Plant Growth Regulation31: 113–119, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

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Fruit thinning of peach trees

Guglielmo Costa1 & Giannina Vizzotto21Dipartimento di Colture Arboree, University of Bologna, Via Filippo Re 6, 40126 Bologna, Italy;2Dipartimentodi Produzione Vegetale e Tecnologie Agrarie, University of Udine, Via delle Scienze 208, 33100 Udine, Italy

Key words:bloom thinning, bud thinning, correlative inhibition, fruit growth

Abstract

The present review deals with the importance of fruit thinning in peach. The date of treatment, the severity and thecriteria underlying the practice are discussed. Methods of fruit thinning are described, with particular emphasis onthe use of chemical treatment as an alternative to hand thinning. Strategies for chemical thinning are advanced.

1. Introduction

In peach fruit, thinning must be performed every year.The advantages of this technique are well known viz.reducing the number of flowers or fruits increases fruitsize, improves quality, prevents alternate bearing andbalances the fruit-to-shoot ratio, leading to an increasein assimilates to fruits and shoots [18, 33, 39, 40, 42,57]. The practical consequences of thinning includean increase in individual fruit weight, fruit maturityenhancement and better flower bud formation [53], allof which lead to higher prices and crop value.

Thinning responses are closely related to environ-mental and soil conditions and management prac-tices, especially pruning. Reproductive and vegetativeperformance is closely linked to thinning severity andtiming. Research focusing on alternatives to hand thin-ning is topical because of increasing labour costs andthe high fertility of currently grown cultivars.

2. Treatment Scheduling

2.1 Date

Thinning time is critical to achieve the desired results.Manual fruit thinning is usually carried out towardsthe end of fruit growth Stage II (pit-hardening) orthe beginning of Stage III. After natural abscission,only the excess fruits on the tree need to be thinned.During Stage II, the fruits slowly increase in size and

their demand for assimilates remains high for endo-carp hardening. Delaying fruit thinning until after thisstage eliminates fruits and the assimilates that could beused to optimise the current and subsequent season’scropping potential [71].

To ensure a proper supply of assimilates rightfrom the moment of fruit formation, earlier thin-ning, such as during Stage I of fruit growth or evenduring bloom, can be a viable option. While manualflower thinning has only been adopted for experi-mental purposes [26, 42], it can steadily increasefruit yield with respect to thinning later in theseason, although its cost makes it unlikely that itwill become standard orchard management practice. Itcould become cost-effective for a very early-croppingcultivar that commands a high enough market price,although such a cultivar usually yields small-size fruitsbecause of its shortened growing season. Thinning atbloom also reduces competition between fruits andvegetative sinks and enhance fruit size [8, 9, 10].

2.2 Scheduling criteria

The fruit growth curve can be used as a tool to estab-lish the optimum thinning datevis à vis the givencultivar. In peach it has a double sigmoid patternwith three distinguishable stages [14]. Stage I, whichrepresents the period of rapid pericarp and seed devel-opment, is marked by exponential growth. Stage II(the so-called lag-phase) usually is coincident witha reduction in fruit growth, thought to be linked to

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endocarp lignification. Stage III is a second phase ofexponential growth featuring rapid cell expansion andmaturation of the mesocarp. This three-stage modelis based on fresh weight accumulation (or increasein transversal diameter) and can readily be appliedto mid- and late-ripening cultivars. Note, however,that for early ripening cultivars determining the threestages of this construct is difficult, as Stage II istoo short to be distinguishable. Note too that thesethree development stages do not necessarily corre-spond to fruit dry-weight development, the basis foranother model [13]. While the dimensions reachedby fruit during Stage II (14 days after the onset ofpit hardening) have been shown to be well correlatedto final fruit size [3] under optimum or sub-optimumcarbon supply, late-ripening cultivars may be limitedby insufficient assimilate supply (e.g. nutritionalstarvation) [37]. Indeed, for the efficiency of the treeit is better to thin fruits marked by a slower growthrate and, hence, a greater probability of abscising [58].However, as noted above, in the case of early ripeners,the three-stage model is inadequate because of thebrevity of Stage II [17].

