HORTSCIENCE 46(3):432–438. 2011.
Ethephon as a Blossom and Fruitlet
Thinner Affects Crop Load, Fruit
Weight, Fruit Quality, and Return
Bloom of ‘Summerred’ Apple
(Malus ·domestica) Borkh.
Mekjell Meland1 and Lars Sekse
Norwegian Institute for Agricultural and Environmental Research, Bioforsk,
Ullensvang, N-5781 Lofthus, Norway
Clive Kaiser
Extension Faculty, Department of Horticulture, Oregon State University, 418
N. Main Street, Milton-Freewater, OR 97862
Additional index words. chemical thinning, PGR, flowering, abscission, fruit set, yield
Abstract. ‘Summerred’ apples (Malus domestica Borkh.) are highly susceptible to biennial bearing if not properly thinned. This results in erratic yields and also affects fruit
quality adversely. Between 2003 and 2005, ‘Summered’/‘M9’ trees were treated with
ethephon at concentrations of 250, 375, and 500 mgL–1 when most king flowers opened
’20% bloom) or at concentrations of 500, 625, and 750 mgL–1 when the average fruitlet
(’
size was 10 mm in diameter. The experiment was conducted with 2.5-m height slender
spindle trees sprayed to the point of runoff with a hand applicator only when temperatures exceeded 15 8C. Within 2 weeks after the second application, fruit set was
reduced linearly with increasing concentrations of ethephon to less than one fruitlet
per cluster at the highest concentrations used. Most thinning treatments reduced fruit
set significantly compared with unthinned trees. Fruit numbers per tree decreased
significantly with increasing ethephon concentrations, and the highest concentrations of
ethephon applied during bloom or when the average fruitlet size was 10 mm in diameter
resulted in overthinning. Yield results confirmed the fruit set response in which yield
reductions were significant at the highest concentrations of ethephon (2.1 kg/tree)
compared with hand-thinned trees (7.3 kg/tree) in 2005. All thinning treatments resulted
in higher percentage of fruits larger than 60 mm diameter compared with unthinned
control fruit. Thinning resulted in significantly higher soluble solid contents, and this was
especially so for hand-thinned trees. Other fruit quality parameters like yellow–green
background color did not show a clear response to thinning. Return bloom was, however,
improved on all thinned trees. It is recommended that ethephon be applied at a rate of
375 mgL–1 when king flowers open or at a rate of 625 mgL–1 when the average fruitlet
size is 10 mm in diameter. This thins ‘Summerred’ apples to a target of approximately
five fruits/cm2 per trunk cross-sectional area or 50 to 70 fruits per 100 flower clusters
without impacting on fruit quality, yield, or return bloom the next year.
Biennial bearing is a major problem in
many apple-producing areas of the world
(Schmidt et al., 2009) and Norway is no exception. As a result of overcropping in the
‘‘on-year,’’ fruit size and quality are reduced.
The next year, yields are markedly reduced
and oversized fruit may result. This inconsistent yield pattern results in volume and quality
problems both for the growers and the market.
Received for publication 9 Nov. 2010. Accepted
for publication 18 Jan. 2011.
We thank the Norwegian Research Council for
financing the work. We also acknowledge Magne
Eivind Moe for technical assistance with field work
and Sigrid Flatland for fruit quality assessments.
1
To whom reprint requests should be addressed;
e-mail mekjell.meland@bioforsk.no.
432
The main horticultural practices for controlling biennial bearing are renewal pruning,
growth control, and blossom or fruit thinning
chemically, by hand, or a combination of the
two. Blossom thinning is a standard procedure
for improving fruit size, increasing return
bloom, and promoting regular bearing (Byers,
2003; Jonkers, 1979). Reducing the number of
fruits per tree increases the amount of leaf area
per fruit and hence more photoassimilates are
available to the remaining fruit. Thinning can,
however, result in variable responses. Both excessive and inadequate thinning can be costly
for apple growers. Consequently, it is essential
to increase precision of the thinning process.
