Ammonium thiosulfate as blossom thinner in 'Maxi Gala' apple trees

Abstract

The objective of this work was to evaluate the feasibility of using ammonium thiosulfate as a chemical blossom thinner in 'Maxi Gala' apple (Malus domestica) trees and its effects on fruit quality. The experiment was carried out in an experimental orchard in the Southern Brazil, in a randomized complete block design, with five replicates. Ammonium thiosulfate was sprayed on the apple trees at the full bloom stage, at 0, 1.5, 2.5, and 3.5%. Evaluations were performed for the effects on crop load, fruit set, yield efficiency, and fruit quality parameters such as weight, shape, total soluble solids, seed number, flesh firmness, color, and russeting occurrence. Ammonium thiosulfate at 2.5% is effective to reduce crop load and to improve fruit quality. The thinning effect of ammonium thiosulfate is not dependent on the weather conditions during the crop season. The rate of 3.5% of ammonium thiosulfate causes overthinning and does not result in the improvement of fruit quality.

Full text

1132 L. De R. Marchioretto et al.
Pesq. agropec. bras., Brasília, v.53, n.10, p.1132-1139, Oct. 2018
DOI: 10.1590/S0100-204X2018001000006
This is an open-access article distributed under the
Creative Com mons Attribution 4.0 Internat ional License
Ammonium thiosulfate as blossom thinner in 'Maxi Gala' apple trees
Lucas De Ross Marchioretto(1), Andrea De Rossi(2), Micheli Fochesato Michelon(1),
Julio Cesar Orlandi( 1) and Leonardo Oliboni do Amaral(1)
(1)Universidade do Estado de Santa Catarina, Avenida Luiz de Camões, no 2.090, Conta Dinheiro, CEP 88520 -000 Lages, SC, Brazil.
E-mail: lucasdeross@hotmail.com, mickefmichelon@hotmail.com, julioorlandi23@yahoo.com.br, loamaral@ucs.br (2)Embrapa Uva e
Vinho, BR-285, s/no, CEP 95200-000 Vacaria, RS, Brazil. E-mail: andrea.rufato@embrapa.br
Abstract – The objective of this work was to evaluate the feasibility of using ammonium thiosulfate as a
chemical blossom thinner in 'Maxi Gala' apple (Malus domestica) trees and its effects on fruit quality. The
experiment was carried out in an experimental orchard in the Southern Brazil, in a randomized complete block
design, with ve replicates. Ammonium thiosulfate was sprayed on the apple trees at the full bloom stage,
at 0, 1.5, 2.5, and 3.5%. Evaluations were performed for the effects on crop load, fruit set, yield efciency,
and fruit quality parameters such as weight, shape, total soluble solids, seed number, esh rmness, color,
and russeting occurrence. Ammonium thiosulfate at 2.5% is effective to reduce crop load and to improve
fruit quality. The thinning effect of ammonium thiosulfate is not dependent on the weather conditions during
the crop season. The rate of 3.5% of ammonium thiosulfate causes overthinning and does not result in the
improvement of fruit quality.
Index terms: Malus domestica, ATS, blossom stage, ower thinning.
Tiossulfato de amônio como raleante
de oração em macieiras 'Maxi Gala'
Resumo – O objetivo deste trabalho foi avaliar a viabilidade de uso de tiossulfato de amônio, como raleante
químico de oração, em macieiras 'Maxi Gala' (Malus domestica), e os seus efeitos sobre a qualidade dos
frutos. O experimento foi realizado em pomar experimental no sul do Brasil, em delineamento de blocos ao
acaso, com cinco repetições. O tiossulfato de amônio foi aplicado às macieiras em plena oração, a 0, 1,5,
2,5 e 3,5%. Foram feitas avaliações quanto aos efeitos sobre carga de frutos, fruticação efetiva, eciência
produtiva e parâmetros de qualidade de frutos como massa, formato, sólidos solúveis totais, número de
sementes, rmeza de polpa, coloração e ocorrência de “russeting”. O tiossulfato de amônio a 2,5% é efetivo
na redução da carga de frutos e no aumento da qualidade dos frutos. O efeito raleante do tiossulfato de amônio
não é dependente das condições climáticas durante o período de raleio. A dose de 3,5% de tiossulfato de
amônio causa raleio excessivo e não resulta em melhoria na qualidade dos frutos.
Termos para indexação: Malus domestica, ATS, plena oração, raleio de ores.