In an attempt to identify criteria with universalapplication, other growth models can be examined.Four phases of fruit growth (P1–P4) can be distin-guished [17] using the first and second derivatives ofthe double sigmoid curve. These phases cannot beclearly identified in all peach cultivars because thefruit growth pattern is related to the time required toreach maturity. An analysis of fruit growth kineticsof three cultivars with different ripening dates showsthat while the length of P1 was similar in all, P2 andP3 were much shorter in the early ripeners than in themid- and late-ripening ones (Table 1) [17].

Another method to describe peach fruit growthkinetics employs the daily fruit demand for photo-synthates throughout the season (sink strength) [23].Relative growth rates of fruits (in terms of dry weight)are plotted against degree days to yield a biphasicmodel in which the shift between the two phases isrelated to the end of a period of active cell division anddifferentiation and to the onset of a phase primarilymarked by cell expansion. This fruit growth modelcan be readily adopted to predict cultivar behaviour indifferent years, although the physiological processesnormally associated with the two phases in very earlyripeners are not clearly associated with two separateRGR (relative growth rate) phases [55].

Thus, the optimum time to carry out thinning maynot be strictly related to endocarp lignificationper

se, but to the sudden increase in sucrose content ofthe mesocarp or the time after which the “marginalcost” of a fruit greatly increases. To prevent a signifi-cant dispersion of assimilates, it may be better toeliminate excess fruits before the demand for carbo-hydrates exceeds the supply, and before an undesid-erable competition between fruits and other sinks oramong fruits occurs.

3. Severity of thinning

Fruit thinning affects fruit size and yield [68]. Theproper amount of fruit to remove depends on genetictraits and on the value of incremental increases infruits size and yield. The bearing capacity is asso-ciated with the tree’s age and size and is also influ-enced by such external factors as pruning. Clingstonetrees require less severe thinning than freestones ornectarines. Indeed, the standard size required for theformer is usually smaller than the latter. Note too thatnectarines frequently show a natural fruitlet abscissionduring the season.

Regardless of species, an increment in fruit numberreduces fruit size and increases yield [25]. Source-sink relationships and the allocation of assimilates todifferent organs play a central role in the determina-tion of crop yield [25]. Since fruit size depends on theratio of leaf number to fruit number, there is a closerelationship between canopy size and bearing capacity[73]. Reducing the number of fruits per tree by thin-ning increases the leaf-to-fruit ratio and increases fruitsize [27, 70]. That early thinning – at bloom or soonafter pollination – results in larger fruits [71] indicatesthat peach fruit growth is source-limited during theearly and late periods of development [41, 56]; duringthe mid-period of fruit growth sink limitations maybe present (at least in late-maturing cultivars) [24].However, the period in which such growth limita-tions take place differs depending on cultivar bloomto harvest dates. Competition among fruits may bemore evident in early ripeners and may be presentthroughout fruit development, although such otherfactors as environmental conditions (water and carbo-hydrate availability, plant nutrition, etc.) can stifle fruitgrowth with respect to its genetic potential [56].

An important factor in determining tree responseis light – shading influences several vegetative andreproductive plant traits [12, 52]. That flower budsare mainly found at the top and near the outer edgeof the canopy means that these zones are conducive

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Table 1.

S P FW DW

I II III 1 2 3 4 1 2 3 4 1 2 3 4 Total

Springcrest 35 7 23 28 14 14 9 33 7 15 10 33 9 13 10 65

Redhaven 35 22 36 30 19 31 3 33 27 20 13 48 12 20 13 93

Cresthaven 33 27 52 28 31 41 12 33 37 30 12 48 17 35 12 112

to fruit development in regard to final size, colorand soluble solids content (shading induces early fruitdrop). However, as vegetative and reproductive budsare usually present on the same node in peach, therecan be competition in the apical parts of the treebetween young fruits and growing shoots [15].

A study on the thinning effect of shading wascarried out on cultivars of different ripening dates [12].The period of maximum sensitivity ranged from 45 to58 days after full bloom (AFB), depending on harvestdate and crop load. This approach suggests that thesensitivity of the developing fruit to canopy manipu-lation (e.g. pruning, shading) differs and provides apotential factor controlling reproductive or vegetativegrowth that can be used as a tool to determine thinningdate and severity.