Apple seeds are a major source of gibberellic acid, which can be translocated into the
tree and may inhibit flower formation for the
next season (Chan and Cain, 1967; Luckwill
et al., 1969). Blossom thinning results in early
removal of flowers, eliminating their competitive effects on photosynthetic assimilates or
on the endogenous hormonal balance, important for flower bud induction. Blossom thinners
are more effective than fruitlet thinning
for encouraging consistent annual bearing
(Meland and Gjerde, 1993). Blossom thinning
improves fruit size and fruit quality at the same
cropping level (Meland, 2011). In Norway,
ethephon is the only chemical registered for
blossom thinning. The only other registered
product, a liquid fertilizer, ammonium thiosulphate, is, however, less effective.
For blossom thinning agents to be effective,
certain minimum weather requirements must
be met during the application time. In the event
of unfavorable weather during bloom, growers
should be afforded another opportunity for
thinning during the young fruitlet stage.
In Europe, ethephon is used as a flower
thinner for biennial-bearing apple trees; however, it has been shown to be risky because it can
easily result in overthinning (Jonkers, 1979).
Ethephon also promotes flowering the next year
(Buban and Sagi, 1976; Stopar and Zadravec,
2004).
‘Summerred’ apples are highly prone to
biennial bearing in Norway if not properly thinned resulting in poor fruit quality and erratic
yields. Ethephon can be an effective thinner;
however, results are dependent on a.i. concentration, ambient temperatures, the volume of
water applied per unit area, and time of application (Jones et al., 1983, 1989, 1990; Stover
and Green, 2005). The objective of this study
was to investigate the effects of ethephon as a
blossom and fruitlet thinning agent for ‘Summered’ apple trees.
Materials and Methods
Field trials with the apple cultivar Summerred grafted on ‘M9’ rootstocks were conducted in a commercial orchard near Bioforsk
Ullensvang, western Norway (60 N) in 2003,
2004, and 2005. Productive, 10-year-old, uniform slender spindle trees, spaced at 1.4 m ·
4 m apart and pruned to a maximum height
of 2.5 m were selected in 2003. The soil
was a sandy-loam with 4% organic matter.
Orchard floor management consisted of frequent mowing of the interrows and a 1-m wide
herbicide strip was maintained in the intrarow.
Trees were irrigated by drip irrigation when
water deficits occurred. All trees received the
same amount of fertilizers based on soil and
leaf analysis. In 2003, trees were sprayed in
May and June when maximum daily temperatures were 15.6 and 15.3 C, respectively.
Solar radiation on both days was 3900
kWm–2. In 2004, trees were sprayed in May
and June when maximum daily temperatures
were 20.8 and 18.4 C, respectively. Solar
radiation on both days was 5908 and 5665
kWm–2, respectively. In 2005, trees were
sprayed in May and June when maximum daily
temperatures were 14.1 and 16.8 C, respectively. Solar radiation on both days was 6001
and 3812 kWm–2, respectively. On the days
succeeding all sprays, daily temperature maxima did not exceed more than 3 C above or
HORTSCIENCE VOL. 46(3) MARCH 2011
GROWTH REGULATORS
below the maximum daily temperature on the
day of application.
Blossom clusters were counted on each tree
and trunk circumference was recorded at 25 cm
above the soil level. The experiment was
blocked by the number of blossom clusters per
square centimeter trunk cross-sectional area
(TCSA) each year. The experimental design
was a completely randomized block design
with six whole tree plots per replication. Unsprayed control and hand-thinned trees were
compared against trees sprayed with ethephon
at concentrations of 250, 375, and 500 mgL–1
when king flowers opened (20% bloom) or
sprayed with ethephon at concentrations of
500, 625, and 750 mgL–1 when the average
fruitlet size reached 10 mm in diameter. A
commercial formulation of ethephon ‘Cerone’
48% a.i. ethephon (w/v) (Bayer Crop Science,
Monheim am Rhein, Germany) was used for
this study in each of the 3 years. Ethephon was
applied on the day closest to the indicated phenological stage, when conditions were considered best for chemical thinning applications,
i.e., when maximum temperatures were between 15 and 20 C, slow drying conditions,
and no rain expected for at least 6 h.
