Fruit thinning of peach trees | SpringerLink

Plant Growth Regulation 31: 113–119, 2000.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
113
Fruit thinning of peach trees
Guglielmo Costa1 & Giannina Vizzotto2
1 Dipartimento
di Colture Arboree, University of Bologna, Via Filippo Re 6, 40126 Bologna, Italy; 2 Dipartimento
di 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 the
criteria underlying the practice are discussed. Methods of fruit thinning are described, with particular emphasis on
the 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 fruit
size, improves quality, prevents alternate bearing and
balances the fruit-to-shoot ratio, leading to an increase
in assimilates to fruits and shoots [18, 33, 39, 40, 42,
57]. The practical consequences of thinning include
an increase in individual fruit weight, fruit maturity
enhancement and better flower bud formation [53], all
of which lead to higher prices and crop value.
Thinning responses are closely related to environmental and soil conditions and management practices, especially pruning. Reproductive and vegetative
performance is closely linked to thinning severity and
timing. Research focusing on alternatives to hand thinning is topical because of increasing labour costs and
the high fertility of currently grown cultivars.
their demand for assimilates remains high for endocarp hardening. Delaying fruit thinning until after this
stage eliminates fruits and the assimilates that could be
used to optimise the current and subsequent season’s
cropping potential [71].
To ensure a proper supply of assimilates right
from the moment of fruit formation, earlier thinning, such as during Stage I of fruit growth or even
during bloom, can be a viable option. While manual
flower thinning has only been adopted for experimental purposes [26, 42], it can steadily increase
fruit yield with respect to thinning later in the
season, although its cost makes it unlikely that it
will become standard orchard management practice. It
could become cost-effective for a very early-cropping
cultivar that commands a high enough market price,
although such a cultivar usually yields small-size fruits
because of its shortened growing season. Thinning at
bloom also reduces competition between fruits and
vegetative sinks and enhance fruit size [8, 9, 10].
2. Treatment Scheduling
2.2 Scheduling criteria
2.1 Date
The fruit growth curve can be used as a tool to establish the optimum thinning date vis à vis the given
cultivar. In peach it has a double sigmoid pattern
with three distinguishable stages [14]. Stage I, which
represents the period of rapid pericarp and seed development, is marked by exponential growth. Stage II
(the so-called lag-phase) usually is coincident with
a reduction in fruit growth, thought to be linked to
Thinning time is critical to achieve the desired results.
Manual fruit thinning is usually carried out towards
the end of fruit growth Stage II (pit-hardening) or
the 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
114
endocarp lignification. Stage III is a second phase of
exponential growth featuring rapid cell expansion and
maturation of the mesocarp. This three-stage model
is based on fresh weight accumulation (or increase
in transversal diameter) and can readily be applied
to mid- and late-ripening cultivars. Note, however,
that for early ripening cultivars determining the three
stages of this construct is difficult, as Stage II is
too short to be distinguishable. Note too that these
three development stages do not necessarily correspond to fruit dry-weight development, the basis for
another model [13]. While the dimensions reached
by fruit during Stage II (14 days after the onset of
pit hardening) have been shown to be well correlated
to final fruit size [3] under optimum or sub-optimum
carbon supply, late-ripening cultivars may be limited
by insufficient assimilate supply (e.g. nutritional
starvation) [37]. Indeed, for the efficiency of the tree
it is better to thin fruits marked by a slower growth
rate 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 the
brevity of Stage II [17].
In an attempt to identify criteria with universal
application, other growth models can be examined.
Four phases of fruit growth (P1–P4) can be distinguished [17] using the first and second derivatives of
the double sigmoid curve. These phases cannot be
clearly identified in all peach cultivars because the
fruit growth pattern is related to the time required to
reach maturity. An analysis of fruit growth kinetics
of three cultivars with different ripening dates shows
that while the length of P1 was similar in all, P2 and
P3 were much shorter in the early ripeners than in the
mid- and late-ripening ones (Table 1) [17].
