Principles of Thinning in Fruit Tree Crops – Needs and Novelties G. Costa

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17 Principles of Thinning in Fruit Tree Crops – Needs and Novelties

G. Costa^{1, a}, M.M. Blanke^{2, b} and A. Widmer³

1Department of Fruit Trees and Woody Plant Science, Alma Mater Studiorum, University of Bologna, viale G. Fanin 46, 40127 Bologna, Italy

2INRES – Horticultural Science, University of Bonn, Auf dem Huegel 6, D-53121 Bonn, Germany

3Research Station Changins-Wädenswil ACW, Research Department Plant Protection and Fruit and Vegetable Extension, Schloss 1, P.O. Box, 8820 Wädenswil, Switzerland Keywords: alternate bearing, biennial bearing, fruit colouration, fruit firmness, fruit

quality Abstract

Fruit trees bear an abundance of flowers, which they cannot support until fruit maturation; ca. 7% of flowers are necessary in apple for a sufficient harvest while 25% of flowers are necessary in peach. Thinning is hence a prerequisite in fruit crops i) to achieve high quality fruit with sufficient size and colouration for class I marketing including sugar (as a parameter for taste) and sufficient firmness (as a parameter for good storability), ii) to reduce labour-intense hand thinning and iii) to overcome alternate bearing (change of low and high yielding years) by providing regular moderate yields.

INTRODUCTION

Many fruit species require a reduction of fruit load to reach a vegetative/

reproductive equilibrium; normally this objective is achieved with the fruit thinning operation. This operation is performed yearly mainly in peach and nectarines among the stone-fruit species and in apple among the pome-fruit specie. Fruit thinning is also performed on other fruit specie such as apricot, cherry, plum, kiwifruit, orange, table and grape vine, etc, although in some of these, pruning might represent an effective tool to achieve the desired fruit load. However many fruit trees bear an abundance of flowers, which they cannot support until fruit maturation; ca. 7% of flowers are necessary in apple to guarantee a satisfactory yield (Untiedt and Blanke, 2001) while only 20-25% of flowers are needed in peach (Costa and Vizzotto, 2000). In anticipation of this, the major fruit specie developed an immature fruit physiological drop as self regulatory mechanism.

The physiological fruit drop leads to a reduction of the fruit number per tree, but this is still insufficient to guarantee, at harvest, fruits of good marketable size and maximum commercial yield. For this reason, fruit thinning is performed to achieve the optimum fruit load and quality. Fruit thinning is hence a prerequisite in fruit crops for:

i) to achieve high quality fruit with sufficient size and colouration for class I marketing (Kong et al., 2009) including sugar (as a parameter for taste) and sufficient flesh firmness (as a parameter for appropriate storage length);

ii) to reduce labour-intense hand thinning; and

iii) to overcome alternate bearing (change of low and high yielding years) by providing regular moderate yields.

OBJECTIVE OF FRUIT LOAD CONTROL

The reasons for reduce the crop load are different and are related to the different species. For instance there are beneficial effects of fruit load reduction that can be distinguished between those on long-term whole tree physiology and current year fruit quality (Table 1).

Taking apple as an example, the fruit load reduction via fruit thinning is mainly

a guglielmo.costa@unibo.it

b mmblanke@uni-bonn.de

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performed to overcame or avoid alternate bearing, improving return bloom and fruit quality. Nowadays the achievement of high fruit quality standard is becoming a “must”

and the efficiency of the fruit thinning operation is judged on the fulfil of this objective.

In fact, there is an inverse relation existing between fruit number/tree and final fruit size. Given a certain tree density per hectare, within each tree there is an ideal optimum fruit number that guarantees at harvest the optimum fruit size and the maximum commercial yield. As a consequence of the abundant fruit set exceeding the physiological ability of the tree to sustain seed and fruit development up to maturation and ripening, intra fruit competitions generate the fruitlet physiological drop that in our condition takes place within 4-7 weeks after full bloom. To reach however the maximum commercial yield, fruit thinning must be performed (Costa et al., 2006).

It has also to be considered that any manipulation of the fruit load affect other physiological processes in the tree. In fact fruit thinning performed before fruit set may prevent or overcome alternate bearing. By contrast, any fruit thinning performed after fruit set is normally ineffective on alternate bearing. The exception to this rule is the application of ethylene releasing compounds such as Ethrel, Ethephon or Flordimex (Bangerth, 2001), with a direct positive effect on flower initiation for next year (Seehuber et al., 2011), although the return bloom can be influenced by the active ingredient and the environment.

METHODS OF CROP LOAD MANIPULATION

Pruning performed during the winter time can be regarded as the earliest type of crop load control (Table 2), since it leads to a removal of bearing wood carrying flower buds to reach an equilibrium between vegetative and reproductive sinks on the tree.