Ethephon treatments were applied to whole
trees as dilute sprays at different concentrations with a handgun and a small portable
sprayer. Trees were sprayed to the point of
runoff with 2 L/tree. No surfactants or any
additives were included in the sprays. In all 3
years of the study, hand-thinning was conducted during the latter half of June and fruit
were thinned to between 10 and 15 cm apart.
During the 2 first years of the trial, the
numbers of flowers or fruitlets that dropped
were recorded on a weekly basis for each of 10
clusters of flowers/fruitlets distributed evenly
around the tree. Counting started immediately
after the first application of ethephon and continued until the end of August. Fruit size
measurements were taken weekly for five
tagged fruits/tree. Measurements started when
fruit size was between 10 and 14 mm in
diameter and continued weekly until harvest.
At harvest, fruit were selectively picked
on two occasions, 1 week apart. Fruit were
harvested according to commercial fruit standards and the first selective pick dates were
19 Sept. 2003, 16 Sept. 2004, and 22 Sept.
2005, respectively. All the fruit from each
tree were graded into two classes (greater than
60 mm diameter and less than 60 mm diameter).
Total mass of each class and total number of
fruit per class were recorded. Number of recently dropped fruit were recorded separately
and assumed to be of average fruit weight. A
sample of 10 randomly selected fruits from each
tree was taken at the first selective pick and
this was used to measure external and internal
fruit quality [fruit weight, firmness, scores for
yellow–green background color and red foreground (blush) color, soluble solids, starch
content, and seed weight]. Flesh firmness was
measured by a digital table penetrometer with
a 11-mm probe (PenefelÒ; CTIFL France,
Vandoeuvre-lès-Nancy, France) and percentage soluble solids was measured using a handheld AtagoÒ (Tokyo, Japan) refractometer
HORTSCIENCE VOL. 46(3) MARCH 2011
(using juice collected from the measurements
of flesh firmness). The starch–iodine score
was measured by spraying two apple halves
with 0.1 M iodine solution and giving scores
for starch content (1 = all tissues stained black
to 10 = no staining or starch presents).
Background color was scored on a scale from
1 to 9 in which 1 = dark green and 9 = bright
yellow. Similar scores were recorded for the
red foreground color in which 1 = no red color
and 9 = red color covering the entire fruit
surface. In the spring after the thinning treatments, the total number of flower clusters per
tree was counted as a measure of return bloom.
Statistical analysis. The data were evaluated by general analysis of variance for randomized block designs using the statistical program
Minitab 15 statistical software (MinitabÒ Inc.,
State College, PA) testing for differences between all crop load parameters and effects on
fruit quality. The main effects of thinning time
and ethephon concentration were analyzed for
linear trends. Unless noted otherwise, only
results significant at P # 0.05 are discussed.
Results
2003. Both the trunk section and the
average number of flower clusters per tree were
uniform at the start of the experiment. On
average, there were approximately six flowers
per cluster at bloom. Within 2 weeks after
blossom application, fruit set was reduced
linearly with increasing concentration of ethephon to less than one fruitlet per cluster (Fig.
1). The highest concentration of ethephon
(500 mgL–1) applied during bloom resulted
in overthinning. A similar result was seen with
the highest ethephon concentration (750
mgL–1) when applied to young fruitlets in
which initial fruit set was reduced to less than
one fruit per cluster within 2 weeks of either
treatment. Natural fruit drop on the unthinned
control trees continued throughout the entire
growing season; however, the most severe
drop occurred within 1 month after bloom to
an average to 2.1 fruitlets per flower cluster.
A total of 250 mgL–1 ethephon applied at
bloom, or 625 and 750 mgL–1 applied when the
average fruitlet diameter was 10 mm, thinned
the fruit to the desired target of approximately
five fruit/cm2 TCSA or a fruit set between 60
and 70 fruit per 100 clusters. Yield was reduced
by half compared with unthinned control trees
(19.5 kg/tree versus 42.6 kg/tree) but was
comparable with hand-thinned trees (18.9 kg/
tree) (Table 1).