Introduction
In 2015, Southern Brazil produced 1,163,744 tons of
apples on 36,089 hectares (Anuário…, 2016). Due to the
usual extensive cultivated areas with apples, the labor
availability, and the high-input costs, an improvement
of the efcacy and efciency of the chemical thinning
is a subject requiring more research.
For apple fruit to achieve a marketable size, it is
necessary to reduce up to 95% of the owers to balance
the cell division and fruit size. Blossom thinning
has the greatest potential to increase fruit quality at
harvest, as fruit size (weight) is highly correlated with
cell numbers (Link, 2000; Lakso & Gofnet, 2013;
Jakopic et al., 2015). Therefore, when thinning was
done at blossom stage, there was a signicant increase
of 30 g in fruit weight, in comparison to thinning
done at four weeks after full bloom. Additionally,
ower thinning increases fruit size by 30% when it
is done at owering, in comparison to when thinning
is performed after the “June drop” (Link, 2000), and
improves bloom return for the following year and
increases fruit total soluble solids (Meland, 2009).
The thinning effect of caustic blossom thinners
are not so weather dependent as hormonal chemical
thinners sprayed on fruitlets later in the season,
when cloudy days and high-night temperatures in the
days after spraying may lead to overthinning of fruit

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(Greene, 2002). Blossom thinners such as ammonium
thiosulfate (ATS) or lime sulfur signicantly decrease
the rate of pollen germination or the pollen tube length,
which struggles the ovule fertilization. Thereby, the
fruitlet is not formed and the ower is abscised (Myra
et al., 2007). At 1.5%, ATS shows a strong thinning
which is capable of decreasing half of the amount of
owers produced by European plums, reducing yield
and fruit set more severely than hand thinning, and as
a consequence, increasing the fruit weight and quality
(Meland, 2007). On apple, ATS at 2% signicantly
reduced fruit set and increased fruit weight (Milić et
al., 2011). In the United States and Europe, the use of
ATS is consolidated and there are plenty of research
on this theme. In Brazil, however, little is known about
the thinning properties of this caustic blossom thinner,
and its effects on fruit quality.
The two hormonal chemical thinners currently
registered for apple cultivation in Brazil are
benzyladenine and GA4+7 + benzyladenine (Agrot…,
2017). However, spring time in the apple producing
regions is usually very unpredictable, and occurrences
such as cold temperatures or cloudiness in the aftermath
of spraying chemical thinners are frequent. The lack of
effective options other than hormonal thinners are still
one of the main discussions among apple producers.
There is plenty of information about the potential use
of ATS as a chemical blossom thinner in cropping
systems around the world (Meland, 2007; Milić et
al., 2011; Maas, 2016), and it may have potentially
promising results in Brazil.
The objective of this work was to evaluate the
feasibility of using ammonium thiosulfate as chemical
blossom thinner in 'Maxi Gala' apple trees and its
effects on fruit quality.
Materials and Methods
The experiment was carried out in the municipality
of Vacaria, in the state of Rio Grande do Sul, Brazil,
over two consecutive crop seasons (2015/2016 and
2016/2017), on a mature experimental orchard of
six years with the apple 'Maxi Gala' grafted on 'M9'
rootstock. A randomized complete block design was
used with ve replicates. Each experimental unit (plot)
was formed by a single plant, besides two plants left as
edges. During the rst crop season (September 2015 to
March 2016), the precipitation was 1,376 mm in a total
of 93 days with rain, and in the second crop season
(September 2016 to March 2017), it was 1,000 mm in
a total of 63 days with rain. Mean temperature in the
rst crop season was 18.0ºC, and in the second one it
was 17.5ºC (Embrapa Uva e Vinho, 2017).
Prior to sprouting, in each crop season, three
representative branches were selected on each tree in
the plots. The total number of clusters were labelled
in each branch to determine fruit set, at 70 days after
full bloom, and the number of fruit in each cluster was
recorded to calculate cluster size.
The treatments consisted of three ATS rates and
an untreated control. The tested ATS rates were 1.5,
2.5, and 3.5% (weight/volume of water) added of the
adjuvant Break Thru at 0.2% v/v. The treatments were
sprayed with a CO2 backpack sprayer with a full cone
nozzle tip until runoff on the leaves. During both
years, the treatments were sprayed at once when the
trees were at 70% full bloom.
At harvest, yield was recorded for tree replicates
in which fruit of each one were sorted out with an
industrial grader equipped with an optical color sorter.
