In This Section |
Thinning of Apples
 NOTE:
Some materials available here are in PDF (portable document file) format.
To read them you need the FREE Acrobat Reader installed on your computer.
The program is available for download (with installation instructions)
at http://www.adobe.com/products/acrobat/readstep2.html.
Excerpt from Chapter 7, Publication 360, Fruit Production Recommendations, Order this publication
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Minimum size
|
No. of apples required per tree for tree
density, desired yield and fruit size
|
|||||
|---|---|---|---|---|---|---|
| No. of apples required to fill 18 bu. bin |
Projected yield (Bu/ac)
|
|||||
|
Tray Size
|
wt (g)1
|
in
|
mm
|
400
|
600
|
|
|
196
|
102
|
2 1/4
|
57
|
3,920
|
320
|
480
|
|
175
|
114
|
2 3/8
|
60
|
3,500
|
286
|
429
|
|
163
|
123
|
2 1/2
|
64
|
3,260
|
266
|
400
|
|
150
|
133
|
2 5/8
|
67
|
3,000
|
245
|
368
|
|
138
|
145
|
2 3/4
|
70
|
2,760
|
225
|
338
|
|
125
|
160
|
2 7/8
|
73
|
2,500
|
204
|
306
|
|
113
|
177
|
3
|
76
|
2,260
|
185
|
277
|
|
100
|
200
|
3 1/8
|
79
|
2,000
|
163
|
245
|
|
88
|
227
|
3 1/4
|
83
|
1,760
|
144
|
216
|
|
80
|
250
|
3 3/8
|
85
|
1,600
|
131
|
196
|
|
72
|
278
|
3 1/2
|
89
|
1,440
|
118
|
176
|
|
64
|
313
|
3 5/8
|
92
|
1,280
|
105
|
157
|
|
56
|
357
|
3 3/4
|
95
|
1,120
|
92
|
137
|
|
48
|
417
|
3 7/8
|
98
|
960
|
78
|
118
|
|
Minimum size
|
No. of apples required per tree for tree
density, desired yield and fruit size
|
|||||
|---|---|---|---|---|---|---|
| No. of apples required to fill 18 bu. bin |
Projected yield (Bu/ac)
|
|||||
|
Tray Size
|
wt (g)1
|
in
|
mm
|
500
|
750
|
|
|
196
|
102
|
2 1/4
|
57
|
3,920
|
218
|
327
|
|
175
|
114
|
2 3/8
|
60
|
3,500
|
194
|
292
|
|
163
|
123
|
2 1/2
|
64
|
3,260
|
181
|
272
|
|
150
|
133
|
2 5/8
|
67
|
3,000
|
167
|
250
|
|
138
|
145
|
2 3/4
|
70
|
2,760
|
153
|
230
|
|
125
|
160
|
2 7/8
|
73
|
2,500
|
139
|
208
|
|
113
|
177
|
3
|
76
|
2,260
|
126
|
188
|
|
100
|
200
|
3 1/8
|
79
|
2,000
|
111
|
167
|
|
88
|
227
|
3 1/4
|
83
|
1,760
|
98
|
147
|
|
80
|
250
|
3 3/8
|
85
|
1,600
|
89
|
133
|
|
72
|
278
|
3 1/2
|
89
|
1,440
|
80
|
120
|
|
64
|
313
|
3 5/8
|
92
|
1,280
|
71
|
107
|
|
56
|
357
|
3 3/4
|
95
|
1,120
|
62
|
93
|
|
48
|
417
|
3 7/8
|
98
|
960
|
53
|
80
|
|
Minimum size
|
No. of apples required per tree for tree
density, desired yield and fruit size
|
|||||
|---|---|---|---|---|---|---|
| No. of apples required to fill 18 bu. bin |
Projected yield (Bu/ac)
|
|||||
|
Tray Size
|
wt (g)1
|
in
|
mm
|
750
|
1,000
|
|
|
196
|
102
|
2 1/4
|
57
|
3,920
|
163
|
272
|
|
175
|
114
|
2 3/8
|
60
|
3,500
|
146
|
243
|
|
163
|
123
|
2 1/2
|
64
|
3,260
|
136
|
226
|
|
150
|
133
|
2 5/8
|
67
|
3,000
|
125
|
208
|
|
138
|
145
|
2 3/4
|
70
|
2,760
|
115
|
192
|
|
125
|
160
|
2 7/8
|
73
|
2,500
|
104
|
174
|
|
113
|
177
|
3
|
76
|
2,260
|
94
|
157
|
|
100
|
200
|
3 1/8
|
79
|
2,000
|
83
|
139
|
|
88
|
227
|
3 1/4
|
83
|
1,760
|
73
|
122
|
|
80
|
250
|
3 3/8
|
85
|
1,600
|
67
|
111
|
|
72
|
278
|
3 1/2
|
89
|
1,440
|
60
|
100
|
|
64
|
313
|
3 5/8
|
92
|
1,280
|
53
|
89
|
|
56
|
357
|
3 3/4
|
95
|
1,120
|
47
|
78
|
|
48
|
417
|
3 7/8
|
98
|
960
|
40
|
67
|
1 ppm = 1 g active ingredient per 1000 L water
Accel is an altered form of Promali that contains the growth regulator benzyladenine (BA), which is the active ingredient for thinning, and gibberellin A4+7. Accel is registered for use in Canada at the rate of 2.5-4.0 L/ha, which is equivalent to 49-74 g BA/ha. The use of Accel is limited to 2 sprays per season, each containing no more than 74 g BA/ha. Since the label is based on product/ha, the maximum concentration applied depends on tree size and volume of water used to obtain good coverage. Research at the Simcoe Horticultural Experiment Station and elsewhere indicates that the concentration of BA in the spray solution is also important. Concentrations below 50 ppm BA are generally ineffective for thinning or improving fruit size.