All thinning treatments resulted in significantly higher percentages of fruit larger than
60 mm fruit size (between 89% and 100%)
when compared with unthinned trees (71%)
(Table 2). Average fruit weight also increased
in response to all thinning treatments (between
110 g and 230 g/fruit) compared with unthinned control fruit (98 g/fruit). The largest
fruit were harvested from trees with the lowest
fruit set. Soluble solids content was generally
low in all thinning treatments (between 9.9%
and 12.9%); however, this was significantly
higher than the unthinned control trees (8.7%).
In the current study, ethephon applications had
no effect on red foreground color (results not
shown) of ‘Summerred’ apples but it did improve average background color of the fruit
skins (between 1.4 and 1.9) compared with
unthinned control trees (1.3). Neither the average number of seeds/fruit nor average seed
weight was affected by fruit or flower thinning
(Table 2). Except for the 500 mgL–1 ethephon
applied when the average fruitlet size was
10 mm in diameter (129 flower clusters/tree),
all other ethephon treatments improved return
Fig. 1. Flower/fruitlet drop of ‘Summerred’ apples in response to different ethephon concentrations applied
either when most king flowers were open (Bloom) (19 May 2003) or when average fruitlet size reached
10 mm in diameter (Fruitlet) (13 June 2003).
433
bloom (between 213 to 281 flower clusters/
tree) compared with hand-thinned (160 flower
clusters per tree) or unthinned control trees
(148 flower clusters/tree) (Table 1). Furthermore, the larger the crop load, the lower the
amount of return bloom the next year, which
also explains why trees sprayed with 500
mgL–1 ethephon when average fruit size
reached 10 mm in diameter had the lowest
return bloom. Indeed, the only negative effect
of applying ethephon at all concentrations in
2003 was its effect on fruit firmness, which
ranged from 5.6 to 5.9 kg in fruit treated with
ethephon compared with hand-thinned trees
(6.2 kg) or those on unthinned trees (6.0 kg).
2004. Fruit set was reduced significantly
by ethephon and the highest concentration
at bloom (500 mgL–1) (Table 3). Likewise,
when the average fruitlet size reached 10 mm
in diameter (750 mgL–1), it also resulted
in overthinning. Similar patterns of flower/
fruitlet drop were seen as in 2003 (Fig. 2);
however, the effects of ethephon were not as
marked. Crop load was reduced to approximately one-third that of the unthinned control
trees. The number of flower clusters per TCSA,
final fruit numbers per TCSA, and final crop
load on the unthinned trees were all less than
the year before. Both the 375 mgL–1 and 500
mgL–1 ethephon sprays applied during bloom
and the 625 mgL–1 ethephon spray when
average fruitlet size was 10 mm in diameter
resulted in satisfactory fruit thinning (92, 75,
and 98 fruit, respectively) compared with
hand-thinned trees (75 fruit) and unthinned
trees (133 fruit) per 100 flower clusters at
harvest. Thinning effects increased linearly
with increasing ethephon concentrations and
this was reflected in both yield of bigger fruit
(Table 3) and fruit weight (Table 4). Increase
in fruit size as a function of higher ethephon
concentrations was a function of a smaller
crop load (Fig. 3). It was thus not surprising
that increasing ethephon concentrations resulted in a linear increase in fruit weight and
this was inversely proportional to the number
of fruit/cm2. In addition, the percentage of
small fruits (less than 60 mm diameter) was
reduced, whereas soluble solids and fruit background color were significantly improved
when compared with fruit from unthinned
trees. There were, however, no significant
differences in fruit foreground color (results
not shown) and fruit firmness. Seed weight
was correlated strongly with the number of
seed, but there were no significant differences in either of these parameters (Table 4).
Return bloom was significantly improved
for all thinned trees [averaging between 79
and 93 flower clusters/tree (Table 3)] compared with 67 flower clusters/tree on the
unthinned control trees, which was surprisingly high.
Table 1. Effects of ethephon, applied to ‘Summerred’ apple trees as a thinning agent, either when most king flowers were open (Bloom) or once fruit size reached
an average diameter of 10 mm (Fruitlet), on mean fruit set, mean fruit number, and mean yield in 2003 and on return bloom in 2004.