Then data were recorded for number of fruit, total
weight, and percentage of red color. The number of
fruit and the yield of each replicate were divided by
the trunk cross-sectional area of each tree, in order to
obtain crop load and yield efciency, respectively. In
addition, the number of fruit was divided by the plant
yield to obtain the mean fruit weight. Fruit were sorted
on three groups of color: 50 to 75%, 40 to 50%, and
<40% of skin red color. After grading, a subsample
of 20 fruit per replicate was collected for physical
and chemical analyses of length: diameter ratio, esh
rmness (Newton), total soluble solids (ºBrix), number
of seed per fruit, and russeting level classied into ve
groups – 0, ≤10, 10 to 30, 30 to 50, and ≥50% of the
fruit epidermis covered with russet.
Data were subjected to statistical analysis using
SAS (SAS Institute Inc., Cary, NC, USA). The effects
of ATS rates and years were subjected to the analysis
of variance, at 5% probability, using covariance
structures for repeated measures in Proc Mixed.
In case of signicance, the means were analysed
through orthogonal polynomial contrasts, at 5%
probability. When necessary, the percentage data were
transformed by arcsine square root (arcsine√x/100) to
meet the assumptions of normality and homogeneity
of variances of the analysis of variance.

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DOI: 10.1590/S0100-204X2018001000006
Results and discussion
After ATS spraying, fruit set decreased inversely
proportional to the rate of blossom thinner application
(Table 1). The same pattern was observed with the
variable crop load, which decreased according to the
enhancement of ATS rate. For fruit set, the rate of ATS
was not inuenced by the environmental condition
at spring, making thinning intensity caused by ATS
not to variate yearly, although crop load and yield
efciency varied between years as indicated by the
F-test. Both variables were linearly responsive to the
rate of ATS, even though the higher rates over thinned
the trees. There is no signicant interaction of ATS
with year, indicating that both sources of variation are
independent for these variables. According to Byers
(2003), an ideal yield efciency should be between
1.0 and 1.5 kg of fruit per cm2 in the United States,
to maintain fruit with good storability, market appeal,
and protability. In the present study, the elevation of
the rates of ATS reduced the yield efciency below the
values indicated by the author.
Similar results and trends for crop load and fruit
set were reported, with ATS at rates from 0.5 to 1.5%
that had a linear thinning response according to the
concentration when sprayed at bloom (Bound &
Wilson, 2007). Many authors suggest that ATS from
1 to 2% is effective to reduce crop load at a level that
produces good marketable fruit, while higher rates
overthin several apple cultivars (Basak, 2006; Bound
& Wilson, 2007; Milić et al., 2011). Based on the crop
load data in this experiment, during the rst crop
season, it is unlikely that ATS 1.5% was effective to
reduce crop load, which was the same found in the
untreated control, whereas ATS at 2.5 and 3.5% were
to some extent similar. Thereby, ATS at 2.5% was
more effective to adjust crop load to desirable levels.
Moreover, on the second crop season, ATS at 1.5%
exhibited a similar effect as at 2.5%. Crop load and
fruit set showed over thinning at 3.5%, making this
rate unsafe in the Southern Brazil conditions.
Cluster size in 2015/2016 with single and double
fruit showed a quadratic response to ATS application,
in which ATS at 3.5% promoted more single-fruit
clusters than ATS at 1.5 and 2.5% (Table 2). While
double-fruit clusters were more prominent at 1.5 and
2.5%, ATS decreased the number of single fruit and
increased the number of double ones. On the second
crop season, no ATS effect was observed, according
to the orthogonal polynomial contrast. Based on
the F-test considering both years, ATS increased
the percentage of single-fruit clusters. Double-fruit
clusters showed an year-dependent effect of ATS.
These results agree with those by Hampson & Bedford
(2011), in which on a two-year experiment ATS at 1.6%
had a variable effect of inducing single-fruit clusters,
and signicantly induced the predominance of both
single- and double-fruit clusters, in comparison to a
single-fruit cluster from the control with hand-thinned
blossom. Single-fruit clusters are prone to produce
fruit up to 6 g heavier in 'Gala' apples (Link, 2000)
and signicantly heavier in 'Ambrosia' apple than in
doubles or triples (Hampson & Bedford, 2011).
Table 1. Effect of ammonium thiosulfate (ATS) on fruit set, crop load, and yield efciency of 'Maxi Gala' apple trees.