Pruning can be also performed during bloom: in this case the effect consists in both a flower reduction as well as a temporary reduction of the new vegetation.

Removal of complete flower clusters is a tool of orchard management in expectation of an intense flowering with the aim of preventing alternate bearing, the principal affect is an obvious reduction of fruit yield.

‘Bud extinction’, performed during winter period (Lauri and Lespinasse, 2001) is practised in some situation; it requires several hours/hectare when the winter pruning is normally performed, but trees do not need in general further operation to adapt the reproductive to the vegetative sinks.

Another possibility to control fruit load is represented by the use of gibberellins (GAs) during the flower bud differentiation period. This application although effective on several fruit species (Costa et al., 2000; Byers et al., 2003) has been rarely adopted in the practice because the lack of reliability depending upon several internal and external factors. In addition the low number of fruit that characterize the trees the year after the GAs application might expose the trees to further reproductive reduction if low temperatures occurred during the blooming period.

Flower thinning can also be performed at blooming time: the flower thinning can be extremely effective since it reduces as early as possible the competition among flower and fruitlet; this operation is rarely performed by hand because is extremely time demanding and is characterized by a very high cost. The removal of individual flowers is normally achieved by chemical and mechanical action.

METHODS OF THINNING AND PERFORMING TIME

Fruit thinning can be performed either by hand or mechanically or chemically.

Hand thinning is considered the most reliable, although is the most expensive method. It is usually applied only after the natural drop occurred when the fruitlet reach a dimension of about 20 mm Ø. The hand thinning allows also to eliminate fruits with poor characteristics, such as the undersized, dark-green fruit close to the tree trunk or scab or hail affected fruit.

The fruitlet chemical thinning is the most interesting because it is quick, has a low cost, and it can be applied at proper time. Given a natural fruit drop dynamics, the

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maximum thinning effect is exhibited when the chemical is sprayed at beginning of the fruit drop. Later applications are less effective. The chemical can be totally ineffective if sprayed at the end of the fruit drop. It has to be underlined that the thinning action of the used bioregulators is quite variable and depends on environmental conditions and cultivars. For example in apples there are cultivars easy to thin and others that are very difficult, even if different chemicals or combinations of them are used. It has however to be considered that many compounds exhibits fruit thinning action on apple. In addition, an important feature of the fruit chemical thinners relies on their ability of magnifying the fruitlet natural drop that hast to be evaluated in order to decide the opportunity of applying chemical thinning agents. Several active ingredients, belonging to fertilizers, plant growth substances and growth regulators that have shown to be effective when applied at bloom time and just after fruit set. Most of these active ingredients can be applied in sequence from bloom up to 14-16 mm king fruit Ø having thus the possibility to decide the intensity of the operation. However, all chemicals including proposed for thinning fruit crops in the EU, also those traditionally used require re-registration under the EU 91/414 and listing in its Annex 1. While the present number of thinning chemicals varies greatly between EU countries, the objectives of EU 91/414 is to achieve a uniform registration throughout Europe, re-assess the environmental impacts and only allow environmentally-friendly and secure chemicals. The current list in Annex 1 includes, BA, NAA, NAAm, and Ethephon (Table 3).

These chemicals are the only ones officially recognized as thinning agents in Europe. It is also interesting to note that there are differences in the use and authorization among the different European countries. This can create difficulties: in fact, the availability of active ingredients allowed in some countries and banned in others. It may determine different production costs between the different producing countries. Also there are some concerns on the free movement of fruit within the European Union. These problems leads to exploit other possibility such as the use of friendly chemicals, mainly used in organic methods of production, or the use of mechanical devices. Recently the availability of Darwin and/or Baum (Solomakhin and Blanke, 2010; Seehuber et al., 2011) devices found an increasing use in the apple flower thinning; the obtained results were up to now quite interesting although it has to be considered that an appropriate use of this method must consider both the adaptation of the training system to the machine and of the machine to the training system. The mechanical methods however has advantages: they are not affected by environmental conditions, can be used in organic farming methods and it can be considered a low environmental impact cultural method, and enables selective thinning e.g. more of the inner than outer canopy of more of the lower than upper branches.

ACKNOWLEDGEMENTS

This research was supported by AGER Project, grant n° 2010-2119.

Literature Cited

Bangerth, F. 2000. Abscission and thinning of young fruit and their regulation by plant hormones and bioregulators. Plant Growth Regulation 31:45-59.

Byers, R.E., Costa, G. and Vizzotto, G. 2003. Flower and fruit thinning of peach and other Prunus. p.351-392. In: J. Janick (ed.), Horticultural Reviews 28, Wiley J. &

Sons Publishers, New Jersey, USA.

Costa, G., Bonany, J., Carbo, J., Casals, M., Stopar, M., Lafer, G., Dorigoni, A., Vizzotto, G. and Bomben, C. 2004. Multilocation analysis of ats and ba thinning efficacy on

‘Golden Delicious’. Acta Hort. 636:303-310.