Treatment
Unthinned
Hand-thinned
Bloom (250 mgL–1)
Bloom (375 mgL–1)
Bloom (500 mgL–1)
Fruitlet (500 mgL–1)
Fruitlet (625 mgL–1)
Fruitlet (750 mgL–1)
Least significant difference 5%
TCSA = trunk cross-sectional area.
NS = nonsignificant.
No. of flower
clusters/cm2
(TCSA)
9.0
9.1
9.2
9.2
9.1
8.9
9.5
9.9
NS
No. of fruit
per 100 flower
clusters at harvest
133
66
57
36
19
135
75
63
32
No.
fruits/cm2
(TCSA)
11
5.3
5.1
3.3
1.8
11.2
5.7
5
2.3
Yield
(kg/tree)
42.6
18.9
19.6
13.8
8.3
28.1
18.1
16
12.2
No.
apples/tree
296
131
131
72
39
258
147
112
117
Return bloom
(No. of flower
clusters/tree in 2004)
148
160
254
213
281
129
244
247
65
Table 2. Effects of different ethephon concentrations, applied to ‘Summerred’ apple trees in 2003 as a thinning agent either when most king flowers were open
(Bloom) or once fruit size reached an average diameter of 10 mm (Fruitlet), on percentage bigger fruit, mean fruit weight, mean soluble solids, mean ground
color, mean fruit firmness measured using a PenefelÒ firmometer, mean number of seeds per fruit, and mean seed weight.
Treatment
Unthinned
Hand-thinned
Bloom (250 mgL–1)
Bloom (375 mgL–1)
Bloom (500 mgL–1)
Fruitlet (500 mgL–1)
Fruitlet (625 mgL–1)
Fruitlet (750 mgL–1)
Least significant difference 5%
Percentage of yield of
fruit with diam greater
than 60 mm (%)
71
99
94
100
100
89
95
99
11.0
Fruit wt
(g)
98
144
147
206
230
110
123
141
25
Soluble
solids (%)
8.7
10.2
10.7
11.9
12.9
9.9
10.4
11.3
0.3
Background
color
1.3
1.5
1.4
1.7
1.9
1.6
1.6
1.8
Firmness
(kg)
6
6.2
5.9
5.6
5.7
5.6
5.9
5.7
0.3
No.
seed/fruit
2.8
2.5
1.9
2.4
2.0
2.7
3.5
2.8
0.7
Seed
wt (g)
0.186
0.186
0.122
0.173
0.133
0.190
0.259
0.207
Table 3. Effects of ethephon, applied to ‘Summerred’ apple trees as a thinning agent either when most king flowers were open (Bloom) or once fruit size reached
an average diameter of 10 mm (Fruitlet), on mean fruit set, mean fruit numbers, and mean yield in 2004 and on return bloom in 2005.
Treatment
Unthinned
Hand-thinned
Bloom (250 mgL–1)
Bloom (375 mgL–1)
Bloom,(500 mgL–1)
Fruitlet (500 mgL–1)
Fruitlet (625 mgL–1)
Fruitlet (750 mgL–1)
Least significant difference 5%
TCSA = trunk cross-sectional area.
434
No. of flower
clusters/cm2
(TCSA)
7.4
7.4
7.5
7.9
8.1
8.2
8.2
8.5
NS
No. of fruits
per 100 flower clusters
at harvest
133
75
92
75
42
98
47
43
34
No. of
fruits/cm2
(TCSA)
9.1
5.0
6.1
5.0
3.4
7.3
3.7
3.3
2.3
Yield
(kg per tree)
27.6
21.4
19.5
19.9
12.3
20.4
9.9
8.9
8.0
No.