Accel is not a satisfactory thinning compound for all cultivars but has exhibited effectiveness for Empire, McIntosh, Idared, Gala and others. See Table 7-20, Suggested rates for chemical thinning of mature apple trees.
Unlike other chemical thinners currently available in Canada, such as NAA and carbaryl, Accel has the potential to improve fruit size independent of thinning. Accel causes increased cell division shortly after flowering during the early stages of fruit growth and development, and this translates into larger fruit at harvest.
The window of best response for Accel is between 5- 12 mm fruitlet size. Dilute Accel in 500-1,000 L of spray solution/ha. Uniform and thorough coverage is essential.
| Top of Page |
To optimize plant uptake of the spray solution, apply Accel during periods of slow drying (for example, early morning). Best results are obtained when warm temperatures greater than 20° C occur during and following application. Always read the label before using.
First, determine the concentration of BA to use. Secondly, determine the water volume required to wet the foliage just to the point of runoff. Then, use Table 7-18, Accel rate, to determine the volume of Accel to add to deliver the correct amount of BA in the appropriate concentration to be effective.
Table 7-18. Accel Rate: The relationship between spray water volumes required to thoroughly wet trees, grams active ingredient (BA) per hectare, and concentration of BA and per hectare rate of Accel.
| Spray
Volume required to thoroughly wet trees (litres/ha) |
grams BA per hectare |
||||
|---|---|---|---|---|---|
|
25
|
37
|
49
|
62
|
74
|
|
|
litres of Accel per hectare |
|||||
|
1.3
|
2.0
|
2.6
|
3.3
|
3.9
|
|
|
Concentration of BA in parts per million (ppm) |
|||||
|
200
|
124
|
185
|
247
|
309
|
371
|
|
300
|
82
|
124
|
165
|
206
|
247
|
|
400
|
62
|
93
|
124
|
154
|
185
|
|
500
|
49*
|
74
|
99
|
124
|
148
|
|
600
|
41*
|
62
|
82
|
103
|
124
|
|
700
|
35*
|
53
|
71
|
88
|
106
|
|
800
|
31*
|
46*
|
62
|
77
|
93
|
|
900
|
27*
|
41*
|
55
|
69
|
82
|
|
1000
|
25*
|
37*
|
49*
|
62
|
74
|
*Note: These concentrations of BA are not recommended.
| Top of Page |
NAD, such as Amid Thin, is a hormone-type material that is absorbed by foliage, where it induces formation of the abscission layer between the spur and the fruitlet. NAD is a relatively safe material. Apply NAD at first petal fall; i.e. when petals on the king blossom are falling. Use concentrations of 50-100 ppm, depending on variety and growing conditions. NAD is particularly effective on hard to thin varieties such as Golden Delicious, if used at the proper timing. NAD is mostly absorbed from the original solution. Once dry, the effect it has on thinning is over. Therefore use large water volumes applied under slow drying conditions. Weather that promotes soft, succulent and rapid growth leads to increased absorption. Avoid windy, cool, fast drying conditions. Temperatures between 21°C-24°C are considered optimum.
