sci 75: 1–4 or using clubs to remove excess fruit, etc. These 15. Beach SA (1903) Thinning apples. New York State Expt Sta Bul 239 methods are seldom if ever used with pome fruits, 16. Bergh O (1990) Proposed regression model for calculating for immature fruit are easily bruised, whereas peaches optimum crop levels of apple trees. S Afr J Plant soil 7: 19–25 and apricots are still hard when thinning is performed. 17. Bertschinger L, Stadler W and Weibel F (1997) 6. Behangs- For fruit growers to remain competitive and fruit to und Alternanzregulierung. Schweiz Z Obst-Weinbau 133: 12, 293–294 remain relatively inexpensive for the consumer, new, 18. Black BL and Bukovac MJ (1996) Plant growth regulator “environmentally friendly” chemicals must be found application technology, uptake and action. In: Maib K (ed) to replace those eliminated because of environmental Tree Fruit Physiology: Growth and Development, Ch. 4. concerns. Other areas of research that may provide Yakima, WA: Good Fruit Grower, pp 41–49 19. Black BL, Bukovac MJ and Hull J Jr (1995) Effect of new insights are discussed more thoroughly in other spray volume and time of NAA application on fruit size and chapters of this book. These include the use of several cropping of Redchief ‘Delicious’ apple. Scientia Hort 64: chemicals, together or sequentially, surfactants to 253–264 improve penetration through the cuticle, modeling, 20. Bobb AC and Blake MA (1938) Annual bearing in the Wealthy apple was induced by blossom thinning. Proc Amer spray application technology, etc. Soc Hort Sci 36: 321–327 21. 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