fruit/tree
253
144
159
136
84
191
77
67
71
Return bloom
(no. of flower
clusters/tree in 2005)
67
85
79
88
88
88
92
93
13
HORTSCIENCE VOL. 46(3) MARCH 2011
2005. Number of flower clusters/cm2 of
TCSA at bloom for each treatment was significantly higher than either of the 2 previous
years reaching as many as 10.9 flower clusters/
cm2 of TCSA (Table 5). Again, the different
concentrations of ethephon applied either
when the king flowers opened or when the
average fruit diameter reached 10 mm in size
resulted in a linear reduction in the average
number of fruit/cm2 per TCSA (ranging from
18.5 to 8.9) compared with unthinned control
trees (23.4) and hand-thinned trees (21); the
average number of fruits/tree (ranging from 54
to 22) when compared with unthinned control
trees (101) and hand-thinned trees (80); and
average yield (ranging from 5.5 to 2.1 kg/tree)
compared with unthinned control trees (10.5)
and hand-thinned trees (7.3). To summarize,
the two highest concentrations of ethephon
applied at bloom, and all three concentrations
applied when the average fruitlet size reached
10 mm in diameter, resulted in fruit overthinning (Table 5). Again, the fruit growth
pattern (Fig. 4) demonstrated that largest fruits
were found on trees with the lowest crop
load and the differences were established
early during fruit development. Here, fruit
from unthinned control trees had the smallest fruit diameter (58 mm) on average at
harvest compared with those fruit from
hand-thinned trees (67 mm) or treated with
ethephon (ranging from 64 to 78 mm at
harvest) (Table 6).
Discussion and Conclusions
Fruit set. In this study, ethephon was an
effective thinning agent for reducing fruit set
of ‘Summerred’ apples when applied at different concentrations either during blossom,
when most king flowers had opened, or when
the average fruitlet size reached 10 mm in
diameter. However, there are many additional
factors that must to be taken into account
before consistent thinning with ethephon may
be achieved. These factors include crop load
(Schmidt et al., 2009), spray volume, and spray
concentration.
Fig. 2. Flower/fruitlet drop of ‘Summerred’ apples in response to different ethephon concentrations applied
either when most king flowers were open (Bloom) (10 May 2004) or when average fruitlet size reached
10 mm in diameter (Fruitlet) (8 June 2004).
Several other studies on the effects of
ethephon as a fruit thinner found marked effects
of both daily maximum temperatures for up to
3 d after application (Stover and Green, 2005)
and total solar radiation (Byers et al., 1990). In
this study, we did not observe any of these effects and we believe that the cool, mesic temperate climate in Norway is responsible for this
anomaly.
After the bloom application of ethephon,
final fruit set was achieved within 2 weeks and
little subsequent fruit drop occurred during the
season. During the first month after bloom,
fruit set was reduced by up to one-third.
Unthinned control trees continue to drop fruit
regularly throughout the growing season and
the final set was approximately two fruits/
cluster. No ‘‘June drop’’ peak was observed.
Similarly, ethephon applications when the
average fruitlet size reached 10 mm in diameter resulted in rapid fruit drop and final
fruit set numbers were established within 2
weeks of application (Figs. 1 and 2).
In the current study, regardless of the time
of application, increasing ethephon concentrations resulted in a linearly reduction in
fruit set. Applying ethephon during bloom,
however, had a stronger thinning effect than
later applications when the average fruitlet
size reached 10 mm in diameter despite higher
concentrations being applied. This corresponded
with the model developed by Koen and Jones
(1985) for ‘Golden Delicious’ apple trees, in
which trees were most sensitive to thinning
during flowering but at the pink bud stage.
Post-bloom applications required higher rates
to achieve similar reductions in fruit set.
Wertheim (1997) suggested that ethephon is
most active when the natural tendency for
apple fruitlet drop is highest. At petal fall,
higher rates were needed than at the start of the
‘‘June drop’’ period. However, in the current
study, flower applications were done well in
advance of the ‘‘June drop,’’ which normally
occurs in early July under Norwegian conditions. Furthermore, during all 3 years of the
current study, 250 mgL–1 ethephon applied
when most king flowers were open or 625
mgL–1 when average fruitlet size reached
10 mm in diameter reduced the crop load to
the target of approximately five fruits/cm2
TCSA, which is equally to 50 to 70 fruits
per 100 clusters. This supports the study on
‘Paulared’/‘MM106’ apple trees, which found
Table 4. Effects of different ethephon concentrations, applied to ‘Summerred’ apple trees as a thinning agent in 2004, either when most king flowers were open
(Bloom) or once fruit size reached an average diameter of 10 mm (Fruitlet), on percentage bigger fruit, mean fruit weight, mean soluble solids, mean ground
color, mean fruit firmness measured using a PenefelÒ firmometer, mean number of seeds per fruit, and mean seed weight.