Another method to describe peach fruit growth
kinetics employs the daily fruit demand for photosynthates throughout the season (sink strength) [23].
Relative growth rates of fruits (in terms of dry weight)
are plotted against degree days to yield a biphasic
model in which the shift between the two phases is
related to the end of a period of active cell division and
differentiation and to the onset of a phase primarily
marked by cell expansion. This fruit growth model
can be readily adopted to predict cultivar behaviour in
different years, although the physiological processes
normally associated with the two phases in very early
ripeners are not clearly associated with two separate
RGR (relative growth rate) phases [55].
Thus, the optimum time to carry out thinning may
not be strictly related to endocarp lignification per
se, but to the sudden increase in sucrose content of
the mesocarp or the time after which the “marginal
cost” of a fruit greatly increases. To prevent a significant dispersion of assimilates, it may be better to
eliminate excess fruits before the demand for carbohydrates exceeds the supply, and before an undesiderable competition between fruits and other sinks or
among fruits occurs.
3. Severity of thinning
Fruit thinning affects fruit size and yield [68]. The
proper amount of fruit to remove depends on genetic
traits and on the value of incremental increases in
fruits size and yield. The bearing capacity is associated with the tree’s age and size and is also influenced by such external factors as pruning. Clingstone
trees require less severe thinning than freestones or
nectarines. Indeed, the standard size required for the
former is usually smaller than the latter. Note too that
nectarines frequently show a natural fruitlet abscission
during the season.
Regardless of species, an increment in fruit number
reduces fruit size and increases yield [25]. Sourcesink relationships and the allocation of assimilates to
different organs play a central role in the determination of crop yield [25]. Since fruit size depends on the
ratio of leaf number to fruit number, there is a close
relationship between canopy size and bearing capacity
[73]. Reducing the number of fruits per tree by thinning increases the leaf-to-fruit ratio and increases fruit
size [27, 70]. That early thinning – at bloom or soon
after pollination – results in larger fruits [71] indicates
that peach fruit growth is source-limited during the
early and late periods of development [41, 56]; during
the mid-period of fruit growth sink limitations may
be present (at least in late-maturing cultivars) [24].
However, the period in which such growth limitations take place differs depending on cultivar bloom
to harvest dates. Competition among fruits may be
more evident in early ripeners and may be present
throughout fruit development, although such other
factors as environmental conditions (water and carbohydrate availability, plant nutrition, etc.) can stifle fruit
growth with respect to its genetic potential [56].
An important factor in determining tree response
is light – shading influences several vegetative and
reproductive plant traits [12, 52]. That flower buds
are mainly found at the top and near the outer edge
of the canopy means that these zones are conducive
115
Table 1.
Springcrest
Redhaven
Cresthaven
I
S
II
III
1
2
P
3
4
1
2
35
35
33
7
22
27
23
36
52
28
30
28
14
19
31
14
31
41
9
3
12
33
33
33
7
27
37
to fruit development in regard to final size, color
and soluble solids content (shading induces early fruit
drop). However, as vegetative and reproductive buds
are usually present on the same node in peach, there
can be competition in the apical parts of the tree
between young fruits and growing shoots [15].
A study on the thinning effect of shading was
carried out on cultivars of different ripening dates [12].
The period of maximum sensitivity ranged from 45 to
58 days after full bloom (AFB), depending on harvest
date and crop load. This approach suggests that the
sensitivity of the developing fruit to canopy manipulation (e.g. pruning, shading) differs and provides a
potential factor controlling reproductive or vegetative
growth that can be used as a tool to determine thinning
date and severity.
Pruning can elicit contrasting effects [34, 35]. It
reduces the tree’s total dry matter and rate of accumulation, stimulating vegetative growth in the local
area of the cut. The manipulation of the plant’s sourcesink ratio can alter carbohydrate and phytohormone
levels [43], which in turn can affect the relationship
between vegetative and reproductive growth. Stonefruit response to pruning has been studied extensively
[7, 36, 59], the effect being dependent on the time
of application, tree vigor and planting density [50].