| Top of Page |
NAA, such as Fruitone-N, is a hormone-type thinner that is absorbed primarily through the leaves. NAA affects auxin activity and interferes with proper fruit development by accelerating ethylene biosynthesis. The fruitlet being thinned usually drops within 10-14 days after spraying. Plants absorb NAA only when it is dissolved in water. Almost all NAA is absorbed from the original solution. Once dry, the thinning effect is stopped. Therefore, use larger volumes of water under slow drying conditions. The longer the time required to dry the spray and the higher the drying temperatures, the greater the absorption of the NAA. NAA has a short life due to breakdown by sunlight. The most effective time to spray is under warm, dull, slow drying conditions. The preferred time is in the morning when the trees are damp, drying is slow, and the temperature is on the rise. The material is absorbed and the tree has an immediate growing day to use the thinner. NAA is considered to be rain fast after 2-3 hours. NAA is not effective at temperatures less than 10oC. NAA can result in leaf flagging (wilting) but these leaves recover in a few days.
| Top of Page |
Carbaryl is the active ingredient in the insecticide Sevin. Carbaryl is absorbed primarily through the fruit, not the foliage. The presence of carbaryl in the vascular system of the fruitlet interferes with biochemical processes. Certain important fruit growth processes cease, and the fruitlet drops. This fruitlet drop usually starts 5-10 days after application. Thinning with Sevin increases with increasing concentrations up to 1.0 kg active carbaryl (2.0 L Sevin XLR Plus)/1,000 L of water. There may be little or no increase in thinning with higher concentrations. Carbaryl is relatively insoluble in water and low concentrations produce a saturated solution. This insolubility limits its effectiveness because absorption by the fruitlet is restricted to materials in true solution. It is an advantage however, because the hazard of over thinning through concentration errors is reduced. Surface residues of the XLR Plus formulation of Sevin (unlike the 50 W formulation) do not provide additional thinning effects when rain or dews cause re-wetting of the tree canopy. Sevin, although the easiest and safest thinner to use can, depending on the amounts used, be harsh on beneficial insects and mite predators.
Note: 1 L of Sevin XLR Plus contains approximately the same amount of active ingredient (carbaryl) as 1 kg of Sevin 50 W.
Bee Warning: When Sevin is used in fruit thinning, extensive bee kills can occur if weeds or legumes are blooming in the ground cover. To reduce the chance of poisoning, remove bees from the orchard prior to treatment. Do not spray when the wind will carry Sevin to adjacent weedy or crop areas in bloom. Sevin XLR Plus is much safer to use than Sevin 50 W around honeybees. Advise local beekeepers of your spraying activity.
| Top of Page |
This combination has been used successfully on hard-to-thin cultivars such as Paulared, Golden Delicious, Fuji and Gala strains. Use a constant rate of 1 L Sevin XLR Plus (product)/1,000 L of water with varying rates of NAA from 3-15 ppm. Because NAA only has 1 chance to work, use high water volumes.
| Top of Page |
Plants absorb thinning materials more quickly at higher temperatures than lower ones. Best temperatures for absorption are in the 20°C-25°C range. At these temperatures, drying time is usually much faster than early morning or evening when temperatures are 10°C-15°C cooler. Absorption under cooler conditions appears to be slower and stretches over a longer period or until the canopy dries.
The total amount of thinning chemical absorbed by the tree canopy appears to be more or less equal regardless of drying time (warm and fast, or cool and slower). Time of day is not a critical factor from a drying perspective. Research on selected cultivars indicate no differences in success based on timing of sprays during day or night. However spraying in early morning or evening reduces the likelihood of blowing thinners off target. Wind can become more of a factor in the day.
The following weather conditions may increase the thinning response by permitting greater absorption of chemicals:
A decrease in thinning response can result from sunny and warm daytime temperatures accompanied by relatively cool nighttime temperatures before and after thinning treatments have been applied. This results in a high net accumulation of carbohydrate reserves in the tree.
Pollination and bee activities are closely tied with weather conditions. Pollinating conditions and bee populations may render fruit either more or less difficult to thin. Fruit that is set under good pollinating conditions and results in increased seed count is harder to thin. Fruit that is set on king blossoms is more difficult to thin than fruit set on side blossoms. Single fruit are more difficult to thin than clustered fruit. A light or spotty set, with clustered fruit, should be thinned but at low rates in order to break up the clusters.