ATS rate
(%)
Fruit set (%) Crop load (fruit number cm-2 TC SA) Yield efciency (kg cm-2 TCSA)
2015/ 2016 2 016/2017 2015/ 2016 2 016/2017 2015/2016 2016/ 2017
014 16 12.00 7. 58 1.36 1.01
1.5 10 912.0 0 4.26 1. 23 0.71
2.5 8 6 7.9 6 4.57 1. 00 0 .75
3.5 6 4 7.0 7 2 .79 0.81 0.44
Linear ** *** ** *** ** ***
Quadratic ns ns ns ns ns ns
Coefcient of variation (%) 34.95 32.75 2 3.18 29.88 21.89 26.83
F-test
ATS *** *** ***
Season ns *** ***
ATS x Season ns ns ns
(–)Dash: no data. nsNonsignicant, by the orthogonal polynomial contrasts and F-test. *, **, ***Signicant at 5, 1, and 0.1% probability, respectively.
TCSA, trunk cross-sectional area.

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In the rst year, the reduction of single-cluster
size might have taken place due to insufcient time
from the spraying to the effective pollination of the
king owers, while at the second year this timing was
respected, indicating that ATS application should be
done after the king owers are fully pollinized. In
'Elstar' apples, pollen tube is required to have grown at
least 80% to reach the ovary to form fruit; when ATS
is sprayed in pollen tube with 50% growth, there is a
50% probability of fertilization, and it can take 49 to
95 hours for 100% pollen tube growth, depending on
the weather conditions (Maas, 2016).
For fruit weight, ATS expressed a linear effect of
increasing fruit weight as the rate was increased, in the
rst year; however, in the following year, the response
was quadratic, with less weight gain at the highest
ATS rate (Table 3). Flesh rmness was not affected
by ATS in the rst season, and at the second season,
it showed a quadratic response. Additionally, in the
second year, esh rmness was inversely proportional
to fruit weight. The F-test conrms that the effect of
ATS varies along the years for fruit weight. These
results agree with those by De Salvador et al. (2006),
who concluded that in 'Golden Delicious' and 'Red
Chief' apples, heavy crop loads lead to lighter fruit,
but with high-esh rmness.
Caustic blossom thinners have a second indirect mode
of action as a thinning agent by affecting leaf-stomatal
conductance and diminishing photosynthetic rate, which
can last over 30 days after the treatment with LS, and it
is linear with the elevation of rate and timing, which in
turn might be involved in the trade-off between elevated
rates of caustic thinners and lag in fruit development
(McArtney et al., 2006). Another point to consider is that
at bloom, leaves are still developing, have a thin cuticle,
and are consequently more susceptible to injury caused
by caustic thinners. In sweet cherry, ATS affected leaf-
net carbon exchange rate up to 17 days until the leaves
fully recovered. Caustic thinners increase the chlorophyll
uorescence, indicating that damage occurs in the light
harvesting complex of the photosystem II, leading to
dissipation of electrons through nonphotochemical
processes, in addition to the reduction of chlorophyll
content up to seven days to recover (Lenahan & Whiting,
2006, 2008).
Fruit shape (length:diameter ratio) was not altered
by ATS and, in our trial, a highly signicant effect of
year was found, and there was a signicant difference
for fruit shape caused by season as shown by the
F-test (Table 3). These results agree with those by
Bound & Wilson (2007), in which no difference for
fruit shape was found for any rate of ATS (0.5 to
1.5%). Fruit shape can be inuenced by sunlight
exposure throughout the season, which promotes more
elongated 'Golden Delicious' apple, in comparison to
a more shaded environment (Noè & Eccher, 1996).
In cloudy crop seasons, the denser atmosphere lters
a part of the photosynthetic active radiation, which
increases the proportion of ultraviolet radiation, and
reduces the biosynthesis of endogenous gibberellins,
Table 2. Effects of ammonium thiosulfate (ATS) on the cluster size of 'Maxi Gala' apple trees.
Rate of ATS
(%)
Cluster size
Sin gle (%) Dou ble (%)(1) Triple or more (%)(1)
2015/ 2016 2 016/2017 2015/ 2016 2016/2 017 2015/2016 2016 /2 017
087. 5 74. 2 10.0 24.8 2.5 1.0
1.5 67.1 64.8 26.6 26.0 6.3 9.2
2.5 60.1 75.4 29. 2 19.4 10.7 5.4
3.5 70.2 59.2 23.7 31. 8 6.1 9. 0
Linear ** ns ** ns ns ns
Quadratic ** ns *ns ns ns
Coefcient of variation (%) 12 .11 -2 7.14 ---
F-test
ATS *ns ns
Season ns ns ns
ATS x Season ns *ns
(1)Data were transformed by arcsine square root. (–)Dash: no data. nsNonsignicant, by the orthogonal polynomial contrasts and F-test. *, **Signicant at
5 and 1% probability, respectively.