Summer pruning reduces growth to a greater extent,
without inducing an invigorating effect as compared to
winter (dormancy) pruning [35]. It also promotes new
shoot development, improves light penetration into
the canopy and can reduce reproductive development
the following season, thereby acting as a contributory
factor to limiting flower number [36].
FW
3
15
20
30
DW
3
4
1
2
10
13
12
33
48
48
9
12
17
13
20
35
4
Total
10
13
12
65
93
112
competition among fruits [40], although this period is
not appropriate for all cultivars. Especially for early
ripening cultivars, which are generally characterized
by small fruit size [17], flower as opposed to fruit thinning is preferred since early reduction of competition
among reproductive sinks leads to large fruit size [9,
42, 69]. Yet the economic benefits of bloom thinning
are to be weighed against the higher prices paid for
larger fruits, the desired yield level, the risk of spring
frost, etc. [8]. Thereafter, other parameters like the
leaf-to-fruit ratio, lignification, type of shoot, position
of fruit in the canopy and type of winter pruning can
be taken as additional criteria.
4.2 Mechanical
Both flowers and fruits can be removed by mechanical
means. Flower number can be reduced by dormancy
pruning, physical removal by hand or specialized
brushes, rope drags, high pressure water streams [8].
Fruit removal has also been performed by mechanical
shaking [16], although because it selectively thins fruit
based on their mass it removes the largest fruit and
thereby decreases fruit yield and value [4].
4.3 Chemical
No satisfactory chemical thinning results in peach and
nectarine have been achieved despite the numerous
agents (3-CPA, CGA, Orthonil, Morphactins, NAA,
NAAm, Ethrel) employed and the extensive body of
research devoted to the subject up to the 1980s [1, 6,
16, 19, 29, 60]. However, at present, there are several
viable ways of reducing fruit or flower number by
applying chemicals with a thinning effect at specific
phenological stages.
4. Thinning Methods
4.1 Manual
Thinning by hand is usually carried out 40 to 60 days
AFB when natural abscission takes place because of
4.3.1 Flower-bud differentiation. Gibberellin (GA)
sprays reduces flower bud number when applied from
bloom 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
116
Table 2.
Treatment
Hand-thinned
Bloom thinned
Bloom thinned + GA3
Bloom thinned + GA3
Bloom thinned + GA3
Bloom thinned + GA3
Table 5.
Treated No. fruit/TCA Total fruit wt/
(DAFB)
(n/cm2 )
TCA (g/cm2 )
44
0
0
12
36
47
3.3
6.4
4.9
4.3
1.7
1.3
325.2
455.5
390.8
327.0
207.0
159.3
Treatment
Fruit abscission
(%)
Hand thinned
1% Hydrogen cyanamide
2% Hydrogen cyanamide
3% Hydrogen cyanamide
18
73
97
100
Table 6.
Table 3.
Hours from
pollination
Application date GA conc Thinning Reduction in
(mg/L) time/tree thinning time
(min)
(%)
June 15
July 9
July 27
Hand thinned
50 to 120
50 to 120
50 to 120
–
0
15.1
20.2
21.4
100
28.2
5.5
–
differentiation can be affected. Some investigations
have indicated that GA application from 0 to 47
days AFB inhibits flower bud formation and reduces
the subsequent year’s cropping [9] (Table 2). More
recently, Southwick et al. [62] show that flower
reduction occurred when GA3 was applied from
mid-June to early July; spray applied in mid-June
means no thinning the following year (Table 3).
The inhibition of flower bud differentiation is also
clearly related to spray concentration (Costa et al.,
unpublished) (Table 4). Flower bud inhibition has
not become widely accepted because of the potential
subsequent winter or spring frost damage to buds,
which further reduces cropping [9].
Table 4.