1 Petal fall - when 81% of the petals have fallen off but before calyx lobes are closed.
| Cultivar | Diameter of largest fruitlets in cluster |
|---|---|
| McIntosh |
8.0-9.5 mm
|
| Red Delicious |
6.5-8.0 mm
|
| Northern Spy |
10.0-11.0 mm
|
| Empire, Idared, Jonagold |
8.0-10.0 mm
|
| Gala, Golden Delicious, Fuji |
8.0-9.0 mm
|
| Top of Page |
Collect 50-60 fruitlets of each cultivar to provide a good estimate of fruit development in a reasonably uniform orchard block. Select the 2 largest developing fruits from each of 25-30 randomly selected clusters. Measure the greatest diameter of each and determine the average for the sample. Vernier calipers or a thinning template provide a simple means for taking measurements rapidly and accurately.
| Top of Page |
| Top of Page |
Depending on the severity of the damage, growers may thin sections or areas of the orchard that appear to have escaped the worst of the frost damage. Differences in elevation and airflow in the orchard can lead to great variations in frost damage to tree foliage and potential fruit set. Trees in the same block but on lower ground can be exposed to air 2°C-3°C colder than trees on higher ground in the same frost event.
In making a decision to apply thinners, the grower may plan to spray some of the block because of what appears to be a fairly normal set on trees on higher ground. Nozzles can be shut off in areas of the block that are showing significant potential losses in crop load from frost. Likewise, nozzles directing thinning sprays into the lower canopy can be shut off if necessary.
Carefully evaluate damage to the bloom. Petals, stamens, pistils and ovaries can exhibit a significant difference in their response to freezing temperatures. (See Figure 7-6, Anatomy of an apple bloom). Although flower petals are browned off by frost, the process of set can still take place from pollination that occurred 1-2 days before the frost event.
Frost around bloom usually produces a variety of symptoms. Bands of russetting that run either around the fruitlet (frost rings) or lengthways, from stem to calyx, are a symptom of frost damage. Frost can also reduce seed count and result in a high proportion of misshapen fruit at harvest.
Exercise extreme caution when thinning after a significant frost. Leaves damaged by frost absorb more of the thinner. Fruitlets with reduced seed count abscise more easily. Consider applying lower rates of thinner for many varieties under these circumstances. Or, on a badly damaged block, do not apply a thinner but rely on hand thinning if necessary. However, a light amount of frost damage should not prevent the use of thinning treatments that worked in the past for any given cultivar.
| Top of Page |
Response to thinning treatments is generally based on size and dominance differences within the cluster of set fruitlets. The king fruitlet of most cultivars is usually larger, stronger and more dominant than side-bloom fruitlets. (An exception is Paulared). The king will normally hold on when weaker, smaller fruitlets drop in response to treatments.
If the king bloom or king fruitlet is lost to frost, size and strength differences amongst remaining fruitlets can be minimal, making them equally weak and vulnerable to thinners. The best approach in this case may be to wait until size differences appear in the cluster (provided the largest fruitlet is not greater than 12-14 mm). These size differences may become more apparent several days past full bloom. A difference of a few hours in pollination timing can show up in size differences of side-bloom fruitlets as the post-bloom period progresses.
| Top of Page |
Weather conditions are often not suitable for applying thinners even though the fruitlet diameter is at the recommended stage of development. Poor conditions can include cold, wet, windy or rainy weather that can persist for days at a time. Growers are better off waiting for warmer temperatures, more sunlight and less wind, even though the fruitlet diameter could be larger than the diameter suggested for application of thinners. Thinners applied when king fruitlets are 12-14 mm in diameter will give a better response under warmer conditions than thinners applied to king fruitlets at 8-10 mm in persistent cold, wet and rainy conditions. Warmer temperatures include temperatures at or above 18°C during the day of application or for 24 hours after spraying.
| Top of Page |
In most cases apply thinners even when the crop is unevenly set. Single fruitlets are much tougher to remove from the canopy. These fruitlets can be present at the same time as multiple-set clusters in the same tree or block. In comparison, fruitlets found in clusters of 3-5 are easier to thin because of competition within the group. Ultimately, only 5%-10% of the bloom, in any cultivar, produces a commercial crop of fruit. Therefore in situations of uneven set on unstressed trees and in the absence of frost injury, follow the recommended thinning treatments.
| Top of Page |
Ambrosia
This spury upright growing cultivar has been successfully
thinned in Ontario with moderate rates of Sevin XLR. A well-thinned
crop of Ambrosia finishes strongly with regard to size and colour
development. The picking window for Ambrosia is narrow if the crop
is to be held in long-term storage.
Cameo
Trees can normally handle heavy crop loads in a vigorous
canopy. This cultivar has been shown to thin adequately with Sevin
XLR (1L/ 1000L water) on established trees.