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as it is modulated by light perception (phytochromes),
leading to a reduction of cell elongation, which results
in attened fruit, increasing the likelihood of russeting
(Hajnajari & Eccher, 2006).
ATS affected total soluble solids (TSS) of fruit only
in the second year, by increasing linearly with the rate.
TSS was also affected by the weather conditions of
the crop year, showing greater means at the second
season as shown by the F-test. Bound & Wilson (2007)
reported a TSS increase when ATS rate was increased.
Similar results were found by Meland (2009), who
reported a both TSS and fruit weight decreases when
crop load increased in 'Elstar' apple.
Seed number had a linear increase with the
increment of ATS rate in the rst season, and a linear
decrease with increased ATS rate in the second season
(Table 3). The effect of ATS on seed number is
dependent on weather conditions annually. These
results differ from those reported by Bound &
Wilson (2007), whose experiment showed that ATS
did not affect seed number in 'Hi-Early Delicious'
apple. Additionally, McArtney et al. (2006) stated
that caustic thinners eventually shrink the number
of fertilized ovules, which in turn could reduce seed
number. Seed number in apple fruit is an important
indicator of fruit quality, as it is strongly related to fruit
shape and weight (Buccheri & Di Vaio, 2004; Garratt
et al., 2014). Therefore, fruit with a higher number
of seeds lead toward a heavier fruit, as observed in
this experiment in the second year, when fruit weigh
showed a quadratic response, and mean seed number
was lesser than that in the previous crop season.
Skin russeting of apple fruit in the rst year was
more pronounced especially in the class ≤10%, in
the rst season, with a high increment, although this
level of russeting does not compromise the marketable
value of fruit. In the second season, there was some
occurrence of russeting in the classes from 10 to 30%
and, in both cases, a quadratic effect was signicant,
with more fruit showing russeting at the rate of 3.5%
in the rst year, and of 1.5 and 2.5% in the second
year that induced more russeting. However, this
phenomenon was caused by year effect rather than ATS
as shown by the F-test (Table 4). Apple fruits are more
susceptible to skin russet at early stages, when fruitlets
are growing at the highest rate, creating an elevated
strain in the cuticle (Hajnajari & Eccher, 2006).
Fruit was not affected by ATS for red color from
50 to 75%, but by season, as shown by the F-test
(Table 5). In the red color group from 40 to 50%, there
was a quadratic effect of ATS in which it decreased
the percentage of fruits at the rates 1.5 and 2.5%, being
the unfavourable weather condition of the rst season
the responsible for enhancing the percentage of fruits
with less color. In a less favorable year (less sunlight),
ATS was effective only at the elevated rate to promote
color. In the red color group of <40%, in both crop
seasons, the trend inversely followed the pattern of the
red color group from 40 to 50%, in which ATS at 1.5
and 2.5% increased the amount of fruit of this class of
Table 3. Effect of ammonium thiosulfate (ATS) on fruit weight, length:diameter ratio, esh rmness, total soluble solids,
and seed number of 'Maxi Gala' apples.
Rate of ATS
(%)
Fruit weight (g) Length: diameter Flesh rmness (Newton) Total soluble solids (ºBrix) Seed number
2015/ 2016 2 016/2017 2015/2016 2016/ 2017 2015/2 016 2016/ 2017 2015/2 016 2016/ 2017 2015/2 016 2016/ 2017
0107 134 0.93 0.95 77. 4 4 87. 5 8 11.3 0 13.12 5.55 7.09
1.5 110 167 0.94 0.97 75.70 86 .15 10.9 0 14.0 0 5.68 6.6 4
2.5 115 166 0.93 0.96 75.88 84.78 11. 0 0 13.64 7.0 3 5.38
3.5 119 159 0.93 0.95 75.35 8 7.40 11 .01 14. 28 6 .16 5.43
Linear ** *** ns ns ns ns ns *** ***
Quadratic ns *** ns ns ns *ns ns ns ns
CV (%) 6.04 5. 21 - - 7.27 5.78 -5.24 35.8 0 34.84
F-test
ATS *** ns ns ns ns
Season *** *** *** *** ns
ATS x Season *** ns ns ns ***
CV, coefcient of variation. (–)Dash: no d ata. nsNonsignicant, by the orthogonal polynomial contrasts and F-test. *, **, ***Signicant at 5, 1, and 0.1%
probabilit y, respect ively.