Pruning can elicit contrasting effects [34, 35]. Itreduces the tree’s total dry matter and rate of accu-mulation, stimulating vegetative growth in the localarea of the cut. The manipulation of the plant’s source-sink ratio can alter carbohydrate and phytohormonelevels [43], which in turn can affect the relationshipbetween vegetative and reproductive growth. Stone-fruit response to pruning has been studied extensively[7, 36, 59], the effect being dependent on the timeof application, tree vigor and planting density [50].Summer pruning reduces growth to a greater extent,without inducing an invigorating effect as compared towinter (dormancy) pruning [35]. It also promotes newshoot development, improves light penetration intothe canopy and can reduce reproductive developmentthe following season, thereby acting as a contributoryfactor to limiting flower number [36].

4. Thinning Methods

4.1 Manual

Thinning by hand is usually carried out 40 to 60 daysAFB when natural abscission takes place because of

competition among fruits [40], although this period isnot appropriate for all cultivars. Especially for earlyripening cultivars, which are generally characterizedby small fruit size [17], flower as opposed to fruit thin-ning is preferred since early reduction of competitionamong reproductive sinks leads to large fruit size [9,42, 69]. Yet the economic benefits of bloom thinningare to be weighed against the higher prices paid forlarger fruits, the desired yield level, the risk of springfrost, etc. [8]. Thereafter, other parameters like theleaf-to-fruit ratio, lignification, type of shoot, positionof fruit in the canopy and type of winter pruning canbe taken as additional criteria.

4.2 Mechanical

Both flowers and fruits can be removed by mechanicalmeans. Flower number can be reduced by dormancypruning, physical removal by hand or specializedbrushes, rope drags, high pressure water streams [8].Fruit removal has also been performed by mechanicalshaking [16], although because it selectively thins fruitbased on their mass it removes the largest fruit andthereby decreases fruit yield and value [4].

4.3 Chemical

No satisfactory chemical thinning results in peach andnectarine have been achieved despite the numerousagents (3-CPA, CGA, Orthonil, Morphactins, NAA,NAAm, Ethrel) employed and the extensive body ofresearch devoted to the subject up to the 1980s [1, 6,16, 19, 29, 60]. However, at present, there are severalviable ways of reducing fruit or flower number byapplying chemicals with a thinning effect at specificphenological stages.

4.3.1 Flower-bud differentiation.Gibberellin (GA)sprays reduces flower bud number when applied frombloom to September. This was shown some years ago[28, 44, 66, 67] and recently reproposed [9, 48, 63,64]. Gibberellins must be applied when flower-bud

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Table 2.

Treated No. fruit/TCA Total fruit wt/

Treatment (DAFB) (n/cm2) TCA (g/cm2)

Hand-thinned 44 3.3 325.2

Bloom thinned 0 6.4 455.5

Bloom thinned + GA3 0 4.9 390.8

Bloom thinned + GA3 12 4.3 327.0

Bloom thinned + GA3 36 1.7 207.0

Bloom thinned + GA3 47 1.3 159.3

Table 3.

Application date GA conc Thinning Reduction in

(mg/L) time/tree thinning time

(min) (%)

June 15 50 to 120 0 100

July 9 50 to 120 15.1 28.2

July 27 50 to 120 20.2 5.5

Hand thinned – 21.4 –

differentiation can be affected. Some investigationshave indicated that GA application from 0 to 47days AFB inhibits flower bud formation and reducesthe subsequent year’s cropping [9] (Table 2). Morerecently, Southwick et al. [62] show that flowerreduction occurred when GA3 was applied frommid-June to early July; spray applied in mid-Junemeans no thinning the following year (Table 3).The inhibition of flower bud differentiation is alsoclearly related to spray concentration (Costa et al.,unpublished) (Table 4). Flower bud inhibition hasnot become widely accepted because of the potentialsubsequent winter or spring frost damage to buds,which further reduces cropping [9].

Table 4.

Treatment Concentration No. of flowers/

m shoot

Control – 32.64a

Mid-June 60 ppm 21.38

80 ppm 17.34

Mid-July 60 ppm 28.46

80 ppm 16.33

a Average of three cultivars

Table 5.

Treatment Fruit abscission

(%)

Hand thinned 18

1% Hydrogen cyanamide 73

2% Hydrogen cyanamide 97

3% Hydrogen cyanamide 100

Table 6.