Treatment
Unthinned
Hand-thinned
Bloom (250 mgL–1)
Bloom (375 mgL–1)
Bloom (500 mgL–1)
Fruitlet (500 mgL–1)
Fruitlet (625 mgL–1)
Fruitlet (750 mgL–1)
Least significant difference 5%
NS = nonsignificant.
Percentage of yield
of fruit with diam
60 mm or greater
83
96
89
97
88
80
94
95
HORTSCIENCE VOL. 46(3) MARCH 2011
NS
Fruit
wt (g)
109
153
130
153
158
113
136
137
29
Soluble
solids (%)
10.2
11.0
10.7
11.6
11.8
11.2
12.8
12.9
1.0
Background
color
3.2
2.9
3.6
3.6
3.3
3.3
4.1
4.1
Firmness
(kg)
4.6
4.8
4.4
4.7
4.6
4.8
5.0
5.0
No.
seed/fruit
2.4
2.2
2.5
2.0
2.1
2.7
3.2
3.2
NS
NS
Seed
wt (g)
0,153
0,150
0,165
0,132
0,143
0,17
0,213
0,207
435
Fig. 3. Fruit growth pattern of ‘Summerred’ apples in response to different ethephon concentrations applied either when most king flowers were open (Bloom) on
10 May 2004 or when average fruitlet size reached 10 mm in diameter (Fruitlet) on 8 June 2004.
Table 5. Effects of different ethephon concentrations, applied to ‘Summerred’ apple trees as a thinning agent, either when most king flowers were open (Bloom) or
once fruit size reached an average diameter of 10 mm (Fruitlet), on mean fruit set, mean fruit numbers, and mean yield in 2005 and on return bloom in 2006.
Treatment
Unthinned
Hand-thinned
Bloom (250 mgL–1)
Bloom (375 mgL–1)
Bloom (500 mgL–1)
Fruitlet (500 mgL–1)
Fruitlet (625 mgL–1)
Fruitlet (750 mgL–1)
Least significant difference 5%
TCSA = trunk cross-sectional area.
NS = nonsignificant.
No. of flower
clusters/cm2
(TCSA)
9.9
9.9
10.1
10.2
10.2
10.1
10.3
10.9
NS
that crop load of mature trees was reduced
significantly when treated with 550 mgL–1
ethephon once the average fruitlet size reached
13 mm in diameter (Embree et al., 2001).
However, in contrast, a previous study found
that ethephon applied at 200 mgL–1 at the start
of flowering reduced fruit set in ‘Summered’
apple trees but did not enhance fruit weight
(Stopar and Lokar, 2003). In another field experiment on ‘Golden Delicious’ apple trees, a
500 mgL–1 ethephon spray applied once the
average fruitlet size reached 5 to 11 mm in
diameter resulted in effective fruit thinning but
had no effect on fruit weight (Bukovac et al.,
2006).
Fruit size and quality. Reducing apple crop
load usually increases fruit weights as a result
of less competition for carbohydrates among
the remaining fruit on the tree (Wu}nsche and
Ferguson, 2005). In the current study, fruit
weight increased significantly on hand-thinned
436
No. of fruits per 100
flower clusters
at harvest
255
177
144
84
88
83
104
62
85
No.
fruits/cm2
(TCSA)
23.4
21.0
18.5
11.7
13.2
9.6
10.0
8.9
8.2
and all ethephon-treated trees compared with
unthinned trees in all three seasons and was
negatively correlated with crop load. Furthermore, fruit weight was positively correlated
with the amount of fruitlet drop and fruit
weight was larger when ethephon thinning
took place during bloom rather than when the
average fruitlet size reached 10 mm in diameter. Fruit soluble solid content was positively correlated with the amount of thinning
achieved and increased with reduced fruit set.
Timing of ethephon sprays, however, had no
effect on the soluble solid content of the fruit in
all 3 years of the study in ‘Summerred’ apples.