Costa, G., Bucchi, F., Montefiori, M., Bregoli, A.M. and Corelli Grappadelli, L. 2004.

Thinning activity and fruit quality of Gala and Fuji apple varieties as affected by cytokinins. Acta Hort. 653:107-113.

Costa, G., Dal Cin, V. and Ramina, A. 2006. Practical, physiological and molecular aspects of fruit abscission. Acta Hort. 727:301-310.

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Costa, G., Corelli Grappadelli, L. and Bucchi, F. 2001. Studies on apple fruit abscission and growth as affected by cytokinins. Acta Hort. 557:243-252.

Costa, G. and Vizzotto, G. 2000. Fruit thinning of peach trees. Plant Growth Regulation 31:113-119.

Lauri, P. and Lespinasse, M. 2001. Genotype of apple trees affects growth and fruiting responses to shoot bending at various times of the year. Amer J. Hort. Sci. 126(1):169- 174.

Seehuber, C., Damerow, L. and Blanke, M.M. 2011. Regulation of fruit set and source:

sink relationship and fruit quality of (European) plum- using thinning as crop load management (CLM). Plant Growth Regulation 65(2):335-341.

Solomakhin, A. and Blanke, M.M. 2010. Mechanical thinning improves fruit quality.

Journal of the Science of Food and Agriculture 90(5):735-740.

Untiedt, R. and Blanke, M.M. 2000. Effects of fruit thinning agents on apple tree canopy photosynthesis and dark respiration. Plant Growth Regulation 35(1):1-9.

Tables

Table 1. Beneficial effects of fruit load control in fruit crops.

On tree physiology Fruit quality

Overcome alternate bearing Improved fruit size

Regular yields Improved fruit colouration

Restrict excessive vegetative growth Improved sugar (taste)

Less pruning Improved firmness (storability)

Table 2. Times and types of thinning.

Phenological stage Type of thinning

Winter Pruning

Bud stage Mechanical and manual

Full bloom Mechanical, manual, chemical

12-18 mm fruitlet Chemical only

Walnut size Manual hand-thinning only

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Table 3. List of the chemical thinners for apple: registrations and specification in EU countries (Widmer 2012 - EUFRIN WG on Fruit Chemical Thinning).

NAA - Annex I (1107/2009): Registered.

Application Country Fruit crop Officially

registered (as thinning agent)

Phenology Specifications Further remarks

Austria Apple Yes Petal fall – 7 mm king fruit Ø (10-12 DAFB)

50% NAAm, (Dirigol N) 10-20 g/100 L

Registration limited to 30-06-2012

Belgium Apple Yes 5-7 days after FB 7.5 g/100 L a. i.

Denmark - No - -

France Apple Yes Petal fall 5-10 DAFB

50 ppm Germany Apple No

Italy Apple Yes 1-6 days after FB 50-100 ppm Registration limited to 31-12-2012 Netherlands Apple No

Norway - No - -

Poland - No - -

Slovenia Apple Yes Petal fall – 7 days later

25-67 ppm NAD (AmidThin)

Registration limited to the end of 2015 Spain Apple Yes Petal fall – 10 days

later

8,4% NAAm (Amid-Thin, Clerthin,

Clerthin D) 60 g/100 L Switzerland Apple Yes Petal fall 100-200 ppm

United Kingdom - No - -

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Table 3. Continued.

NAA - Annex I (1107/2009): Registered.

registered (as thinning

agent)

Austria Apple Yes Petal fall – 8-12 mm king fruit Ø

10% NAA (Luxan Late Val)

10-20 ppm

Registration limited to 31-12-2013;

preferably used as tank mix with BA Belgium Apple Yes 10 mm fruit Ø 1 g/100 L a.i. (Fixor 100 SL) Preferably used in combination with BA Denmark Apple, pear Yes At petal fall 10-30 ppm (Pomoxon) Registration limited to 31-12-2015 France Apple Yes 8-14 mm, 15-25

DAFB

7-15 ppm

Germany - No

Italy Apple Yes 10-14 mm fruit Ø 5-15 ppm, NAA 4% (Nokad) 10-12 mm, 300-375 ml/ha

Registration limited to 31-12-2012 Netherlands Apple No Not registered for thinning but

only for prevention of fruit drop before harvest Apple, pear Yes Permitted only as

drop spray

Max. 10 ppm (10 ml/100 L)

Norway - No

Poland Apple, pear,

plum

No

Slovenia Apple Yes 8-10 mm fruit Ø 5-13 ppm NAA (Dirager) Registration limited to the end of 2015 Spain Apple Yes 10-15 mm fruit Ø

(15-21 days after FB)

1% ANA (Rhodofix, Rhodofix N, Etifix, Etifix WP, Vifix, Creciforte, Hormonax, Clar-

Frut) 0, 1-0, 15%

Used from 8 to 10 mm fruit Ø normally

Switzerland Apple Yes 10-12 mm fruit Ø 20-30 ppm

United Kingdom - No - -

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Table 3. Continued.