Hours from Hours between pollination and

pollination 3% ArmoThin treatment

Ca 0b 0c 24 48 72 96 120

Pollen tube growth (% of total style length)

0 – – –

24 33 – – 40

36 71 – – 60

48 78 – – 83 55

60 83 – – 42 62

72 85 – – 79 67 75

84 81 – – 84 79 100

96 100 – – 85 100 86 100

108 91 – – 90 93 100 96

120 95 – – 93 100 100 95 100

132 100 – – 100 87 100 100 100

144 100 – – 100 100 100 100

a untreated controlb ArmoThin applied before pollinationc ArmoThin applied immediately after pollination

4.3.2 Autumn and dormancy. In October andNovember, GA and Ethrel sprays have been used todelay flowering and prevent the risk due to winteror spring frost. These sprays also caused flowerbud-mortality [74], an effect related both to date ofapplication and concentration of the active ingredients.Gianfagna et al. [38] found that 100 and 200 ppmof Ethrel applied in autumn can delay flowering byseveral days and, at the same time, have a direct effecton flower bud mortality.

During the dormant period, bud-dormancybreaking agents are applied when the chillingrequirement is not completely met. Normally, undersuch conditions, compounds like hydrogen-cyanamideor nitrogen and surfactant mixtures are applied 60–40days before expected bud-break. The application ofhydrogen cyanamide close to bloom (less than 40days) can inhibit flower-bud burst. Where the chilling

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Figure 1.

requirement is usually met, treating with hydrogencyanamide may result in flower bud abscission orinhibition of flower opening [32] (Table 5).

4.3.3 Bloom.Some chemicals [surfactants, endothal,X–77, D–88, and fertilizers such as ammoniumthiosulfate (ATS)], when applied 2 or 3 days afterpeach flower opening, can interfere with pollinationand fertilization of the ovule [8, 11], causing flowersand fruits to abscise. ATS burns blossoms and youngshoots, especially when applied with fungicides intank mix [54]. Thiourea and urea applied at thebeginning of bud swell have caustic effects on flowerparts, resulting in thinning of early ripeners [26, 30].Other compounds applied in full bloom as endothalicacid (Endothal) or pelargonic acid (Thinex) effectivelythinned blossoms reducing peach fruit set [31].

In the last 5–6 years, one of the most exten-sively studied surfactants is a fatty amine polymercalled ArmoThin, which has been tested on severalstonefruit species with interesting results [20, 21,48, 49, 61]. This compound induces early antherdehiscence, a marked reduction in pollen germinationand pollen tube growth within the stylar tissue soonafter germination has occurred (Table 6). The dataon pollen tube growth suggest the efficiency of thecompound in relation to flower stage: the earlier thetreatment, the more effective the compound (Figure 1)[2]. Application of the chemical at a 2–3% concen-tration when 70–80% of the flowers have opened hasyielded interesting results in several climatic areas ofcultivation and on different cultivars.

4.3.4 Fruit thinning by bioregulators.Since 1970,numerous trials in several countries have studiedethephon as a chemical fruitlet thinner for peach andother stonefruits. However, while some trials haveprovided very interesting results, ethephon use has notbecome widespread since its effectiveness is closelyrelated to a number of internal and external factors [16,29, 45, 46, 47, 57, 65].

Compounds capable of inhibiting photosynthesis(terbacil) cause fruit abscission when applied 30 to40 days AFB [12] and even thereafter [22]. PP333,a well known growth retardant for stonefruit, alsoinduces some fruit abscission in peach when treatmentis performed at the shuck-off stage [5], although theeffects of such sterol- and gibberellin inhibitors do nothold for all cultivars and growing conditions [51].

5. New perspectives

It is difficult to find a winning strategy for chem-ical thinning in peach. The attempts to find a singlechemical compound as an alternative to hand fruitthinning have failed. A possible approach to solvingthis problem in peach may lie in the strategy adoptedfor other species, e.g. apple [72]. In this species,it is possible to thin fruits using substances (i.e.DNOC, NAA, NAAm and SEVIN) which are appliedat specific and successive phenological stages on thesame trees. With the chemicals already available, itmight be possible even in peach to carry out treatmentsat flower differentiation (GA), or during bud dormancy

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(dormancy-breaking agents), evaluate their efficacy atbloom and still be able to spray at bloom (surfactant asblossom thinner) and even repeat the application withethephon at the fruitlet stage.

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