Creston
Creston has been shown to overthin easily with low to moderate
rates of Sevin XLR. It is a productive large size cultivar that
requires two pickings.
Empire
Before application of NAA, make sure the king bloom fruitlet
is set through a check of sepal posture. Petal fall of king bloom
can take place as early as 3-4 days past peak pollination. If NAA
is used too early (full bloom to petal fall), the overall fruit
size of unthinned apples may be reduced compared to treatment at
a stage when the king fruitlet is 8-10 mm in diameter.
Enterprise
Enterprise appears to thin adequately with moderate rates
of Sevin XLR.
Fortune
Fortune is a larged sized, highly coloured, dual-purose
apple that requires thinning to avaoid biennial bearing. Fortune
requires a strong treatment of NAA in combination with Sevin XLR.
Experienced growers have reported success with 10 ppm NAA plus 1L
Sevin XLR/ 1000 L water.
Fuji
This cultivar exhibits strong biennial characteristics
and unthinned Fuji normally returns very little bloom the next year.
Fuji normally returns very little bloom the next year. Fuji must
be thinned aggressively for best results. Mature trees usually thin
adequately with combinations of Sevin and NAA but follow-up hand
thinning is usually necessary to further reduce the number of king
fruitlets. Excessive crop may prevent the fruit from maturing to
premium quality.
Gala
Gala is relatively difficult to thin successfully. On mature
trees, the approach must be aggressive. Good results have been achieved
using a combination of NAA and Sevin at the rates listed for Golden
Delicious. The suggested timing for this application is when king
fruitlets are at 8-9 mm. Fruit size potential is determined within
2 weeks following calyx. Clusters must be thinned to 1 fruitlet
(king bloom) by mid to late June. A follow-up hand thinning within
30 days of full bloom is normally required to further reduce crop
load.
Gingergold
This cultivar appears to be more sensitive to thinners
than McIntosh, Northern Spy and Empire. It can be overthinned with
high rates of NAA or strong combinations of NAA and Sevin XLR or
Accel plus Sevin XLR. Most growers would consider this cultivar
to be uncomplicated as far as thinning requirements go.
Golden Supreme
Unlike Golden Delicious, this mid season Golden Delicious
type will thin adequately with 1 L of Sevin XLR/ 1000 L water.
Goldrush
Goldrush requires a combination of NAA and Sevin XLR (10
ppm NAA + 1L Sevin XLR/ 100 L water). This cultivar is much tougher
to thin adequately but will mature a well-thinned crop more easily.
Honeycrisp
This cultivar seems to be more sensitive to thinning treatments
than Empire. Thin Honeycrisp cautiously, as it can easily become
oversized and is very prone to bitterpit. Use of growth regulators,
which enhance fruit size, may not be warranted for this large fruited
cultivar. Use no more than 2.5 ppm NAA with 1 L Sevin XLR per 1,000
L water as a starting point on established trees. Many growers report
good fruit size distribution when the largest fruitlets are removed
by hand thinning following the spray thinning treatment. Honeycrisp
is highly biennial in nature and may not return an adequate crop
year after year.
For a discussion on cropload and trunk circumference with respect to thinning see OMAFRA Factsheet: Commercial Production of Honeycrisp, Order No. 05-047.
JonagoldShizuka
Shizuka thins similarly to Crispin with moderate rates
of Sevin XLR.
Silken
Silken thins with 1-1.5L of Sevin XLR/ 1000 L water, depending
on tree age. This cultivar bruises easily; therefore even fruit
distribution in the canopy, which enables careful picking and handling,
is desirable.
| Top of Page |
The rates in Table 7-20, Suggested rates for chemical thinning of mature apple trees, are suggested guidelines only. There is no substitute for personal experience. It is important to keep yearly records on weather conditions, rates, etc., and above all, that some trees be left unsprayed to help assess the thinning response. Use only one of the suggested chemical treatments. For example, with Wealthy, apply NAD or NAA or Sevin, or the combination. Before altering or modifying a treatment that has proven to be effective, refer to a crop consultant.
In some cases, it is desirable to defruit immature apple trees to encourage proper tree development. A dilute spray of NAA at 15 ppm combined with Sevin XLR Plus at 1.5-2 L/1,000 L water 5 days after petal fall, gives acceptable results. Well established, supported trees under minimal stress can carry some crop load in the 2nd year following planting.
For highly-valued cultivars, hand thinning immature trees is less risky than spray thinning. Hand thin promptly following June drop.
Hand thinning:
Table
of Contents 