Ethephon - Annex I (1107/2009): Registered.

Austria Apple Yes Pink bud – open king flower max. 20-22 mm

king fruit Ø

150-200 ppm; commercial product: Flordimex 420,

30-40 ml/hl

Belgium - No - -

Denmark - No

France Apple Yes Pre and post

bloom

240 to 360 ppm Commercial product:

PRM12 RP

Registration for the use “ripeness and colour” - Number of sprays restricted Germany Apple Yes Pink bud – open

king flower 20-24 mm fruit Ø

Flordimex 420 30-40 ml/hl

Plum Yes Flordimex, 0.15 L/ha,

m crown height

Pre harvest interval 80 days Sour cherry Yes Flordimex, 0.5 L/ha,

m crown height

Pre harvest interval 7 days Italy Apple Yes At the beginning

of bloom or 20- 22 mm fruit Ø

100-150 ppm, 39.6%

(Ethrel), 20-22 mm, 300-600 ml/ha

Peach Yes 9-11 mm seed length

30-60 ppm

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Table 3. Continued.

Ethephon - Annex I (1107/2009): Registered.

Application Country Fruit crop Officially registered

(as thinning agent) Phenology Specifications Further remarks Netherlands Apple Yes Blossom thinning

between balloon stage and 10%

open flowers

50 ml/100 L Ethrel A Registration nr. 6355 N. Owner registration:

Bayer Crop Science B.V. (www. ctb- wageningen.nl)

Apple Yes Improvement fruit colour

50 ml/100 L Registration nr. 6355 N. Owner registration:

Bayer Crop Science B.V.

(www. ctb-wageningen.nl) Sour cherry Yes Enhancement fruit

ripening 0.75 L/ha Registration nr. 13046 N, expiration date: 31- 07-2011. Owner registration: NFO (Dutch fruit

growers organisation) (www. ctb-wageningen.nl) Norway Apple Yes Improvement fruit

colour 50 ml/100 L Apple Yes Pink bud –10%

bloom 10-15 mm fruit Ø

250-500 ppm 500-750 ppm

Poland Plum Yes Full bloom 200 ppm

Apple, pear,

plum No

Slovenia - No - -

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Table 3. Continued.

BA - Annex I (1107/2009): Registered.

Austria Apple Yes 10-14 mm king

fruit Ø

BA 2% (MaxCel) 100-150 ppm or tank mix with

NAA (100 ppm + 10 ppm)

MaxCel registration in Austria is based on NL Registration nr. 13147 N.

Registration limited to 31-12-2013 Belgium Apple Yes 10-12 mm fruit Ø Globaryl 100 SL,

10 g/100 L a.i.

Exilis, 5 L/ha Pear Yes 10-12 mm fruit Ø 200 ppm Denmark - No

France Apple Yes 10-12 mm king

fruit Ø

One application, max 150 ppm MaxCel® and Exilis®

Registration limited to 120 days

Germany Apple Yes MaxCel Registration limited to 120 days

Italy Apple Yes 10-14 mm fruit Ø 75-150 ppm BA 2% (MaxCel, Exilis), 8-14 mm BA 9.4% (Brancher

Dirado), 8-14 mm

Registration nr. 6324, Valent BioSciences/Sumitomo Chemical Agro

Europe SAS.

Netherlands Apple Yes 7-15 mm fruit Ø MaxCel. One application, 375-750 ml/100 L

Registration nr. 13147 N. Owner registration: Valent Biosciences Corporation. Expiration date: 01-01-

2019 (www. ctb-wageningen.nl) Norway No

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Table 3. Continued.

BA - Annex I (1107/2009): Registered.

Slovenia Apple Yes 7-15 mm fruit Ø 70-150 ppm BA (MaxCel) MaxCel registration Nr. 3433-124 in the name of Sumitomo Chemical Agro Europe

SAS

Registration limited to the end of 2015 Spain Apple Yes 7-15 mm fruit Ø

(15-21 days after FB)

75-150 ppm BA (MaxCel®

500-750 cc/100 L, Exilis®

480-715 cc/100 L)

Preferably used in tank mixture with NAA MaxCel registration Nr. 24415 in the name

of Kenogard SA Switzerland Apple Yes 10-12 mm fruit Ø MaxCel 2% BA,

70-150 ppm

MaxCel registration Nr. W6529 in the name of SCAE-Valent BioSciences Sàrl United Kingdom Apple, pear Yes 7-15 mm fruit Ø Exilis, 7.5 L/ha Registration limited to 120 days

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