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color, according to the orthogonal polynomial contrast.
ATS showed some effect by slightly increasing the
percentage of fruit with less red color. Even though,
in the other groups, the variable was inuenced by
weather conditions rather than ATS. There is a high
positive correlation between fruit color and crop load
where decreasing the latter, there is an increment in
the trend to enhance red color of apples and European
plums (Link, 2000; Meland, 2007, 2009). Anthocyanin
biosynthesis is highly dependent on light, temperature,
and TSS levels. In an environment with low light and
elevated temperature, most of the sucrose is consumed
through respiration, while at lower temperatures and
high luminosity there is more sucrose available, which
leads to elevated TSS and enhanced anthocyanin
biosynthesis in apples (Shü et al., 2001).
Tab le 4. Effect of ammonium thiosulfate (ATS) on the russeting occurrence on 'Maxi Gala' apples skin in the 2015/2016 and
2016/2017 crop seasons.
Rate of ATS
(%)
Russeti ng on 'Maxi Gala' apples
0% ≤10% 10 to 30% 30 to 50%(1) ≥50%( 1)
2015/ 2016 2 016/2017 2015/2016 2016/ 2017 2015/2 016 2016/2017 2015/ 2016 2016/2 017 2015/2016 2016/ 2017
027. 0 64.0 40.5 34.0 25.0 2.0 7.0 00.5 0
1.5 29.1 36.0 39.3 48.0 20.2 16 .0 8.0 03.4 0
2.5 30.3 41.4 34.8 42.6 23.8 16 . 2 8.6 02.6 0
3.5 28.6 41.0 48.1 50.0 20.0 9.0 7. 0 00.5 0
Linear ns ns ns ns ns ns ns -ns -
Quadratic ns ns *ns ns ** ns -ns -
CV (%) - - 28.25 - - 37. 32 ----
F-test
ATS ns ns ns ns ns
Season ** ns *** *** *
ATS x Season ns ns ns ns ns
(1)Data were tra nsformed by arcsine square root. (–)Dash: no data . nsNonsignicant, by the orthogonal polynomial contrasts and F-test. *, **, ***Signicant
at 5, 1, and 0.1% probability, respectively. CV, coefcient of variation.
Tab le 5. Effect of Ammonium thiosulfate (ATS) on the percentage of red color on 'Maxi Gala' apple skin in the 2015/2016
and 2016/2017 crop seasons.
Rate of ATS
(%)
Red color on 'Maxi Gala' apple skin
50 to 75%(1) 40 to 50% <40%(1)
2015/ 2016 2016/2017 2015/2016 2016/ 2017 2015/2 016 2016/ 2017
06.4 71.8 64.4 26.0 2 9.8 2.8
1.5 9.4 65.2 44.0 31.0 46.8 4.2
2.5 4.6 65.8 51.6 2 9.8 43. 8 4.6
3.5 19.2 79.6 58.8 17.0 22.0 3.4
Linear ns ns ns ns ns ns
Quadratic ns ns ** ns ** *
Coefcient of variation (%) - - 19.18 -21.50 18. 38
F-test
ATS ns ns ***
Season *** *** ***
ATS x Season ns ns ns
(1)Data were tra nsformed by arcsine square root. (–)Dash: no data . nsNonsignicant, by the orthogonal polynomial contrasts and F-test. *, **, ***Signicant
at 5, 1, and 0.1% probabilit y, respect ively.

1138 L. De R. Marchioretto et al.
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DOI: 10.1590/S0100-204X2018001000006
Conclusions
1. Ammonium thiosulfate spray at 2.5% is effective
to reduce crop load and to improve fruit quality in
'Maxi Gala' apples (Malus domestica).
2. The thinning effect of ammonium thiosulfate on
'Maxi Gala' apples is not dependent on the weather
conditions during the crop season.
3. High rates of ammonium thiosulfate cause
overthinning on 'Maxi Gala' apples trees and is not
associated with improved fruit quality.
Acknowledgments
To Coordenação de Pessoal de Nível Superior
(Capes), for scholarship grant; and to Conselho
Nacional de Desenvolvimento Cientíco e Tecnológico
(CNPq), for nancial support.
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Received on August 16, 2017 and accepted on March 27, 2018