Embree et al. (2001) found that ethephon
applied to small fruitlets advanced fruit maturity at harvest and increased the level of fruit
color and return bloom the next year. Whale
et al. (2008) found that red foreground color of
‘Cripps Pink’ was markedly increased by
applications of ethephon or aminoethoxyvinyl-
Yield
(kg/tree)
10.5
7.3
5.5
3.5
3.6
3.6
3.9
2.1
3.2
No.
fruit/tree
101
80
54
31
33
36
40
22
31
Return bloom
(no. of flower
clusters/tree in 2006)
80
84
72
76
79
76
83
75
NS
glycine followed by ethephon. In the current
study, foreground color was unaffected by any
of the ethephon applications; however, background color was significantly improved in
2003 and 2004. Fruit softness of ‘Summerred’
apples was mostly unaffected by any of the
ethephon applications when compared with
unthinned trees or those thinned by hand.
Return bloom. A general aspect of crop
management is that heavy thinning will promote an increase in return bloom the next
season. Several studies have shown that
treating apple trees with ethephon during
bloom or right afterward in an ‘‘on-year’’
will promote return bloom (Bukovac et al.,
2006; McArtney et al., 2007; Meland and
Gjerde, 1993; Stopar and Zadravec, 2004).
This observation was also found to be true in
this present study with significant effect in
the 2 first years. In the last season, there was
little effect on return bloom, but this was
HORTSCIENCE VOL. 46(3) MARCH 2011
Fig. 4. Fruit growth pattern of ‘Summerred’ apples in response to different ethephon concentrations applied either when most king flowers were open (Bloom)
on 12 May 2005 or when average fruitlet size reached 10 mm in diameter (Fruitlet) on 21 June 2005.
Table 6. Effects of different ethephon concentrations applied to ‘Summerred’ apple trees in 2005 as a
thinning agent, either when most king flowers were open (Bloom) or once fruit size reached an average
diameter of 10 mm (Fruitlet), on mean yield, mean fruit weight, mean soluble solids, mean fruit firmness
measured using a PenefelÒ firmometer, mean number of seeds per fruit, and mean seed weight.
Percentage of yield
Fruit
with fruit of 60 mm Fruit
Soluble firmness
No.
Seed
Treatment
or greater diam
wt (g) solids (%)
(kg)
seed/fruit wt (g)
Unthinned
58
92
10.1
7.0
2.9
0.201
Hand-thinned
92
121
10.7
7.9
3.7
0.290
–1
94
133
11.2
7.1
2.7
0.216
Bloom (250 mgL )
97
144
12.0
7.3
2.8
0.217
Bloom (375 mgL–1)
97
151
12.3
7.3
2.1
0.154
Bloom (500 mgL–1)
88
111
12.7
7.0
4.3
0.293
Fruitlet (500 mgL–1)
–1
91
121
12.1
7.8
3.5
0.255
Fruitlet (625 mgL )
98
139
13.4
7.4
3.7
0.246
Fruitlet (750 mgL–1)
Least significant difference 5%
10.6
19
1.1
0.3
0.4
most likely the result of a balance being
achieved the previous two seasons.
Under cool temperate conditions like those
in Norway during bloom, daily temperature
maxima do not always exceed 15 C. Consequently, it is imperative for growers to have
a second window of opportunity for fruit
thinning and in ‘Summerred’ apples this was
found when the average fruitlet size reached
10 mm in diameter. However, based on the
findings of the current study, it is recommended that ethephon applications during bloom,
when most king flowers were open, be favored
over later applications when the average fruitlet size reached 10 mm in diameter. Furthermore, it is suggested that a full cover spray of
ethephon at a rate of 250 mgL–1 be applied to
‘Summerred’ apples when most king flowers
are open and maximum daily temperatures are
HORTSCIENCE VOL. 46(3) MARCH 2011
15 C or higher on the day of application and
the subsequent 2 d. If the weather does not
permit this application, then a full cover spray
of 625 mgL–1 ethephon should be applied to
‘Summerred’ apples when the average fruit
size reaches 10 mm in diameter and when
the daily maximum temperatures are 15 C
or greater on the day of application and the
subsequent 2 d. This should achieve the desired
target of approximately five fruits/cm2 TCSA
or 50 to 70 fruits per 100 flower clusters with
little effect on the return bloom the next year
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