Treatment
Concentration
Control
Mid-June
–
60 ppm
80 ppm
60 ppm
80 ppm
Mid-July
a Average of three cultivars
No. of flowers/
m shoot
32.64a
21.38
17.34
28.46
16.33
Hours between pollination and
3% ArmoThin treatment
Ca
0b
0c
24
48
72
96
120
Pollen tube growth (% of total style length)
0
24
36
48
60
72
84
96
108
120
132
144
–
33
71
78
83
85
81
100
91
95
100
100
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
40
60
83
42
79
84
85
90
93
100
55
62
67
79
100
93
100
87
100
75
100
86
100
100
100
100
100
96
95
100
100
100
100
100
a untreated control
b ArmoThin applied before pollination
c ArmoThin applied immediately after pollination
4.3.2 Autumn and dormancy. In October and
November, GA and Ethrel sprays have been used to
delay flowering and prevent the risk due to winter
or spring frost. These sprays also caused flower
bud-mortality [74], an effect related both to date of
application and concentration of the active ingredients.
Gianfagna et al. [38] found that 100 and 200 ppm
of Ethrel applied in autumn can delay flowering by
several days and, at the same time, have a direct effect
on flower bud mortality.
During the dormant period, bud-dormancy
breaking agents are applied when the chilling
requirement is not completely met. Normally, under
such conditions, compounds like hydrogen-cyanamide
or nitrogen and surfactant mixtures are applied 60–40
days before expected bud-break. The application of
hydrogen cyanamide close to bloom (less than 40
days) can inhibit flower-bud burst. Where the chilling
117
Figure 1.
requirement is usually met, treating with hydrogen
cyanamide may result in flower bud abscission or
inhibition of flower opening [32] (Table 5).
4.3.3 Bloom. Some chemicals [surfactants, endothal,
X–77, D–88, and fertilizers such as ammonium
thiosulfate (ATS)], when applied 2 or 3 days after
peach flower opening, can interfere with pollination
and fertilization of the ovule [8, 11], causing flowers
and fruits to abscise. ATS burns blossoms and young
shoots, especially when applied with fungicides in
tank mix [54]. Thiourea and urea applied at the
beginning of bud swell have caustic effects on flower
parts, resulting in thinning of early ripeners [26, 30].
Other compounds applied in full bloom as endothalic
acid (Endothal) or pelargonic acid (Thinex) effectively
thinned blossoms reducing peach fruit set [31].
In the last 5–6 years, one of the most extensively studied surfactants is a fatty amine polymer
called ArmoThin, which has been tested on several
stonefruit species with interesting results [20, 21,
48, 49, 61]. This compound induces early anther
dehiscence, a marked reduction in pollen germination
and pollen tube growth within the stylar tissue soon
after germination has occurred (Table 6). The data
on pollen tube growth suggest the efficiency of the
compound in relation to flower stage: the earlier the
treatment, the more effective the compound (Figure 1)
[2]. Application of the chemical at a 2–3% concentration when 70–80% of the flowers have opened has
yielded interesting results in several climatic areas of
cultivation and on different cultivars.
4.3.4 Fruit thinning by bioregulators. Since 1970,
numerous trials in several countries have studied
ethephon as a chemical fruitlet thinner for peach and
other stonefruits. However, while some trials have
provided very interesting results, ethephon use has not
become widespread since its effectiveness is closely
related 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 to
40 days AFB [12] and even thereafter [22]. PP333,
a well known growth retardant for stonefruit, also
induces some fruit abscission in peach when treatment
is performed at the shuck-off stage [5], although the
effects of such sterol- and gibberellin inhibitors do not
hold for all cultivars and growing conditions [51].
5. New perspectives
It is difficult to find a winning strategy for chemical thinning in peach. The attempts to find a single
chemical compound as an alternative to hand fruit
thinning have failed. A possible approach to solving
this problem in peach may lie in the strategy adopted
for 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 applied
at specific and successive phenological stages on the
same trees. With the chemicals already available, it
might be possible even in peach to carry out treatments
at flower differentiation (GA), or during bud dormancy
118
(dormancy-breaking agents), evaluate their efficacy at
bloom and still be able to spray at bloom (surfactant as
blossom thinner) and even repeat the application with
ethephon at the fruitlet stage.
20.
21.
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